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| − | =The right way to kiss: directionality bias in head-turning during kissing=
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| − | ''' The full paper '''
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| − | A. K. M. Rezaul Karim, Michael J. Proulx, Alexandra A. de Sousa, Chhanda Karmaker, Arifa Rahman, Fahria Karim & Naima Nigar
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| − | Humans have a bias for turning to the right in a number of settings. Here we document a bias in head-
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| − | turning to the right in adult humans, as tested in the act of kissing. We investigated head-turning bias in
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| − | both kiss initiators and kiss recipients for lip kissing, and took into consideration differences due to sex
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| − | and handedness, in 48 Bangladeshi heterosexual married couples. We report a significant male bias in
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| − | the initiation of kissing and a significant bias in head-turning to the right in both kiss initiators and kiss
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| − | recipients, with a tendency among kiss recipients to match their partners’ head-turning direction.
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| − | These
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| − | interesting outcomes are explained by the influences of societal learning or cultural norms and the
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| − | potential neurophysiological underpinnings which together offer novel insights about the mechanisms
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| − | underlying behavioral laterality in humans.
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| − | It is well documented that humans have a bias for turning to the right in a number of settings. For example, most
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| − | newborn infants prefer to lie with their heads turned to the right, rather than to the left1.
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| − | Strikingly, even human
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| − | fetuses prefer to turn their heads to the right during the final weeks of gestation at an age for freely moving the head
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| − | . This preference is also exhibited by newborns of both vaginal and caesarean deliveries4–6
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| − | and maintained
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| − | for the first six months of birth2, 3
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| − | . The rightward bias in head-turning may occur in response to both aversive
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| − | and nonaversive stimulation7
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| − | and is more frequent among the children of two right-handed parents8
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| − | . This is one
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| − | of the earliest behavioral asymmetries in humans and is thought to predict handedness1, 2, 9–13, and also affect the
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| − | subsequent development of spatial-perceptual and motor preferences by increasing visual orientation to the right
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| − | side14–17.
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| − | Empirical evidence of this effect has been demonstrated in a number of studies in both real life and artificial
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| − | situations. For example, Güntürkün18 was the first to test this effect in 124 couples kissing (unless otherwise
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| − | specified, kissing refers to lip-kissing in this paper) in public places like international airports, railway stations,
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| − | beaches and parks in the United States, Germany and Turkey. He recorded the head-turning behavior of each cou-
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| − | ple during a single kiss and reported that 64.5% (80) of the couples turned their heads to the right and 35.5% (44)
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| − | turned their heads to the left during kissing. Barrett, Greenwood and McCullagh19 studied laterality in both pub-
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| − | licly kissing couples and in individual participants kissing dolls in a laboratory setting. In line with Güntürkün18,
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| − | these researchers found that 80% of kissing couples and 77% of individual participants kissing dolls turned their
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| − | heads to the right. More recent studies in a laboratory setting also consistently reported a rightward head-turning
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| − | bias in kissing20, 21. A very recent study has suggested that laterality in kissing is contextual, showing a rightward
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| − | head-turning bias in romantic couples (including parent-parent) and a leftward bias in a parent-child kissing
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| − | situation22.
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| − | Theories of laterality have put forward a few postulates to explain the rightward head-turning bias in kiss-
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| − | ing. One of the postulates relates to the innate motor bias in head-turning18, 19. It posits that laterality in kissing
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| − | in adult humans reflects the persistence of a bias for turning the head to the right that is present in fetuses and
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| − | newborns18. A second postulate proposes that the head-turning bias in kissing is related to other sorts of lat-
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| − | eralities, such as handedness and footedness20, 21. A more recent study has challenged both these postulates by
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| − | investigating how cultural factors, such as a predominant reading or writing direction, contribute to laterality in
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| − | kissing23. Using a cross-cultural approach that study examined head-turning direction in both kissing couples
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| − | and doll kissing in a laboratory setting. It revealed that a significantly rightward head-turning bias during kissing
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| − | was apparent in left-to-right readers (e.g., English) and a significantly leftward head-turning bias in right-to-left readers (e.g., Hebrew, Arabic)23. Thus the rightward bias in kissing that is observed in Western societies is turned
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| − | into a leftward bias in Middle-Eastern societies. Based on this finding, Shaki23 suggested that the directional
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| − | bias in head-turning can be shaped by cultural spatial habits, rather than reflecting an innate lateral asymmetry.
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| − | Thus it appears that laterality in kissing is a complex issue and this complexity might result from the influence of
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| − | socio-cultural pressures on behavioral laterality through interactions with its genetic endowment. Socio-cultural
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| − | pressures can implicitly or explicitly force individuals to align their behavior to that of their peers within the pop-
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| − | ulation. In support of this, a recent study which examined cheek kissing in humans in 10 cities of France demon-
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| − | strated that social pressures are involved in modulating laterality at the individual or population level24. Though
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| − | cheek kissing and lip kissing are different in terms of movement, force, emotion, and meaning24 the mechanisms
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| − | through which social pressures might act upon the individuals to determine this laterality are possibly the same.
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| − | However, this should not be confused with the leftward head-turning bias in parent-child kissing which is also
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| − | thought to be learned from society22, but through a different kind of mechanism – cradling infants or babies using
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| − | the left arm by parents25–27.
