Shared-environmental influences on gender-role behaviour may be more important in early childhood than in adolescence and adulthood

Whether little boys prefer to play with trucks or dolls depends more on the family environment than genetics, while genetics seems to play a larger role in little girls' preferences for dolls over trucks, according to a study published in the July/August 2005 issue of the journal Child Development.

The study, conducted by researchers from King's College London and City University London, also found that the shared environments of twins make a unique contribution to gender-role behavior. This may reflect prenatal factors (e.g., unlike non-twin siblings, twins share the same womb) as well as postnatal socialization effects (e.g., unlike non-twin siblings, twins are exactly the same age and may be more likely to share the same toys and partake in the same activities and games).

While a large body of research suggests that a child's genetic heritage is important in explaining variation in many aspects of behavioral development, says lead researcher Alessandra C. Iervolino, PhD, research fellow of the Institute of Psychiatry at King's College London, "we know very little about the extent to which genetic versus social-environmental factors contribute to individual differences in sex-typed behavior, especially during the preschool years."

The researchers set out to understand what underlies the substantial variation within the sexes in terms of the extent to which boys and girls adopt masculine-typical or feminine-typical behaviors.

To reach their conclusions, the researchers used information from the Twin's Early Development Study, a longitudinal study that comprises a representative sample of all twins born in England and Wales in 1994, 1995, and 1996.

To disentangle genetic from social-environmental influences on gender-role behavior, they used the classic twin design, comparing the similarity of identical twins, who share their entire genetic heritage, with that of fraternal twins, who share about half of their genes.

Their findings suggest that shared-environmental influences on gender-role behaviour may be more important in early childhood than in adolescence and adulthood, concluded Dr. Iervolino.