It’s Saturday, my first day with nothing to do for about three weeks and how do I decide to spend it?  Writing a literature review of sex-specific play behaviour in humans…

LEGO – now for girls?

This was inspired by the recent announcement (and mixed reception) of a new LEGO playset clearly marketed at girls, called “LEGO Friends“.  Here are a few links to posts floating around the web at Wired, CFI Ottawa, HuffPo, Deus ex Machinatio, and Jezebel.  It seems to me that LEGO is just following good business practice by marketing a diverse range of products to different segments of its market.  This new range of products was based on five years of market research during which time they ASKED GIRLS WHAT THEY WANTED!  This isn’t a symbol of masculine oppression or forced stereotyping, it’s a manifestation of pre-existing preferences.

However, there is a scientific angle to this: are there genuine sex differences in toy preferences or are toy preferences entirely determined by social pressures?  If girls and boys exhibit innate variation in their preferences for certain toys then there is a little or no (or at least less) blame to be laid at the feet of toy manufacturers for marketing to certain demographics.

Evidence for innate, sex-specific toy preferences

Alexander et al. (2009) showed red trucks and dolls to 30 infants between the ages of 3 and 8 months.  Boys showed greater visual interest in trucks, girls showed greater visual interest in dolls.  Jadva et al. (2010) showed cars and dolls to 120 infants between 12 and 24 months of age and demonstrated that girls showed more interest in dolls while boys showed more interest in cars.  Other studies in both infants (Campbell et al., 2000) and adults (Alexander, 2006) have shown that these patterns are driven primarily by male preference with little preference exhibited by females.  Serbin et al. (2001) showed the same pattern, but also demonstrated that infant boys were unable to associate toys with particular genders.  So that is pretty conclusive: boys, at least, exhibit consistent preferences for masculine toys.

Do girls prefer pink?

Jadva et al. (2010) also tested for an effect of colour and shape, but found no difference in preference between sexes.  In fact, both boys and girls showed a mutual preference for red-like colours.  They suggest that the preference for certain colours is entirely the result of social conditioning as a result of preferred sex-typed toys often being presented in those colours.  In a previous study, Hurlbert and Ling (2007) showed that there are sex differences in adults (aged 20-26) which are consistent with societal stereotypes (see the figure above for the data).  They also suggest that there is a shared preference for blue (contrary to the Jadva et al. 2010 study which suggested an innate preference for red in both sexes) but that female preference for pink overrides that original state.  They propose two (slightly arm-wavy) hypotheses for this observation: (i) females are gatherers and needed to be able to judge when fruit was ripe (often indicated by changing colour), or (ii) females are “empathisers” who need to be able to judge changes in skin colour to evaluate mood and health in conspecifics.  It seems less likely that there are sex-differences in colour preference at birth, but we have pretty good evidence for the colloquial “girls-like-pink” generalisation.

The mechanism

The question has to be asked “what is causing this variation”.  As with most problems in psychology, the answer seems to be coming from those people whose bodies are malfunctioning in some way.  A series of studies have looked at girls suffering from congenital adrenal hyperplasia (CAH), a condition which causes the adrenal glands to secrete large quantities of androgens during development which then masculinise otherwise female infants.  Girls suffering from CAH tend to exhibit a greater preference for male-type toys compared to unaffected sisters (Berenbaum and Hines, 1992; Meyer-Bahlberg et al., 2004; Paterski et al., 2005).  Furthermore, the degree to which CAH sufferers exhibit masculinised behavior is related to the severity of the disease.

The evolutionary context

Finally (and most interestingly), two independent studies (Alexander and Hines,2002; Hassett et al., 2008) have looked at sex-specific variation in non-human primates.  Alexander and Hines (2002) demonstrated that male vervet monkeys preferred male sex-typed toys (car and a ball) and female vervet monkeys preferred female sex-typed toys (doll and a pot).  When they were given toys that were not preferred either by boys or girls, the male and female monkeys didn’t show any preference, either.  Hassett et al. (2008) showed that male rhesus monkeys preferred male sex-typed toys but that females showed no preference.  These two studies are taken as strong evidence for the evolution of sex-specific play behaviour prior to the development of humans as a species (and certainly prior to the development of LEGO…).  For an interesting discussion of the evolution of toy preference, see Williams and Pleil (2008).