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| − | Although prior studies have made important contributions to the understanding of laterality in kissing, a limi-
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| − | tation is that they did not investigate the roles of kiss initiators and kiss recipients in producing the head-turn dur-
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| − | ing kissing. Studies that investigated kissing in romantic couples present findings of ecological significance18, 19, 23,
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| − | but thus far have failed to disentangle the influence of the kiss initiators on the kiss recipients' head turn. Studies
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| − | that examined laterality in a laboratory setting have taken a step forward methodologically by introducing dolls
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| − | or plastic heads as neutrally valenced kiss recipients19–21. This approach enabled researchers to examine kissing
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| − | behavior in a laboratory setting by excluding the influence of one kissing partner upon another, but at the same
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| − | time, it made them unable to examine head-turning bias in kiss recipients and the true nature of head-turning
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| − | among the kissers in such an artificial non-emotional situation. Kissing is a form of non-verbal communication,
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| − | a kind of tactile intimacy, characterized by lively romantic-sexual interactions made by lip-to-lip contact between
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| − | the partners of a couple. Related to this, prior research has shown significant sex differences in other types of
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| − | sexual intimacy or heterosexual interactions (e.g., sexual dating, sexual initiation), with males playing the more
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| − | dominant role of the initiator, more often than females, according to studies conducted in some Western28, 29
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| − | and rich liberal Asian30 societies, that is, W.E.I.R.D. (Western, Educated, Industrialized, Rich, and Democratic)
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| − | societies31. This point reiterates the need for further research including non-W.E.I.R.D. participants in order to
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| − | understand human nature more broadly. It is likely that such a male bias also happens in the initiation of kissing,
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| − | and could be even more pronounced in participants of a non-W.E.I.R.D. patriarchal (male-dominated)32 con-
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| − | servative Muslim society, like Bangladesh. Because kisses are rarely shared between an adult male and an adult
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| − | female in public in this society it is very difficult to anticipate the social influences, if any, on the development
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| − | of laterality during kissing. A benefit of studying this population is that perhaps there will be far less influence
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| − | of social observation and conformity given the private nature of kissing there. However, it is commonly known
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| − | that people in this Asiatic society predominantly read and write in a left–to–right direction which, according to
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| − | Shaki’s23 cultural shaping theory, might be associated with laterality in kissing. But, data from such a society are
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| − | lacking. Thus these aspects of laterality in kissing have been partially and poorly understood, warranting a study
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| − | in such a non-W.E.I.R.D. patriarchal society31.
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| − | In addition to cultural representation, another main concern here is the methodology of studying such
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| − | face-to-face romantic-sexual interactions. We argue that studying the interactions of a kissing pair might only
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| − | demonstrate the head-turning bias of the dominant partner who initiates the kiss: There will be a first kisser, first
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| − | turner! The direction of the kiss initiator’s head-turning to a particular side, which we call spontaneous turning in
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| − | the first single kiss, will exhibit the actual head-turning bias of that partner, but this will not tell us anything about
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| − | the spontaneous and actual head-turning direction of the kiss recipient. The kiss initiator might indeed have a
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| − | bias to turn the head to the right, but at that point the kiss recipient (who might not actually be a right-turner)
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| − | might do the same submissively in order to avoid any potential discomfort which might be felt upon turning the
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| − | head to the opposite side (i.e., orienting at the same line in space). This indicates that the head-turning direction
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| − | of the kiss recipient might not always be in a spontaneous direction, the direction he/she would prefer if he/she
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| − | were to initiate the kiss. The spontaneous head-turning bias can thus be estimated for only one partner in a kiss-
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| − | ing pair, unless otherwise the two partners are biased to turn their heads to the same side (in reference to self).
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| − | The two partners possibly cannot turn their heads to the opposite sides simultaneously; turning the two heads
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| − | to the opposite sides means orienting them at the same line in space which is perhaps conflicting and inconven-
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| − | ient for enjoying the act of kissing. So, when a right-turner kisses a left-turner first, the left-turner will possibly
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| − | turn the head to the right– even though it is not the preferred direction, and similarly when a left-turner kisses
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| − | a right-turner first, the right-turner will also likely turn the head to the left. Thus, in a kissing pair comprising
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| − | partners of spontaneous head-turning biases in the opposite directions (i.e., one partner is a right-turner and
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| − | another partner is a left-turner) the spontaneous head-turning direction of the kiss initiator is likely to alter the
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| − | spontaneous/preferred head-turning direction of the kiss recipient. However, such an impact of one partner on
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| − | the other cannot be anticipated in a kissing pair comprising partners of spontaneous head-turning biases in the
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| − | same direction (i.e., both the partners are right- or left-turners).
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| − | It follows from the above line of reasoning that Güntürkün’s18 data indicate that 32.26% (80) of the kissing
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| − | individuals (248) made a spontaneous head-turn to the right and 17.74% (44) made a spontaneous head-turn to
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| − | the left, while the remaining 50% (124) of the kissing individuals’ spontaneous head-turning direction was not
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| − | actually examined. Similarly, the spontaneous head-turning direction of 50% of the kissing individuals (the kiss
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| − | recipients) was not studied in any other prior studies on kissing couples19, 23 as they used a method which was
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| − | oriented solely on the behavior of the kiss initiators. Thus 100% of the kiss recipients’ (spontaneous) head-turning
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| − | direction was, in fact, unknown in all the prior studies, indicating that the findings they reported were erroneous
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| − | being that an important aspect – the recipient behavior – was missing. Besides the previously unexamined behav-
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| − | ior of the kiss recipients, a number of other variables including sex and handedness that can potentially modulate
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| − | the initiation of kissing and head-turning bias in kissing have not been studied systematically. Males and females
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| − | are different in many ways, and such differences can vary as a function of handedness33–36, prenatal hormone
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| − | levels37 and dopaminergic levels in the brain38–40. A mounting body of evidence suggests that these factors can
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| − | potentially modulate sexual dominance, romantic interactions, and behavioral laterality in humans17, 41, 42. For
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| − | example, testosterone, an endogenous hormone, which is higher in males than females, regulates sex drive (libido
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| − | in humans, courtship behavior in animals)42 and appears to germinate seeds of personality43; a higher level of
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| − | testosterone makes the person more dominant, sexually more active and aggressive41. In face-to-face emotional
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| − | or social interactions, testosterone can also play the role of promoting status-seeking behavior by a modulation of
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| − | reward processing and motivational drive in the dopaminergic system41, 44, 45. Beyond the influence of testoster-
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| − | one, dopamine (DA, a neurotransmitter) has also its unique role to modulate leaning or turning behavior perhaps
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| − | via handedness in both model organisms and humans17.
