Published: 17:20 EST, 29 September 2014 | Updated: 21:51 EST, 29 September 2014
Sit down for a moment. Relax. Then clasp your hands together so your fingers entwine — don't overthink it! Now look at your thumbs. Which one is on top — the left one or the right?
If you are a man, the odds are it will be the left; if you are a woman, it is more likely to be the right. Now unfold your hands and take a look at your fingers, in particular your index finger (next to your thumb) and your ring finger (next to your little finger).
It can be quite subtle, but in men the ring finger (measured from the crease where it joins the hand) is likely to be longer than the index finger. In women the two fingers are typically the same length.
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Strangely enough, your hands give clues to what is sometimes called 'brain sex' — the way your brain reflects your gender.
Of course, we all have different skills and interests, but some are considered more typically male, and occur more commonly in men, while others are described as more typically female, and occur more commonly in women.
According to popular mythology, men tend to be more obsessed by things such as cars and obscure facts. You find men in pubs discussing the top speed of a car they are never going to drive, let alone own. They cling to the TV remote control. They like spending time in sheds.
Women, on the other hand, are said to be better at empathy and understanding what another person is feeling or needs. In an emotional crisis, they are more likely to offer sympathy.
If you are a man you may be surprised to learn that there are more than 400 different human emotions. If you're a woman you probably knew that already.
This is the stuff of jokes and self-help books — but it is also shown to be true through science. The question is, do these tendencies result from nature — with the biological gender we are born with deciding our interests and personalities — or do they result from nurture, with society and upbringing creating the differing ways that men and women behave?
The BBC series Horizon asked Professor Alice Roberts and me to investigate. We started from very different positions.
Alice thinks apparent brain differences between the sexes have been exaggerated by how our culture treats boys and girls. In the programme she carries out fascinating tests to prove her point, such as dressing up little boys as girls and vice versa and watching how people treat them.
Almost immediately, the girls start rough-housing and playing with trucks, while the boys are treated far more gently by the adults around them.
She argues that parents' unconscious actions — such as being gentler with girls and letting boys behave more roughly — often mould children into men and women who embody gender stereotypes.
Do men and women really have different brains?
Systemisters: According to popular mythology, men tend to be more obsessed by things such as cars and obscure facts, like these young train spotters pictured at Chislehurst and Sidcup station
While I agree that lots of wild generalisations about men and women are bandied around, I also think there may be something in claims that our fundamental biology influences how we behave.
Professor Simon Baron-Cohen, an expert on the brain who I visited at Cambridge University, has done a lot of pioneering work on this. He believes, broadly, that people of whatever gender fall somewhere along a 'systemiser' to 'empathiser' spectrum.
Systemisters are people who enjoy breaking down and analysing systems. They are more likely to become train spotters or computer scientists.
They are what he has called 'male brained' — as these qualities occur most frequently, but far from exclusively, in men.
Empathisers, on the other hand, are more typically 'female' brained as they are more typically women.
Although there are exceptions, most men — when tested — come out as more 'systemising' than 'empathising', while for women it is the other way round.
While Professor Baron-Cohen accepts social pressures are important in influencing choices and behaviour, his studies suggest the hormones babies are exposed to in the womb can also shape the brain. Higher levels of testosterone in utero, for example — as measured in long-term studies that took samples from pregnant women then followed their children from birth — are associated with offspring who are less empathetic but better at some mental skills later in life.
In other words, more testosterone during pregnancy produces babies with a more male brain (we're not yet sure why some mothers produce more testosterone).
An indirect way of assessing how much testosterone you were exposed to is by looking at your fingers.
A number of studies have shown that the greater the difference between the length of the ring finger and the index finger, the more 'male' your brain is likely to be.
Mine is slightly more 'male' than average but much less so than, say, a professional footballer, who is typically male brained with great spatial awareness skills.
As you can imagine, this is a controversial area of science. Professor Baron-Cohen does these studies because he is interested in autism, which he describes as an extreme version of the male brain — more interested in systems and often struggling with empathy.
When he started his research there was a lot of criticism, based partly on the fear studies like his will be used to support unhelpful stereotypes — suggesting men can't excel in female areas and vice versa.
Ignoring gender differences, however, comes with its own dangers. Take the problem of pain.
A while ago, when I was making a programme called Pleasure And Pain, we did a survey where we asked people which of the sexes they thought was better at tolerating pain — 81 per cent of women said 'women', while a mere 11 per cent thought men were the tougher breed.
Although men were more inclined to give themselves the benefit of the doubt, the majority, 54 per cent, still agreed 'women' were more stoical. But is this right?
One way to find out is to get male and female volunteers to take part in a cold water immersion test.
This is a standard pain test widely used because it causes acute pain without doing any long-term damage (as long as you don't do it for more than 15 minutes).
In this test, you put your hand in a bucket of ice-cold water and see how long you can keep it there before the pain becomes intolerable.
I've done it a couple of times and oddly enough after the initial shock it doesn't actually feel cold; below about 3 degrees Celsius your pain receptors overwhelm your temperature receptors so you are no longer able to tell if the water is hot or freezing.
All you know is that it is incredibly painful.
When this test is done in a laboratory setting, men almost always outlast women. This may be pure machismo, but Professor Jeff Mogil of McGill University, Montreal, thinks there is more to it.
As he puts it: 'There are all kinds of reasons to believe there are different neural systems in the brain of males and females.'
That is, our brains process pain differently because our brains are different: our varying ability to withstand pain is proof. He thinks it is unfortunate that much basic pain research has, until recently, been done in male animals.
Males don't menstruate or go on heat, which makes them simpler and cheaper to study.
But it also means we have been developing and testing drugs on males, assuming that they will work just as well on females, which may not be the case.
We've come to realise, for example, paracetamol works better on men, while some opioids work better on women. We don't, however, know why.
That situation is now changing as researchers involve more females and start actively looking for differences.
Indeed, Jeff imagines a future where pharmaceutical companies produce pain-killing drugs that are specific to one gender.
So perhaps we'll have pink painkillers for women, and blue ones for boys, each honed to work on the specific male or female brain.
'If that happens,' says Jeff, 'that'll be a first in medicine.' It may also mean far more effective medication for all of us.
Head injury, a leading cause of death and disability in young people, is another area where studying gender differences could bring significant benefits.
A couple of years ago, I was in a large military hospital in Afghanistan, filming a series called Frontline Medicine for the BBC. I saw a number of soldiers, male and female, with serious head injuries.
I was told that the women were likely to make a better recovery than the men.
Why? It may be, in part, because women have higher levels of progesterone.
Progesterone is best known as a female hormone, involved in the menstrual cycle and pregnancy, but it is also important for the development of neurones — the cells that carry messages in the brain.
Animal studies and a few small human trials have shown that giving progesterone soon after suffering a brain injury improves survival and recovery.
Larger studies, some of them funded by the military, are currently underway.
This, I think, is why researching gender differences is worth doing.
It is not because it will help us understand why men struggle to remember their children's birthdays or why there are fewer female darts players, but because it may help us find more effective ways to tackle disease.
Horizon: Is Your Brain Male Or Female? is now available to watch on the BBC iPlayer.