In June 2008, a team of Swedish researchers released the largest study to date of genetic differences in the human brain. They found that 1,349 genes are expressed differently in the brains of men compared with women. They also studied macaques, a kind of monkey in which researchers have noticed some differences in how males and females (just as there are some differences in how human females and males behave); and they studied marmosets, a kind of monkey in which the males and females do NOT behave differently as a rule. In macaques, they found that 486 genes are expressed differently in the brains of females compared with males; while in marmosets, only seven genes are expressed differently in the brains of females compared with males.
Why does this matter?
This study provides the strongest evidence to date that sex differences in the brain are genetically programmed. In the 1970’s and 1980’s, for example, many people believed that sex differences were not hardwired. Gender was believed to be primarily a social construct. Kids learn how to “do gender”: boys learn that boys are supposed to play with trucks, while girls learn that girls are supposed to play with dolls. Until quite recently, even scholars assumed that sex differences in the expression of genes would be confined to the X and Y chromosomes.
We now know that those old beliefs were mistaken. More than a thousand genes, scattered across all 46 chromosomes, are expressed differently in the male brain compared with the female brain, in our species. It’s beginning to look as though the dramatic sex difference in the expression of the human genome is one of the things that make us uniquely human, one of the things that distinguishes us from the other primates.
Reference: Reinius, B., Saetre, P., Leonard, J. and four other authors: “An evolutionarily conserved sexual signature in the primate brain,” PLoS Genetics, volume 4, #6, June 2008, available at no charge at this link: http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1000100