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| − | Here we formulate three hypotheses grounded on the above research findings. First, we hypothesize that there
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| − | is a sex difference in kissing – typically males play the role of an initiator and females play the role of a passive
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| − | recipient. A second hypothesis is that handedness determines head-turning direction, particularly spontaneous
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| − | head-turning direction, in the first kiss which can typically be observed and tested in the partners who initiate the
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| − | kiss rather than those who receive it. Related to this is our third hypothesis which states that irrespective of sex
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| − | and handedness the head-turning direction of the kiss recipients is modulated by the head-turning direction of
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| − | the kiss initiators. In order to test these novel hypotheses, overcome the limitations of prior studies (see above),
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| − | and extend the research to a new understudied non-W.E.I.R.D. population31, we assessed 51 heterosexual married
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| − | couples in Bangladesh. Indeed, this is the first study on laterality in kissing in a private naturalistic context that
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| − | eliminates the effects of an observer or public on such an intimate form of physical interactions. Data about this
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| − | kind of private romantic behavior from such a non-W.E.I.R.D. patriarchal conservative Muslim society which is
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| − | often overlooked in scientific enquiry31 advance our understanding of the aspects of kissing and lateral bias in
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| − | kissing behavior in adult humans.
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| − | ==Results ==
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| − | First, we calculated the proportion of males and females initiating a kiss. We observed that 79.17% of the kiss
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| − | initiators were males (Figure 1), and that the ratio of males and females initiating a kiss was significantly differ-
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| − | ent from 50% (χ2
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| − | (1)=16.33, p<0.001). This has further been corroborated by the outcomes of a binary logis-
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| − | tic regression where we examined the effects of sex and handedness on the likelihood that participants have a
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| − | bias in the initiation of kissing. The logistic regression model was significant (χ2
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| − | (2)=34.878, p<0.001). The
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| − | non-significant value of the Hosmer and Lemeshow test (p=0.993) suggests that our model is a good fit to the
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| − | data. The model explains 40.60% (Nagelkerke R2
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| − | ) of the variance in the initiation of kissing. Table 1 displays the
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| − | logistic regression coefficient (B), Wald, and Odds Ratio (Exp(B)) for each of the two explanatory variables. It
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| − | shows that there is a significant effect of sex (Wald=28.186, df=1, p<0.001), but not the effect of handedness,
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| − | on the initiation of kissing. The Odds Ratio for sex indicates that when holding handedness constant males are
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| − | 14.532 times more likely to initiate kissing as opposed to females.
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| − | Second, we calculated the proportion of participants turning the head to the right and that of those turning
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| − | the head to the left during kissing. Kiss initiator provided data revealed that 72.92% of the kiss initiators and 75%
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| − | of the kiss recipients turned their heads to the right (Figure 2a), the ratio of the right- and left-turners in each cat-
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| − | egory being significantly different from 50% (χ2
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| − | (1)=10.083, p=0.001 for kiss initiator category; χ2
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| − | (1)=12.000,
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| − | p=0.001 for kiss recipient category). Consistently, kiss recipient provided data revealed that 66.67% of the kiss
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| − | initiators and 70.83% of the kiss recipients turned their heads to the right (Figure 2b), and that the ratio of the
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| − | right- and left-turners in each category was significantly different from 50% (χ2
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| − | (1)=5.333, p=0.021 for kiss
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| − | initiator category; χ2
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| − | (1)=8.333, p=0.004 for kiss recipient category). Similar results were demonstrated when
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| − | the data were analyzed again by taking all of the participants together irrespective of who initiated or received
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| − | the kiss. Overall, 68.75% (kiss recipient provided) to 73.96% (kiss initiator provided) of the participants turned
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| − | their heads to the right (Figure 2), and the ratio of the right- and left-turners was significantly different from 50%
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| − | (χ2
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| − | (1)=13.500, p=0.001 for kiss recipient provided data; χ2
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| − | (1)=22.042, p=0.001 for kiss initiator provided
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| − | data).
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| − | In order to see the effects of sex and handedness on such a bias during kissing, we performed binary logis-
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| − | tic regressions on the collective head-turning data for the kiss initiators and the kiss recipients (Table 2). The
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| − | overall logistic regression model was significant (χ2
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| − | (2)=7.876, p=0.019 for kiss initiator provided data) or
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| − | showed a tendency in the direction of significance (χ2
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| − | (2)=5.422, p=0.066 for kiss recipient provided data).
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| − | The non-significant values of the Hosmer and Lemeshow test suggest that our models are a good fit to both the
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| − | kiss initiator provided data (p=0.437) and the kiss recipient provided data (p=0.771). The overall model for
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| − | the kiss initiator provided data explains 11.50% (Nagelkerke R2
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| − | ) of the variance in head-turning bias, and the
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| − | overall model for kiss recipient provided data explains 7.70% (Nagelkerke R2
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| − | ) of the variance in head-turning
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| − | bias. Table 2 shows that there is a significant effect of handedness on head-turning bias as demonstrated by the
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| − | outputs for both the kiss initiator provided data (Wald=6.856, df=1, p=0.009) and the kiss recipient provided
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| − | data (Wald = 4.732, df = 1, p = 0.030). The Odds Ratio for handedness indicates that when holding sex con-
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| − | stant, an increase in handedness score by 1 unit (increased right-handedness) increases the likelihood of overall
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| − | head-turning bias to the right by 1.013 times (for kiss initiator provided data) or 1.010 times (for kiss recipient
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| − | provided data). However, the impact of participants’ sex on head-turning bias was not significant.
