It is said that girls and boys learn their stereotypical behaviour from their social environment. This is not true: such behaviour is already determined in the womb.

Gender differences in the brain and in behaviour are found in areas other than those associated with reproduction. One of the typical gender differences between boys and girls, which are often thought to be learned from the social environment, is the way they play. Boys are more active, wilder and prefer playing with soldiers and cars while girls like playing with dolls.

More than 30 years ago, my partner and I provided our children, a girl and a boy, with both types of toys, but they still made the stereotypical choice every time. Our daughter played only with dolls and our son was only interested in cars. Two children is too small a sample for a scientific publication. But that this gender difference has a biological basis has now been demonstrated by Alexander and Hines, who offered Velvet monkeys dolls, cars and balls to play with.

The female monkeys chose the dolls and began to sniff their ano-genital region. They thus displayed typical maternal behaviour, while the male monkeys were more interested in playing with the cars and a ball. The preference for a particular toy is thus not learned from society but is programmed in our brain to prepare us for our later role in society, like motherhood for girls and fighting and more technical tasks for boys.

The gender difference in the monkey’s choice of toy shows us that the underlying mechanism goes back tens of millions of years in our evolutionary past. The peak in testosterone which normally develops in boys in the womb seems to be responsible for their style of playing. Girls, who produce too much testosterone in the womb due to an adrenal gland disorder (congenital adrenal hyperplasia, CAH) display an unusual preference for boys as playmates, prefer boys’ toys and play wilder than you would expect from girls. As a result, they are often called “tomboys”.

There are also evident gender differences in drawings done spontaneously by children. In their choice of subjects, colours and composition, boys and girls are clearly influenced by the hormones to which they are exposed in the womb. For example, girls tend to draw human figures, particularly girls and women, or flowers and butterflies. They use bright colours, like red, orange and yellow. The subjects are peaceful, and the figures generally stand in a row.

In contrast, boys prefer drawing technical objects, weapons, fights and transport vehicles like cars, trains and planes. Often the composition is viewed from above, and they use dark and cool colours, like blue. Girls, who are exposed to an excess of testosterone in the womb due to CAH, produce drawings like boys do, despite the fact that they received treatment immediately after birth.

Some gender differences in our behaviour are evident so early in our development that they could only have originated in the womb. From the first day of life, girls look mostly at faces, while boys prefer mechanical objects. After one year, girls are making eye contact more often than boys. However girls exposed to excessively high testosterone levels in the womb make less eye contact later as a child.

Thus testosterone in the womb plays a key role here, too. Eye contact in everyday life has a completely different meaning for women than for men. In Western cultures women use eye contact to understand other women better, and they feel better doing so.

For Western men eye contact is involved in testing their position in the hierarchy and can appear quite threatening. Again, that is pure biology. At the exit from the airport in Aspen (Colorado, USA) there is a warning sign. If you run into a bear, do not make eye contact. Because the bear will attack immediately to show you who is the boss.

Experiments conducted by my son and me reveal that the gender differences in eye contact also have consequences for business transactions. Our research has shown that eye contact between two women leads to more creative negotiation results, whereas eye contact between two men has instead a negative effect on the negotiation results. You can profit from this practical tip.

Dit verhaal is geplaatst op vrijdag 24 oktober 2008 om 16:51 uur.