Discussion
In the observational study it was found that in 57% of pairs, men walked on the right side of the pair, and women in 43%. The proportion of men walking on the right was robustly above the chance value of 50%, based on Bonferroni-corrected frequentist inferencing and on Bayesian credible intervals and the Bayes Factor. This shows that humans, like many other species (Regaiolli, Spiezio, Ottolini, Sandri, & Vallortigara, 2021; Zaynagutdinova et al., 2021), exhibit lateralized position preferences when in a pair. In addition, the finding adds to the examples of other human social behaviours, such as kissing, cradling and embracing, which show patterns of lateralization. The greater number of men on the right side is compatible with the readiness to fight hypothesis which predicted that men would want to walk with their dominant hand on the outside of the formation, and because right-handed men are more frequent in a population, there would be more men on the right side of a pair. We evaluate this interpretation in more depth shortly.
For the survey data, the prediction from the readiness to fight hypothesis that men, but not women, would show a preference to have their dominant hand on the outside of the formation, was supported for walking and sitting on a bench. In contrast, there was no evidence for an overall preference to be on the right of a pair, and thus the data were not compatible with a right-hemisphere hypothesis that applies universally to all. The dominant-hand hypothesis also did not receive clear support because the preference to be on the right, when walking and sitting, applied to men but not women. Despite this, the dominant-hand hypothesis could account for the observational and survey data if it is assumed that men get to be on their preferred side more often, perhaps by having a stronger side preference and / or being more assertive. For example, manipulating objects might be more important to men than women, and so men might have a stronger preference to have their dominant hand on the outside of a pair, so it is free to manipulate objects (we are grateful to Dr Tucker Gilman for this suggestion). The stronger effect of handedness for side preferences in bench sitting compared to walking could be related to this. Further research is required to examine these possibilities. Taken together, without including additional moderating influences, the observational data and survey data are more compatible with a readiness to fight hypothesis than with a universal right-hemisphere hypothesis, or the dominant-hand hypothesis.
In the side of the bed preferences, the survey data showed that participants largely avoided “both sides equally” responses. Individual preferences for the side of the bed were not associated with sex or handedness. The bed question is helpful in the interpretation of the data. First, it showed that participants were not responding in a mindless or heuristic way, simply selecting the same response for each question. This provides confidence in the walking and bench data, because it suggests that participants expressed genuine preferences. Second, it showed that the typically habitual, long-lasting, and daily arrangement of a couple in bed did not determine the preferred side when walking and sitting on a bench. Third, although this is debatable, lying in bed is not usually a situation in which a man is required to fight a foe, and therefore this question served as a useful control condition. The observed pattern is compatible with the predictions from a fighting readiness hypothesis, if fighting-related behaviours are context-specific and only instantiated in relevant settings where fighting may be required. However, the bed data may contain some noise, because people tend not to sleep on their backs, and rotating into other positions can reverse the left and right sides, so some caution is required in the interpretation of the bed data.
Some researchers have discussed whether a fighting drive is still relevant in modern Western societies. For example, Faurie and Raymond (2013) suggest that, rather than fighting leading to survival of the individual involved in the fight, in modern non-violent societies, fighting, along with sporting achievements, may instead serve as a ritualized display aimed at attracting mates and promoting procreation. However, evolutionary theorizing accommodates vestigial behaviours that may have conferred Darwinian fitness in the past but that are no longer adaptive (see e.g., Rognini, 2018). A drive to be prepared to fight, even if it is unlikely to be necessary, may still be present in men as a behavioural trait at an implicit level, even if, in most situations, it is not necessary to exhibit the fighting for which the man readies himself.