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| − | Third, we also performed binary logistic regressions to see the impacts of sex and handedness on the
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| − | kiss initiators’ head-turning bias and the kiss recipients’ head-turning bias separately. The regression model
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| − | for the kiss initiators’ head-turning data was significant (χ2
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| − | (2) = 7.100, p = 0.029 for kiss initiator pro-
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| − | vided data) or showed a tendency in the direction of significance (χ2
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| − | (2) = 4.698, p = 0.095 for kiss recipient
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| − | provided data). As revealed by Nagelkerke R2
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| − | values, the models explained 12.90% (kiss recipient provided
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| − | data) to 20.00% (kiss initiator provided data) of the variance in head-turning bias in the kiss initiators. As
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| − | depicted in Table 3, results further showed that the effect of handedness on head-turning bias was signifi-
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| − | cant (Wald = 4.591, df = 1, p = 0.032 for kiss initiator provided data) or showed a tendency in the direction
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| − | of significance (Wald = 3.459, df = 1, p = 0.063 for kiss recipient provided data) in the kiss initiators. The
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| − | Odds Ratio for handedness indicates that when holding sex constant, an increase in handedness score by 1
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| − | unit (increased right-handedness) increases the likelihood of turning head to the right by 1.019 times (for
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| − | kiss initiator provided data) or 1.015 times (for kiss recipient provided data). However, the regression mod-
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| − | els for the kiss recipients’ head-turning data provided by both kiss initiators and kiss recipients fell short of
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| − | statistical significance (data not shown).
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| − | =Two tables here=
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| − | Then we performed another binary logistic regression, adding the kiss initiators’ head-turning direction as a
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| − | third explanatory variable of the kiss recipients’ head-turning direction (Table 4). This analysis was done for the
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| − | kiss initiator provided data only because the other set of data (kiss recipient provided) included some outliers.
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| − | Results showed that the new model for the kiss initiator provided data was significant (χ2
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| − | (2)=12.230, p=0.007),
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| − | indicating that the model with the third explanatory variable is a significant improvement not only over the null
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| − | model, but also over the model which was non-significant with sex and handedness as the explanatory variables.
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| − | The non-significant value of the Hosmer and Lemeshow test (p=0.554) suggests that the new model is a good
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| − | fit to the data. The model explains 33.30% (Nagelkerke R2
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| − | ) of the variance in kiss recipients’ head-turning bias.
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| − | Table 4 shows that there was a significant effect of the kiss initiators’ head-turning direction on the kiss recipients’
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| − | head-turning direction (Wald=9.382, df=1, p=0.002), but the effects of the kiss recipients’ sex and handedness
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| − | were non-significant. The Odds Ratio for the kiss initiators’ head-turning direction indicates that when holding
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| − | the kiss recipients’ sex and handedness constant, a rightward head turn by the kiss initiators is 11.327 times more
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| − | likely to alter the kiss recipients’ head turn to the right, as opposed to a leftward head turn by the kiss initiators
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| − | altering the kiss recipients’ head turn to the left.
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| − | The dependence of the kiss recipients’ head-turning direction on the kiss initiators’ head-turning direction
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| − | was further corroborated by the evidence that 78.12% of the participants reported kissing with the two heads
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| − | turned to the opposite direction (i.e., oriented at the same line in space) as inconvenient (Figure 3). The ratio of
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| − | this (inconvenient) proportion of participants to the remaining who reported such a kissing situation as conven-
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| − | ient was significantly different from 50% (χ2
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| − | (1)=30.375, p<0.001). By further analyzing the data in a binary
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| − | logistic regression we did not find any evidence that this difference can be accounted for sex or handedness of the
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| − | participants. Based on these findings we conclude that despite the normal tendency of handedness to modulate
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| − | head-turning direction during kissing the kiss initiators’ head-turning direction can modify or alter that effect to
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| − | a comfortable direction in the kiss recipients.
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| − | | + | |
| − | =Many tables here=
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| − | =Discussion=
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| − | This study demonstrated some novel and interesting aspects of kissing behaviors and laterality in adult humans.
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| − | Specifically, we found that there was a significant male bias (14.532 times more likely than females) in the ini-
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| − | tiation of kissing (Table 1, Figure 1), and a significant rightward bias in head-turning in both the kiss initiators
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| − | and the kiss recipients. The rightward head-turning bias in the kiss initiators was found to be associated with
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| − | their handedness (Table 3), and that in the kiss recipients was found to be associated with the kiss initiators’
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| − | head-turning direction (Table 4). Though there is no prior study to directly compare the male bias in the initiation
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| − | of kissing it is consistent with other relevant studies which showed that males are dominant over females in other
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| − | forms of sexual activities, such as sexual dating, and sexual initiation28–30. The rightward bias in head-turning
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| − | during kissing is consistent with a similar bias in whole body turning in humans17, 46–48. The proportions of the
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| − | right- and left-turners in the act of kissing (Figure 2) also closely correspond to the proportions reported in prior
| + | |
| − | studies18, 19, 23. The impact of handedness on the spontaneous head-turning direction in the kiss initiators supports
| + | |
| − | the postulate that the head-turning bias in kissing is related to handedness20 though there is also an opposite
| + | |
| − | view18, 23. The impact of the kiss initiators’ head-turning direction on the kiss recipients' head-turning direction
| + | |
| − | during kissing is in line with the evidence that participants felt discomfort while engaged in the act of kissing
| + | |
| − | with the two heads turned to the opposite directions (i.e., oriented at the same line in space; Figure 3). This latter
| + | |
| − | finding leads us to argue that irrespective of sex and handedness, the kiss recipients showed a tendency to match
| + | |
| − | their partners’ head-turning direction during kissing to avoid discomfort.
