Sex differences in response to children's toys in nonhuman primates (Cercopithecus aethiops sabaeus)

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Abstract

Sex differences in children's toy preferences are thought by many to arise from gender socialization. However, evidence from patients with endocrine disorders suggests that biological factors during early development (e.g., levels of androgens) are influential. In this study, we found that vervet monkeys (Cercopithecus aethiops sabaeus) show sex differences in toy preferences similar to those documented previously in children. The percent of contact time with toys typically preferred by boys (a car and a ball) was greater in male vervets (n=33) than in female vervets (n=30) (P<.05), whereas the percent of contact time with toys typically preferred by girls (a doll and a pot) was greater in female vervets than in male vervets (P<.01). In contrast, contact time with toys preferred equally by boys and girls (a picture book and a stuffed dog) was comparable in male and female vervets. The results suggest that sexually differentiated object preferences arose early in human evolution, prior to the emergence of a distinct hominid lineage. This implies that sexually dimorphic preferences for features (e.g., color, shape, movement) may have evolved from differential selection pressures based on the different behavioral roles of males and females, and that evolved object feature preferences may contribute to present day sexually dimorphic toy preferences in children.

Introduction

Boys favor construction and transportation toys, whereas girls favor toys such as dolls Connor & Serbin, 1977, Liss, 1981. Boys are also more active Campbell & Eaton, 1999, Eaton & Enns, 1986 and show more rough physical play than girls (DiPietro, 1981). These sexually dimorphic play styles are thought by many to derive from learning and cognitive mechanisms associated with gender socialization (see Serbin, Poulin-Dubois, Colburne, Sen, & Eichstedt, 2001 for a recent discussion). Learning theories suggest that sex differences in play activities and toy preferences arise from modeling and reinforcement of sex-typical play Bandura, 1977, Fagot & Hagan, 1991, Langlois & Downs, 1990. Cognitive theories suggest further that children develop an understanding of their gender identity that results in schemas or mental representations of socially defined gender appropriate behavior and a positive evaluation of toys and activities associated with this gender identification Maccoby, 1988, Martin, 1999, Martin et al., 1990. Sexually dimorphic play has, in turn, social and cognitive consequences. Children have strong preferences for compatible play styles and playmates of the same sex (e.g., Alexander & Hines, 1994). The resulting same-sex groupings for play appear to encourage sexually dimorphic social interaction styles (Maccoby, 1990) and may promote male-typical cognitive abilities (e.g., spatial abilities) (Sprafkin, Serbin, Denier, & Connor, 1983). Sex-typed play styles, therefore, appear to further human sex differences in social and cognitive development.

In other species, biological factors influence sex differences in related aspects of play behavior. In rats and rhesus monkeys, genetic females treated with androgen during critical periods of pre- or neo-natal development show increased “rough-and-tumble” (male-typical) play Goy et al., 1988, Meaney & McEwen, 1986, assumedly because androgen directs basic processes of neural development in relevant brain regions Arnold & Gorski, 1984, Goy & McEwen, 1980. In humans, genetic females exposed to androgenic hormones prenatally (because of genetic disorders causing increased androgen production or because their mothers were prescribed hormones during pregnancy) also show increased male-typical behavior. In particular, androgenized girls show increased preferences for male playmates and toys typically preferred by boys Berenbaum & Hines, 1992, Hines & Kaufman, 1994 and a reduced interest in activities and toys typically preferred by girls Berenbaum & Hines, 1992, Dittman et al., 1990, Ehrhardt & Baker, 1974, Ehrhardt & Money, 1967. The apparent consistency of androgen effects on play across a variety of mammalian species suggests that gonadal hormones, as well as social and cognitive factors, may influence sex differences in children's play, including interests in specific types of toys, such as trucks or dolls.

However, influences of hormones on human play behavior are not universally accepted. Phenotypic masculinization, which occurs to variable degrees in girls exposed to androgenic hormones prenatally, could alter the social environment (e.g., parents may expect or encourage androgenized girls to play in more masculine-typical ways) (Fausto-Sterling, 1992). In addition, hormone exposure could alter cognitive development related to gender (e.g., androgenized girls may develop a less firm identification as female) rather than directly influencing neural processes related to play. From these perspectives, masculinized toy preferences in androgenized girls could reflect altered learning histories or altered cognitive development.

The present research addressed the hypothesis that toy preferences may be associated with factors other than human social and cognitive development by measuring toy preferences in a nonhuman primate, the vervet monkey (Cercopithecus aethiops sabaeus). Unlike humans, vervet monkeys are not subject to the specific social and cognitive influences proposed to explain human sex differences in toy preferences. To evaluate the possibility that sex differences in toy preferences can arise independent of these social and cognitive mechanisms, we therefore tested the hypothesis that vervet monkeys, like human beings, show sex differences in toy preferences.

Section snippets

Methods

Subjects were 44 male (mean age 39.2±31.1 months) and 44 female (mean age 50.4±46.5 months) vervet monkeys ranging in age from 2 to 185 months, living in seven, stable, social groups at the UCLA/Sepulveda Veterans Administration Non-Human Primate Laboratory. They were housed in enclosures (5×5×2.5 m or larger) in groups of 17–28 animals. Six groups included at least three adult males, four adult females and their offspring; the seventh included only adult males. Animals in each group were

Results

There were no sex differences in absolute approach time or percent approach time to any of the toy groupings (i.e., “masculine,” “feminine,” “neutral”). This finding accords with expectation, because approach is scored when an animal comes near a toy without contacting it. Approach often reflected an animal passing near a toy on the way to another location in the enclosure. As expected, then, total approach time and total contact time scores were uncorrelated: r(44)=.11, ns for males; r

Discussion

Vervet monkeys in this research were presented with six toys, two from a “masculine” category (toys typically preferred by boys), two from a “feminine” category (toys typically preferred by girls), and two from a “gender-neutral” category (toys that are not preferred differentially by girls or boys). Whereas the approach of these Old World Primates to individual toys was unrelated to their sex or to toy category, their contact with the toys was. The proportion of contact time with toys

Acknowledgements

The authors thank Michael J. Raleigh and Deborah B. Pollack for comments on prior versions of the manuscript and for the use of animals at the UCLA Non-Human Primate Laboratory and associated archival data on sex, age, and dominance status for these animals. We also thank Richard Green, Mark G. Packard, and Allan R. Wagner for comments on an earlier version of the manuscript, and Margaret Kemeny for a conversation that motivated the study.

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