Environmental and genetic determinants of sensorimotor asymmetries in mother-infant interaction

Artistic turns: laterality in paintings of kisses and embraces

Many lateral biases exist in human behavior, often implicit and not deliberated. Romantic kissing and embracing received experimental attention in the last three decades. We investigated laterality in paintings depicting these social interactions using two methodologies to assess whether painters depicted such biases and whether these biases could be due to observers' aesthetic preferences or painters' ability in portraying naturalistic interactions. In Study 1, we inspected about 190,000 artworks available online to classify leftward and rightward biases in romantic kisses and embraces. The comparison of 103 paintings depicting clearly lateralized interactions revealed a significant rightward bias in romantic kissing (66%) and a trend toward a leftward bias (62%) for embraces, aligning with naturalistic studies of human interactions. In Study 2, 128 participants expressed their aesthetic preference between the paintings selected in Study 1 and their vertically mirrored versions. A preference for the original paintings over their mirrored versions emerged, especially when presented in the upper portion of the screen, but no significant preference for the typical lateralization of kissing and embracing was found. These findings suggest that artists' alignment with naturalistic biases could be due to familiarity and exposure to asymmetric interactions rather than observers' aesthetic preferences.

Functional lateralization in social-emotional processing: The influence of sexual orientation and gender identity on cradling preferences

The left-cradling bias (i.e., the motor asymmetry for cradling infants on the left side) has often been associated to the right-hemispheric social-emotional specialization, and it has often been reported to be stronger in females than in males. In this study we explored the effects of sexual orientation and gender identity on this lateral bias by means of a web-based investigation in a sample of adults (485 biological females and 196 biological males) recruited through LGBTQIA+ networks and general university forums. We exploited a cradling imagery task to assess participants' cradling-side preference, and standardized questionnaires to assess participants' homosexuality (Klein Sexual Orientation Grid) and gender nonconformity (Gender Identity/Gender Dysphoria Questionnaire for Adults and Adolescents). Results confirmed the expected left-cradling bias across all sexual orientation groups except for heterosexual males. Importantly, higher homosexuality scores were associated with higher proportions of left cradling in males. These results suggest that sexual orientation can influence cradling preference in males, indicating a complex interaction between biological and psychological factors in the laterality of social-emotional processing. Finally, the left-cradling bias seems to confirm its role as a behavioral proxy of social-emotional functional lateralization in humans.

Handedness as a major determinant of lateral bias in human functional cradling

Studies examining infant cradling have almost uniformly concluded with a general human left-side bias for cradling, indicating that people prefer to hold an infant to the left of their body. Explanations for the notion of the left-side cradling bias have traditionally been searched for in a variety of factors, for example, in terms of maternal heartbeat, genetic factors, in the form of an ear asymmetry where auditory information is perceived faster through the left ear, as a result of a right hemispheric functional specialization for perception of emotions and faces, and in identifying a motor bias of the infant, such as the tendency of newborn infants to lie with the face to the right when placed supine. Interestingly, handedness is generally considered an inadequate explanation for the lateralized cradling bias, despite it being an intuitively plausible one. In this brief review, I put forward the cradler's handedness as the most convincing and elegant determinant of the cradling bias. This explanation is consistent with a developmental cascades' framework where the cradling bias can be understood as the result of a multitude of factors across a range of levels and systems.