There's a friend of mine whose five-year-old daughter loves to dress up as Spider-Man; another friend has a toddler son who likes to wear dresses.

The reasons seem obvious: the girl has older brothers who dress up as superheroes and the boy has older sisters who like dresses.

The children are only copying those around them.

There was even more convincing evidence of the role external influences play in the shaping of children's behaviour in Horizon: Is Your Brain Male Or Female? (Monday, BBC Two, 9pm) which staged an experiment with a baby boy and a room full of toys.

Volunteers were sent into the room to interact with the child, and asked which toys the boy preferred: trucks and balls or dolls and teddies?

Invariably they said the boy preferred the trucks, which was when the twist in the experiment was revealed: the little boy was in fact a girl.

The point of the experiment was clear: the volunteers believed the child was making its own choices, but in fact the choices were being made by the adults and, subtly, unconsciously, imposed on the children.

On a bigger scale, this is happening all the time to children, and it's what has always happened.

Victorian boys were made to wear pink because it was considered a boy's colour; now girls are doomed to play with pink dolls in pink rooms because their parents think that's what they want.

The consequences of this for girls and boys when they grow up are serious, and Horizon pointed them out when they asked teenage pupils what they wanted to do after school.

The girls chose creative or caring posts, such as being a vet, and the boys chose jobs related to maths and science.

The conclusion - although it was by no means definitive - was that the teenagers believed they were making a free choice but they were doing nothing of the sort: they were conditioned into their roles.

More worryingly, parents could also - as another experiment showed - be unconsciously holding back their daughters. Volunteers were asked to judge the gradient of a slope their children could crawl down and, time after time, they set the gradient steeper for the boys than the girls.

Which could mean we're all wasting our breath talking about the use of quotas in boardrooms or all-women shortlists to make society fairer.

What we need to do is tell parents to get over their prejudices and give their son a doll and their daughter a truck.