| + | |
| − | | + | |
| − | =Tables here=
| + | |
| − | | + | |
| − | An inspection of our results indicates that the regression models with sex and handedness explain more of the
| + | |
| − | variance in head-turning bias (Nagelkerke R2 : 12.90–20.00%) in the kiss initiators than the variance they explain
| + | |
| − | (Nagelkerke R2: 7.70–11.50%) in overall head-turning bias (i.e., when data for the kiss initiators and the kiss recip-
| + | |
| − | ients were analyzed collectively). The Odds Ratio showed that the predictability of handedness increased when
| + | |
| − | the kiss initiators’ head-turning bias was examined separately from the kiss recipients’ head-turning bias (Exp(B):
| + | |
| − | 1.015–1.019; Table 3) than when they were analyzed collectively (Exp(B): 1.010–1.013; Table 2); yet either way
| + | |
| − | the Odds Ratios suggested that there was not a strong effect overall in these cases. Consistent with the low Odds
| + | |
| − | Ratios, the regression models with the same two explanatory variables were non-significant for the kiss recipients’
| + | |
| − | head-turning data.
| + | |
| − | | + | |
| − | =Figure 2=
| + | |
| − | | + | |
| − | | + | |
| − | But, when the kiss initiators’ head-turning direction was taken as a third explanatory variable
| + | |
| − | in the regression model we found that the new model is significant, and explains a large amount of the variance
| + | |
| − | (Nagelkerke R2 : 33.30%) in their head-turning bias. Here the Odds Ratio showed that a rightward head turn by
| + | |
| − | the kiss initiators is 11.327 times more likely to alter the kiss recipients’ head turn to the right, as opposed to a
| + | |
| − | leftward head turn by the kiss initiators altering the kiss recipients’ head turn to the left. These findings lead us to
| + | |
| − | reiterate the argument that the kiss recipients’ head-turning direction is modulated by the kiss initiators’ sponta-
| + | |
| − | neous head-turning direction (see Introduction), and therefore it is not appropriate to analyze the head-turning
| + | |
| − | data for both partners in a collective fashion as done by prior studies18. Thus we suggest that in order to better
| + | |
| − | understand the lateral bias in kissing one should separately examine the data for the kiss initiators and those for
| + | |
| − | the kiss recipients.
| + | |
| − | At the outset, societal learning or cultural norms can account for the findings reported here. In most tra-
| + | |
| − | ditional societies, males are expected to take on the more dominant role in sexual interactions while females
| + | |
| − | are typically expected to be responsive to males’ desires and wait for them to initiate and orchestrate sexual
| + | |
| − | activities28, 29, 49, 50. This impact of socialization is perhaps more intense in a conservative Muslim society like
| + | |
| − | Bangladesh where religious norms restrict females to be less sexually active and more submissive as opposed
| + | |
| − | to males51–53. These male-favoring religious or cultural norms are commonly and often strictly followed in both
| + | |
| − | indoor and outdoor activities32. It has been suggested that females have a greater level of erotic plasticity (chang-
| + | |
| − | ing nature of sex drive) than males due to these sociocultural influences51–53. Perhaps this has been reflected in
| + | |
| − | the act of kissing by the participants in our study. Secondly, people in this Asiatic society typically have a habit of
| + | |
| − | scanning information (e.g., reading and writing text, drawing pictures) from a left to right direction which might
| + | |
| − | cause them to turn their heads to the right while engaged in kissing with romantic partners. As in other Asiatic
| + | |
| − | and Muslim societies, most of the people in this society have a right-handed tendency in daily activities (the esti-
| + | |
| − | mated proportion is about 92% in this study; see Methods section), and therefore we contend that the association
| + | |
| − | of right-handedness with the rightward head-turning bias during kissing might have been further strengthened
| + | |
| − | by the traditional cultural spatial habits.
| + | |
| − | | + | |
| − | | + | |
| − | However, cultural norms or spatial habits might not be the only factors to exclusively modulate the male bias
| + | |
| − | in the initiation of kissing or the rightward bias in head-turning during kissing. An in-depth review of the liter-
| + | |
| − | ature reveals that the male-favoring cultural or societal norms of sexual initiation in Bangladesh are in line with
| + | |
| − | the differences in biological makeup between males and females, and the right-hand bias in head-turning during
| + | |
| − | kissing is in line with the potential biological differences between the right- and left-handers. Specifically, one
| + | |
| − | potential biological factor that might interact with the cultural or societal norms to modulate the male bias in the
| + | |
| − | initiation of kissing is endogenous testosterone. Research has shown that testosterone is involved in regulating
| + | |
| − | sex drive41, 42, and also appears to affect face-to-face emotional interactions and social status-seeking motives or
| + | |
| − | behavioral dominance41, 44, 45. How could this hormone affect face-to-face emotional interactions during kissing?
| + | |
| − | =Figure 3=
| + | |
| − | =Table 4=
| + | |
| − | | + | |
| − | Few data are available to answer this question. It has been suggested that baseline testosterone reflects a person’s
| + | |
| − | personality traits43, a higher level of testosterone indicating that the person is sexually more dominant and aggres-
| + | |
| − | sive41. Relevant to this, neuroimaging studies have shown that testosterone enhances amygdala activation in both
| + | |
| − | males54 and females55, 56. In a face-to-face situation, when the two partners are spatially close to each other they
| + | |
| − | work as sexual social stimuli for each other which induce their testosterone to a larger scale relative to the base-
| + | |
| − | line57, 58. Because of a preexisting innate discrepancy in the amount of testosterone between males and females it is
| + | |
| − | plausible that the elevation of this hormone level in such a romantic situation is much higher in the male than the
| + | |
| − | female partner and that this can differentially affect their amygdala activation (though it has not yet been tested).
| + | |
| − | | + | |
| − | Thus at the preparatory stage of the act of kissing, the increased level of testosterone perhaps motivates the male
| + | |
| − | partner to take it as a challenge59, and to assert power. The female partner, due to a lower testosterone level is
| + | |
| − | instead the passive recipient by this account.
