Gorgeous guy walks into a cafe. Heads spin round. The girls smile coyly, and giggle among themselves - he's so cute. The guys frown, brandishing muscles or harsh words - who does that guy think he is anyway?
The curious thing about this is that even though everyone sees the exact same thing (the handsome chap), somehow the female brain produces female responses, whereas the male brain produces male responses.
The reason for this has to do with the hormones that our brains were exposed to as they were forming. As we know, adult males make testosterone - it's what makes them hairy with deep voices. But the reason males and females also show behaviours that are unique to their sex has to do with differences in their brains; differences that came about very early in life, and which also involve testosterone.
In very young males, soon after birth, testosterone is made by the testes, and the blood system carries it to the brain. When it gets there, it is transformed into another substance that acts on certain nerve cells and gives them important instructions - like whether to make more of themselves and how to connect with other parts of the brain.
Females don't have testosterone, their brains don't get this same education, and so they develop differently. The upshot is that male and female brains are different - not massively different, not different everywhere - just in a few places that control behaviours, like sex and aggression, that are unique to either males or females.
At this point you may be wondering, as the scientists did: if it's really testosterone, what happens if we take it away from the young male? Does he become like a female?
Using experimental animals, researchers changed the amount of testosterone that the young brain was exposed to. When they did this they found that when the young male was deprived of testosterone, his brain looked female and he behaved in some ways like a female, for example he was less aggressive. I say "in some ways" because the change to female brain and behaviour is not 100 per cent.
This tells us that there is probably something else going on in the brain that is important for male behaviour.
Research published last week suggests that the female hormone progesterone is important. That's right: a female hormone influences the male brain! How it fits in remains unknown.
This is a great illustration of the scientific process: Discovery happens in small steps. With each step we become more certain about one thing, only to reveal heaps more questions about others. And sometimes we find the unexpected, causing us to change our view of the past; a bit like uncovering family secrets.
* Dr Christine Jasoni is director of the Otago Neuroscience Programme, a senior lecturer in anatomy at the University of Otago, and president of the Otago Institute of Arts and Sciences, the Otago/Southland branch of the Royal Society of New Zealand.