Whether or not nonhuman primates exhibit population-level handedness remains a topic of considerable scientific debate. Here, we examined handedness for coordinated bimanual actions in a sample of 777 great apes including chimpanzees, bonobos, gorillas, and orangutans. We found population-level right-handedness in chimpanzees, bonobos and gorillas, but left-handedness in orangutans. Directional biases in handedness were consistent across independent samples of apes within each genus. We suggest that, contrary to previous claims, population-level handedness is evident in great apes but differs among species as a result of ecological adaptations associated with posture and locomotion. We further suggest that historical views of nonhuman primate handedness have been too anthropocentric, and we advocate for a larger evolutionary framework for the consideration of handedness and other aspects of hemispheric specialization among primates.

This study, involving a community-based sample of 45 predominantly white primary caregivers of 45 trans and gender-nonconforming (TGNC) children between 6 and 12 years of age, provides descriptive data on children's gender presentations, peer relations, and well-being. Most (n = 31; 69%) of the children were cross-gender identified (CGI). That is, 17 of 28 children assigned male at birth explicitly and consistently identified as girls, and 14 of 17 children assigned female at birth explicitly and consistently identified as boys. The 14 remaining children appeared to have nonbinary gender identities (e.g., "boy-girl") or to identify with the sex and gender they were assigned at birth but were gender-nonconforming, or their gender identities were uncertain. This subgroup was labeled non-CGI. Most of the children were in the normal range for internalizing (64%), externalizing (67%), and total behavior problems (62%), yet a sizable minority were in the borderline-clinical/clinical range for these symptoms. Children in the CGI group had fewer internalizing and total problems than children in the non-CGI group. Child's degree of gender conformity, caregiver's level of anxiety, and child's peer relations were correlated with children's well-being; children in the CGI group were reported to have better peer relations than children in the non-CGI group. Caregivers' rates of depression and anxiety appeared to be similar to normative samples, although anxiety may have been slightly elevated. Findings from this study add to a small but growing body of literature that documents the well-being of TGNC children growing up in supportive and affirming familial environments. (PsycINFO Database Record

Whether lateralization of communicative signalling in non-human primates might constitute prerequisites of hemispheric specialization for language is unclear. In the present study, we examined (i) hand preference for a communicative gesture (clapping in 94 captive chimpanzees from two research facilities) and (ii) the in vivo magnetic resonance imaging brain scans of 40 of these individuals. The preferred hand for clapping was defined as the one in the upper position when the two hands came together. Using computer manual tracing of regions of interest, we measured the neuroanatomical asymmetries for the homologues of key language areas, including the inferior frontal gyrus (IFG) and planum temporale (PT). When considering the entire sample, there was a predominance of right-handedness for clapping and the distribution of right-and left-handed individuals did not differ between the two facilities. The direction of hand preference (right-versus left-handed subjects) for clapping explained a significant portion of variability in asymmetries of the PT and IFG. The results are consistent with the view that gestural communication in the common ancestor may have been a precursor of language and its cerebral substrates in modern humans.

Cognitive and behavioral biases, which are widespread among humans, have recently been demonstrated in other primates, suggesting a common origin. Here we examine whether the expression of one shared bias, the endowment effect, varies as a function of context. We tested whether objects lacking inherent value elicited a stronger endowment effect (or preference for keeping the object) in a context in which the objects had immediate instrumental value for obtaining valuable resources (food). Chimpanzee subjects had opportunities to trade tools when food was not present, visible but unobtainable, and obtainable using the tools. We found that the endowment effect for these tools existed only when they were immediately useful, showing that the effect varies as a function of context-specific utility. Such context-specific variation suggests that the variation seen in some human biases may trace predictably to behaviors that evolved to maximize gains in specific circumstances.

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