| + | |
| − | | + | |
| − | Reward processing is a crucial element in emotional or social interactions60 and social hierarchies61 and can
| + | |
| − | be influenced by testosterone. It has been suggested that testosterone might promote status-seeking behavior by
| + | |
| − | a modulation of reward processing and motivational drive in the DA system41. Thus a second potential candidate
| + | |
| − | in the biological realm that can explain the male bias to initiate kissing is DA which has a variety of dominant
| + | |
| − | behavioral or motivational functions62 and can vary across sexes. Research has shown that DA modulates pleasure
| + | |
| − | seeking behaviors, such as sex, in active males63–67 rather than receptive females68–70. In line with this, studies in
| + | |
| − | humans38–40, 71 have shown that DA levels are markedly greater in males than females. Taken together, we con-
| + | |
| − | clude that males were likely more active, and more motivated, to initiate kissing than females perhaps due to the
| + | |
| − | imbalance of testosterone and DA levels between males and females.
| + | |
| − | | + | |
| − | In addition to the modulation of motivational or pleasure seeking behaviors, DA also plays a key role in the
| + | |
| − | development of spatial attentional or orienting bias and directional bias in turning or rotational behaviors17,
| + | |
| − | 72–75. Research in both healthy humans and patients with neurological disorders (e.g., Parkinson’s disease) has
| + | |
| − | suggested that presumably, left-handers have greater dopaminergic content in the right hemisphere, whereas
| + | |
| − | for right-handers it is greater in the left hemisphere13, 76, 77, 79, 80; however, this inter-hemispheric imbalance of DA
| + | |
| − | cannot be associated with sex. Thus we contend that during kissing in a normal condition participants having
| + | |
| − | greater right-handedness might have shown a greater tendency to turn their heads to the right, and those with
| + | |
| − | greater left-handedness might have shown a greater tendency to turn their heads to the left in our study. However,
| + | |
| − | this biological factor-based interpretation of our results does not necessarily undermine the role culture or society
| + | |
| − | plays in shaping the directional bias in head or whole body turning in humans. To reiterate, a recent study has
| + | |
| − | suggested that head-turning bias during kissing is an acquired behavioral asymmetry, probably shaped by spatial
| + | |
| − | experience, such as reading or writing habits23. On the other hand, Scharine and McBeath’s81 study suggested
| + | |
| − | that the directional bias in walking is an additive function of both learning (e.g., driving practice) and genetic
| + | |
| − | handedness. Thus though inter-hemispheric imbalance of DA is an innate brain property, it is not resistant to
| + | |
| − | behavioral modification; rather it can change or even alter reversely due to explicit or implicit societal learning
| + | |
| − | or cultural habits.
| + | |
| − | | + | |
| − | According to the above discussion, the lateral bias in kissing (in the form of head-turning) develops in a
| + | |
| − | dynamic fashion where the neurogenetic factors (e.g., DA, handedness) and cultural factors (e.g., reading or
| + | |
| − | writing direction) may interplay to determine the direction and extent of bias – a view that finely fits with our
| + | |
| − | recent dynamic model that accounts for the development of a rightward (clockwise) versus a leftward (anticlock-
| + | |
| − | wise) bias in visuospatial functioning in general and turning behavior in particular17. However, the impact of the
| + | |
| − | kiss initiators’ head-turning direction on the kiss recipients’ head-turning direction demonstrated in this study
| + | |
| − | leads us to argue that the head-turning bias developed in such a dynamic manner may not always be apparent
| + | |
| − | in behavioral expression or may be apparent in a reversed direction depending on the immediate environmental
| + | |
| − | situation17. Thus in our study the kiss recipients’ tendency to match their partners’ head-turning direction during
| + | |
| − | kissing might have been to avoid discomfort that could potentially be felt upon turning the head to the opposite
| + | |
| − | side (i.e., orienting at the same line in space).
| + | |
| − | | + | |
| − | | + | |
| − | In conclusion, following an ecologically valid approach we examined lateral bias and other aspects of lip
| + | |
| − | kissing behavior in heterosexual married couples in Bangladesh. As noted earlier in this paper, Bangladesh is a
| + | |
| − | non-W.E.I.R.D.31 and patriarchal32 conservative Muslim society, where kissing is considered very private and is
| + | |
| − | not typically allowed in public places82. It was therefore preferred for us to not directly observe this, and at this
| + | |
| − | stage we could not yet make physiological recordings that directly assess the testosterone and DA hypotheses of
| + | |
| − | kissing and lateral bias when the partners of each couple were freely engaged in kissing each other at home, a
| + | |
| − | very private naturalistic setting. Secondly, until this study, there were no direct studies on the neural correlates
| + | |
| − | of kissing or lateral bias in kissing shared by the members of a romantic couple. So, backed by the relevant prior
| + | |
| − | studies and our recent theoretical work17, we propose here a number of explanatory conjectures (see above) about
| + | |
| − | the potential associations of testosterone and DA with the initiation of kissing and lateral bias in kissing, within
| + | |
| − | the context of socio-cultural milieu. These are speculative, but reasonable in terms of their known functions and
| + | |
| − | distributions across sexes and across hemispheres (see above). Thus the exact causative factors of the male bias
| + | |
| − | in the initiation of kissing and rightward bias in head-turning during kissing are still unclear and merit further
| + | |
| − | investigation. So, if feasible, future studies can attempt to measure the two suggested neurogenetic factors (testos-
| + | |
| − | terone and DA) in the kissing couples, and directly associate them with the initiation of kissing and head-turning
| + | |
| − | direction during kissing. This will enable scientists to delineate the roles testosterone and DA play in the initiation
| + | |
| − | of kissing and lateral bias in this very private behavior. However, another shortcoming of this study is that only
| + | |
| − | 7.29% of the participants sampled had a left-handed tendency (an LI score<0, see Methods section). Perhaps this
| + | |
| − | is why handedness was found to explain (though significantly) just a small part of the variance in head-turning
| + | |
| − | bias (up to 20.00%; see Results section). So, future replication studies on a sample including more participants of
| + | |
| − | a left-handed tendency may provide sufficient data to test the handedness hypothesis about the lateral bias in kiss-
| + | |
| − | ing behavior (i.e., can explain more variance in head-turning bias). A related and final limitation of this study is
| + | |
| − | the reliance on a sample of small size. Though the sample size was justified based on a review of the relevant prior
| + | |
| − | studies and was good enough for logistic regression analysis (see Methods section), future replication studies with
| + | |
| − | a large scale sample can confirm the present findings.
