title: Experience-independent sex differences in newborn macaques: Females are more social than males.
abstract: Human females exhibit greater social interest and skills relative to males, appearing in infancy, suggesting biological roots; however, male and female infants may be treated differently, potentially causing or amplifying sex differences. Here, we tested whether sex differences in social motivation emerge in infant monkeys (n = 48) reared in a controlled postnatal environment. Compared to males, females at 2-3 weeks looked more at conspecifics' faces (d = 0.65), especially the eyes (d = 1.09), and at 4-5 weeks exhibited more affiliative behaviors (d = 0.64), including gesturing, looking, and proximity to familiar and unfamiliar human caretakers. In sum, converging evidence from humans and monkeys suggests that female infants are more social than males in the first weeks of life, and that such differences may arise independent of postnatal experience. Individual differences in social interest have wide-ranging developmental consequences, impacting infants' social interaction quality and opportunities for learning. Understanding the evolution of sex differences and their developmental emergence is necessary to best support infants with varying levels of sociality.
authors.affiliation: Department of Psychology, University of Miami, Coral Gables, Florida, USA.|Laboratory of Comparative Ethology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Department of Health and Human Services, Poolesville, Maryland, USA.|Dipartimento di Neuroscienze, Università di Parma, Parma, 4300 Italy.
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Simpson
authors.affiliation: Dipartimento di Neuroscienze, Università di Parma, Parma, 4300 Italy.
authors.forename: Ylenia
authors.initials: Y
authors.lastname: Nicolini
authors.affiliation: Unit on Computer Support Services, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Department of Health and Human Services, Bethesda, Maryland, USA.
authors.forename: Melissa
authors.initials: M
authors.lastname: Shetler
authors.affiliation: Laboratory of Comparative Ethology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Department of Health and Human Services, Poolesville, Maryland, USA.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Suomi
authors.affiliation: Dipartimento di Neuroscienze, Università di Parma, Parma, 4300 Italy.
authors.forename: Pier F
authors.initials: PF
authors.lastname: Ferrari
authors.affiliation: Laboratory of Comparative Ethology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Department of Health and Human Services, Poolesville, Maryland, USA.
authors.forename: Annika
authors.initials: A
authors.lastname: Paukner
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D005133:Eye Movements; D005260:Female; D006801:Humans; D007398:Interpersonal Relations; D008253:Macaca mulatta; D008297:Male; D012727:Sex Characteristics; D012919:Social Behavior; D013647:Task Performance and Analysis