Other potential explanations of these findings cannot be discounted. One interpretation is that they were caused by sex and handedness differences in emotional lateralization, which caused a stronger side preference in right and left-handed males. There is evidence that males are more strongly lateralized than females (Hirnstein, Hugdahl, & Hausmann, 2019), and stronger leftward perceptual asymmetries in males have been reported with line bisection (Jewell & McCourt, 2000) and the chimeric faces task (Innes, Burt, Birch, & Hausmann, 2016). There is also evidence for sex differences in the lateralization of emotion perception (Bourne, 2005; Burton & Levy, 1989; Rodway et al., 2003; Van Strien & Van Beek, 2000; but see Borod et al., 2001), and in social behaviours involving emotional connections, such as cradling (Packheiser, Schmitz, et al., 2019). It is possible that stronger emotional asymmetries in men may predispose them to more strongly prefer to occupy one side of the pair, because it aids social interaction or the monitoring threats from other males. In relation to this, Marzoli, Prete, and Tommasi (2014) propose that the leftward gaze bias could facilitate the monitoring of the dominant hand of other people, either for aiding communication or for monitoring potentially aggressive acts. A stronger leftward gaze to the hands than to other body parts, when looking at angry bodily postures, is in accord with this suggestion (Calbi, Langiulli, Siri, Umiltà, & Gallese, 2021; see also Lucafò et al., 2021). In addition, men have been found to be more strongly lateralized than women when looking at facial emotions of threat in male faces (Rahman & Anchassi, 2012). However, for stronger emotional lateralization in males to account for the current findings, it would have to be assumed that left-handed males have opposite emotional asymmetries to right-handed males. Some research has found this reversal in left-handers (Willems et al., 2010) while other evidence indicates weaker right hemisphere emotional lateralization, but not a reversal (Elias et al., 1998). In sum, it is apparent that an explanation of the side preference in terms of differences in lateralized emotion processing, due to sex and handedness, is consistent with the current findings and other research.
More research is needed to determine which of these alternative explanations is supported by the evidence. The fighting readiness explanation predicts that men will show a stronger side preference in more threatening environments, such as when it is dark, or when walking through a crowd of unfamiliar people. In contrast, a sex and handedness difference in lateralized emotion processing would not predict a stronger side preference in men under more threatening circumstances, if the asymmetry was operating to facilitate social communication with a partner. However, if it was functioning to facilitate the monitoring of the environment for potential threats from aggressive conspecifics, then a stronger side preference could be expected. In this case, the men’s preference for the side that facilitates the use of their dominant hand, and the monitoring of threats in the environment, could be the manifestation of a general behaviour to be “fight ready”.
Small effect sizes for some of our data could be said to be a limitation of the findings. For example, in the observational study, we observed a Cohen’s h of .13 which is a very small effect size. However, the preference was statistically robust, and the proportion of men on the right (.57) was meaningfully higher than chance (.50). The small effect size is most probably a reason why this subtle bias had not yet been discovered, because it is not noticeable via everyday non-systematic observation. If the effect had been larger, it is likely that people would have noticed the bias routinely in their daily lives. The survey data showed a medium effect size for the key interaction between Sex and Handedness for the walking and bench data (OR = 6.388). This may be because in the survey, people could express their preferences, which were not diluted to the same extent as the observational data, where additional random effects and conflicting inter-individual preferences could interfere with individual preferences. In this respect, the observational data and survey data are most usefully considered together because they complement each other’s limitations.
There are other limitations with the present research. First, it is unclear whether the side preferences in the survey reflect actual preferences, or the side that a person typically occupies, reflecting a memory rather than a preference, or a mixture of these influences. Clarifying this will be relevant to understanding the cause of the side bias. Second, the observational study was conducted in a particular location in the UK. While we have no reason to believe that the findings do not generalize to other locations where male–female pairs are able to walk freely, this requires testing. However, it can be noted that the survey data were from participants from throughout the UK, and the side preferences complemented those obtained in the observational study. This provides confidence in the generalizability of the observational data. Finally, it would have been desirable for all survey images to be matched for the presence of stick figures, so that all conditions were matched along that dimension.
In sum, our observations show that in male–female pairs more men walk to the right of a female than to the left from the pair’s perspective. Unlike women, men report significant side preferences when walking or sitting with a female partner, and this preference is dependent on their handedness. The side that men prefer, when with a partner, facilitates the use of their dominant hand and this might be because men want to be in the best position to fight effectively. There are other plausible explanations of the side preference exhibited by men, including an effect of sex and handedness on the strength and direction of emotional lateralization, or a stronger desire in males to be able use their dominant hand. Further research is desirable to clarify the cause of the side preference in human dyads.
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