| + | |
| − | | + | |
| − | =Methods=
| + | |
| − | All methods were carried out in accordance with the Declaration of Helsinki guidelines83, and in accordance with
| + | |
| − | the legal requirements of Bangladesh and the institutional protocols for research at the University of Dhaka. All
| + | |
| − | participants provided informed verbal consent before participation.
| + | |
| − | | + | |
| − | ''' Participants and measures. '''
| + | |
| − | | + | |
| − | We selected 51 heterosexual married couples purposively from the city of
| + | |
| − | Dhaka. They were highly educated (Bachelor or Master’s degree) and opportunistically selected in a convenient
| + | |
| − | manner from a series of key locations, such as an office lounge, a university lounge, and socio-cultural gatherings
| + | |
| − | (wedding anniversaries, birthday parties) of our friends and relatives. The sample size (N=102; 51 males and
| + | |
| − | 51 females) was justified based on a review of the relevant literature. Prior studies on kissing and head-turning
| + | |
| − | behavior used various sample sizes, ranging from 57 to 248 individuals18, 21. To administer the survey on our
| + | |
| − | participant group we used two measures, a Head Turning Questionnaire (HTQ, designed in this study) and a
| + | |
| − | Bangla version (translated and modified) of the Edinburgh Handedness Inventory (EHI)84. The HTQ comprises
| + | |
| − | 6 items with a binary response (Me/My Partner or Left/Right or Convenient/Inconvenient). One of the items
| + | |
| − | measures spontaneous initiation of kissing (In that romantic situation, who was the first kisser?), two items meas-
| + | |
| − | ure head-turning direction in both the partners during spontaneous kissing (responded by the kiss initiator or
| + | |
| − | first kisser; If you were the first kisser, (a) which direction did you turn your head to on the first kiss? (b) which
| + | |
| − | direction did your partner turn his/her head to while receiving your first kiss?), two items measure head-turning
| + | |
| − | direction in both the partners during spontaneous kissing (responded by the kiss recipient; If your partner was
| + | |
| − | the first kisser, (a) which direction did he/she turn his/her head to on the first kiss? (b) which direction did you
| + | |
| − | turn your head to while receiving his/her first kiss?), and the last one item measures the perceived quality of the
| + | |
| − | act of kissing when the two heads were oriented at the same line in space (Was your lip kissing in such a situation
| + | |
| − | convenient or inconvenient?).
| + | |
| − | | + | |
| − | ''' Procedure. '''
| + | |
| − | | + | |
| − | The questionnaires were distributed to the selected couples in closed envelopes. Each couple
| + | |
| − | was provided with two envelopes, one for a husband and one for a wife. Each envelope contained two question-
| + | |
| − | naires— the HTQ and the BEHI (Bangla EHI). Each of the two questionnaires included standard instructions for
| + | |
| − | the participants to read silently and privately before answering the items. Following the instructions, the members
| + | |
| − | of each couple independently answered the first five items of the HTQ immediately once after finishing the course
| + | |
| − | of spontaneous lip kissing as they do naturally in a face-to-face and standing situation at home, and to answer
| + | |
| − | the last item after sharing a lip kiss with each other, following another course of lip kissing, in a guided manner.
| + | |
| − | That is, before answering the last item, the members of each couple shared a lip kiss with each other for a while in
| + | |
| − | a face-to-face and standing situation, turning one partner’s head in a direction opposite to the turning direction
| + | |
| − | of another partner’s head, the direction of head-turning being defined in reference to each partner’s self. Thus if
| + | |
| − | one partner turned the head to her/his right the other partner turned the head to his/her left and vice versa, such
| + | |
| − | that both their heads were oriented at the same line in space during lip kissing. In any kissing situation (spon-
| + | |
| − | taneous or guided), the members of each couple were not allowed to hold any objects in their hands during lip
| + | |
| − | kissing and to discuss or compare any answers with each other prior to completing the questionnaire. Thus each
| + | |
| − | individual reported about the direction of his/her own head-turning as well as the direction of his/her partner’shead-turning while initiating or receiving a lip kiss, but without having any knowledge of the actual responses
| + | |
| − | made by the other partner prior to finishing. In this way we got two sets of head-turning data for all the partici-
| + | |
| − | pants (kiss initiator provided data and kiss recipient provided data). After finishing the HTQ the members of each
| + | |
| − | couple independently filled out the BEHI following the standard instructions provided on it, and returned both
| + | |
| − | the questionnaires to the survey administrator in a closed envelope. Thus data collection from all the participants
| + | |
| − | was finished approximately in three months. Then to process the data for statistical analysis we checked the par-
| + | |
| − | ticipants’ responses to both the questionnaires, in particular, we cross-checked the participants’ responses as they
| + | |
| − | reported in the HTQ who initiated kissing. Three couples were excluded from further processing and statistical
| + | |
| − | analyses due to providing inconsistent and/or incomplete responses.