Conclusion

Sex differences in play behaviour, including toy preference, do exist and are relatively consistent across studies in boys.  They have been detected at such an early age that it is reasonable to conclude that these differences are innate.  Furthermore, the effect appears to be present in non-human animals, casting more doubt on the role of social conditioning in the development of sex-related biases in play behaviour.  Finally, we appear to have a strong candidate for the causal factor in this phenomonen: pre- or post-natal androgen exposure.

The science supports the idea of “boys toys” and “girls toys” (as well as an intermediate class of neutral toys) and, therefore, the targeted marketing of these products at their appropriate groups.  Needless to say there are subtleties (and I am aware of the ecological fallacy) at work here, but we should not shy away from the differences (“diversity”?) inherent within the human condition.

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References

Alexander, G.M. (2006) Associations among gender-linked toy preferences, spatial ability, and digit ratio: evidence from eye-tracking analysis, Archives of Sexual Behaviour, 35: 699-709. (http://www.ncbi.nlm.nih.gov/pubmed/16708283)

Alexander, G.M., Wilcox, T. and Woods, R. (2009) Sex differences in infants’ visual interest in toys, Archives of Sexual Behaviour, 38: 427-433. (http://www.springerlink.com/content/d5610r1q3x13l0h5/)

Alexander, G.M. and Hines, M. (2002) Sex differences in response to children’s toys in nonhuman primates (Cercopithecus aethios sabaeus), Evolution and Human Behavior, 23: 467-479. (http://www.ehbonline.org/article/S1090-5138(02)00107-1/abstract)

Berenbaum, S.A. and Hines, M. (1992) Early androgens are related to childhood sex-typed toy preferences, Psychological Science, 3: 203-206. (http://pss.sagepub.com/content/3/3/203.short)

Campbell, A., Shirley, L. And Heywood, C. and Crook, C. (2000) Infants’ visual preference for sex-congruent babies, children, toys and activities: a longitudinal study, British Journal of Developmental Psychology, 18: 479-498. (http://onlinelibrary.wiley.com/doi/10.1348/026151000165814/abstract)

Hassett, J.M., Siebert, E.R. and Wallen, K. (2008) Sex differences in rhesus monkey toy preferences parallel those of children, Hormones and Behavior, 54: 359-364. (http://www.sciencedirect.com/science/article/pii/S0018506X08000949)

Hurlbert, A.C. and Ling, Y. (2007) Biological components of sex differences in color preferences, Current Biology, 17: R623-R625. (http://www.cell.com/current-biology/retrieve/pii/S096098220701559X)

Jadva, V., Hines, M. And Golombok, S. (2010) Infants’ preferences for toys, colors, and shapes: sex differences and similarities, Archives of Sexual Behaviour, 39: 1261-1273. (http://www.ncbi.nlm.nih.gov/pubmed/20232129)

Meyer-Bahlburg, H.F.L., Dolezal, C., Baker, S.W., Ehrhardt, A.A. and New, M.I. (2004) Prenatal androgenization affects gender-related behavior but not gender identity in 5–12-year-old girls with congenital adrenal hyperplasia. Archives of Sexual Behaviour, 33: 97–104. (http://www.ncbi.nlm.nih.gov/pubmed/15146142)

Nordenstrom, A., Servin, A., Bohlin, G., Larsson, A. and Wedell, A. (2002) Sex-typed toy play behaviour correlates with the degree of prenatal androgen exposure assessed by CYP21 genotype in girls with congenital adrenal hyperplasia, Journal of Clinical Endocrinology and Metabolism, 87: 5119-5124. (http://www.ncbi.nlm.nih.gov/pubmed/12414881)

Paterski, V.L., Geffner, M.E., Brain, C., Hindmarsh, P., Brook, C. And Hines, M. (2005) Prenatal hormones and postnatal socialisation by parents as determinants of male-typical toy play in girls with congenital adrenal hyperplasia, Child Development, 76: 264-278. (http://www.ncbi.nlm.nih.gov/pubmed/15693771)

Serbin, L.A., Poulin-Dubois, D., Colburne, K.A., Sen, M.G. and Eichstedt, J.A. (2001) Gender stereotyping in infant: visual preferences for and knowledge of gender-stereotyped toys in the second year of life. International Journal of Brain Development, 25: 7–15. (http://jbd.sagepub.com/content/25/1/7.short)

Williams, C.L. and Pleil, K.E. (2008) Toy story: Why do monkey and human males prefer trucks>  Comment on “Sex difference in rhesus monkey toy preferences parallel those of children” by Hassett, Siebert and Wallen, Hormones and Behaviour, 54: 355-358. (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2755553/)