| + | |
| − | | + | |
| − | ''' Data analyses. '''
| + | |
| − | | + | |
| − | The BEHI comprises 15 items. The responses of each participant to this inventory were used
| + | |
| − | to estimate his/her Laterality Index (LI) using the formula: LI=(RH−LH)/T×100 (RH=Number of tasks done
| + | |
| − | with the right hand, LH=Number of tasks done with the left hand, T=Total number of tasks/items). The value
| + | |
| − | of LI on the full inventory ranges from −100 (extreme left-handedness) to +100 (extreme right-handedness).
| + | |
| − | The actual LI scores of our participants also ranged from −100 to +100, with only 7.29% of the participants
| + | |
| − | having an LI score <0, and 91.67% of them having an LI score >0, indicating that most of the participants
| + | |
| − | had a right-handed tendency. However, unlike prior studies19–21, 23 that divided handedness data into a couple
| + | |
| − | of groups (left-handed, right-handed) we retained participants’ handedness data in their original, continuous
| + | |
| − | format. In order to code the data measured by the HTQ we reformatted the original binary response ‘Me/My
| + | |
| − | Partner’ as ‘Yes/No’ and also Convenient/Inconvenient as ‘Yes/No’. We coded participants’ sex, initiation of kiss-
| + | |
| − | ing, self-reported head-turning direction, and (perceived) convenience/inconvenience (of lip kissing with the
| + | |
| − | two heads oriented at the same line in space) as dummy variables. Then we studied interdependence of all these
| + | |
| − | variables by computing a correlation matrix (data not shown). The purpose of this initial analysis was to have an
| + | |
| − | overall idea of the nature of our data.
| + | |
| − | | + | |
| − | The main analyses were done using a χ2
| + | |
| − | -test and binary logistic regression. Because our criterion or depend-
| + | |
| − | ent variables (initiation of kissing, head-turning, convenience) were binary and the explanatory variables were
| + | |
| − | categorical (sex) or continuous (handedness) we chose binary logistic regression analyses to examine the impacts
| + | |
| − | of the explanatory variables on the criterion variables. Though large samples are preferable for a logistic regression
| + | |
| − | analysis, simulation studies have offered a rule of thumb that for stable regression models one requires 10 to 15
| + | |
| − | observations per explanatory variable85–88. A more recent simulation study has suggested that this rule of thumb
| + | |
| − | is too conservative, and that results from any logistic model with the number of observations per explanatory
| + | |
| − | variable ranging from 5 to 9 can be reliable89. Some authors have suggested that results from less than 10 obser-
| + | |
| − | vations per explanatory variable should be cautiously interpreted87, 90. Briefly, a small sample can be problematic
| + | |
| − | when there are a large number of explanatory variables in the study. However, in our study, there were only 2 or 3
| + | |
| − | explanatory variables and the number of observations/participants per explanatory variable ranged from 16 to 48.
| + | |
| − | Thus the sample size required for a logistic regression analysis was satisfied in our study. We carried out a series of
| + | |
| − | binary logistic regression analyses (in addition to a χ2
| + | |
| − | -test where appropriate) after ensuring that the data satisfied
| + | |
| − | the underlying assumptions (absence of collinearity of the explanatory variables, no outliers in the data). Because
| + | |
| − | all the interaction terms fell short of statistical significance we excluded them from the final regression models in
| + | |
| − | order to make the models simple and more comprehensive. Through this analysis our study goes beyond all prior
| + | |
| − | studies that were mostly based on division of data into a few handedness categories, calculating some descriptive
| + | |
| − | statistics (percentages, correlations) and subjecting data to a χ2
| + | |
| − | -test only18, 20, 21, 23.
| + | |
| − | | + | |
| − | Availability of Materials and Data.
| + | |
| − | | + | |
| − | A copy of the full HTQ designed and used in the current study can be
| + | |
| − | obtained from the corresponding author via email. The datasets generated and analyzed during the study are also
| + | |
| − | available in SPSS and/or Excel format from the same author upon reasonable request.
| + | |
| − | | + | |
| − | =References=
| + | |
| − | | + | |
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| − | Acknowledgements
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| − | A. K. M. Rezaul Karim was supported by Envision Research Institute with a postdoctoral fellowship during the preparation and submission of this work, and Michael J. Proulx is supported with a grant from the EPSRC (EP/J017205/1).
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| − | Author Contributions
| + | |
| − | | + | |
| − | A. K. M. R. Karim designed the study and the study questionnaires, reviewed the literature, analyzed and
| + | |
| − | interpreted the data, and prepared the initial and final drafts of the manuscript. M. J. Proulx and A. A. de Sousa
| + | |
| − | significantly contributed in the interpretation of data and critical appraisal of the manuscript drafts. C. Karmaker,
| + | |
| − | A. Rahman, F. Karim and N. Nigar administered the survey, and C. Karmaker and A. Rahman also contributed in
| + | |
| − | the organization of data (data checking, coding and entry), all under the supervision of the first author.
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| − | Additional Information
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| − | Competing Interests: The authors declare that they have no competing interests.
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| − | Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and
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| − | institutional affiliations.
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| − | Open Access
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| − | © The Author(s)
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| − | 1
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| − | Department of Psychology, University of Dhaka, Dhaka, 1000, Bangladesh. 2
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| − | Envision Research Institute, 610 N.
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| − | Main St, Wichita, KS, 67203, USA. 3
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| − | The Smith-Kettlewell Eye Research Institute, 2318 Fillmore St, San Francisco,
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| − | CA, 94115, USA. 4
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| − | Department of Psychology, University of Bath, Bath, BA2 7AY, UK. 5
| + | |
| − | Culture and Environment:
| + | |
| − | Psychology, Bath Spa University, Bath, BA2 9BN, UK. Correspondence and requests for materials should be
| + | |
| − | addressed to A.K.M.R.K. (email: karim.akmr.monscho06@gmail.com)
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