Gender differences have been a long debated subject and are often prone to misunderstandings and misinterpretations. In this article however, I want to write about research that has been conducted on typical sex differences from a neuroscientific standpoint.
Simon Baron Cohen, a psychologist and a professor at Cambridge University, explained that when looking at the neuroanatomy, the structure and composition of the brain, of boys and girls one can find major structural differences. Scanning the brains of various boys and girls revealed that, on average, the male brain is 8% larger than the female brain. This is a volumetric difference and doesn’t necessarily lead to any inference, however it is an occurrence that has been found even with babies as young as 2 weeks old.
Another difference between the brains can be deduced by looking at the human brain in terms of post-mortem tissue. This reveals that the average male brain has 30% more connections, synapses, between nerve cells than the average female brain.
Cohen could also dissect the brain to look at specific brain regions. The amygdala, which serves the processing of memory, decision–making and emotional responses, tends to be larger in the male brain on average. On the other hand, the brain region known as planum temporale, an area involved in language, is larger in the female brain.
Cohen states that the benefit of having these findings are that these differences exist and are rooted in biology and therefore there is less room for misinterpretation or disagreement.
These differences in neuroanatomy indicate differences in the psychology of men and women. Research has shown that girls develop and exhibit empathy much faster and earlier than boys and that boys have a stronger drive to understand the underlying mechanisms of how the world works. Boys are more fascinated with “systems”, as Cohen calls it, these could be mechanical systems like a computer or natural systems like the weather or an abstract system like mathematics.
But where do these sex differences come from?
Cohen and his lab were of the strong opinion that testosterone could be a major driver of these differences and therefore conducted various experiments and analysis with this concentration.
In the year 2000 they published a study involving new-born babies. The main aim of this study was to investigate whether the differences in the mind and behaviour of humans are solely due to cultural, postnatal experiences or if biology also contributes to this difference.
For this experiment, Cohan and his team studied 100 babies that were just 24 hours old. These babies were presented with two objects: a human face or a mechanical mobile phone. The team looked to see whether babies aged one day old looked longer at the human face, a social stimulus or looked longer at the mechanical mobile. The result of this experiment showed that boys seemed to look longer at the mechanical stimulus and girls seemed to look longer at the social stimulus. This result was a strong case for the hypothesis that biological factors were indeed a contributing factor to sex differences. After just one day, a behavioural pattern had been established that showed that girls were on average more inquisitive about people and boys, on average, being more oriented to the physical environment of the world.
Now that it has been established, that behavioural differences also seem to have biological roots, we should understand what these biological roots are and what hormones or molecules play a key part in this process.
Cohan and his team choose to investigate testosterone, as animal research has shown that directly before birth there is a sudden spike in the production of testosterone. Testosterone is suddenly generated in high quantities and then drops off again around birth and the researchers argued that this production of testosterone had “permanent and organizing effects” on the development of the brain. Several experiments on the effects of testosterone levels have been conducted on rats.
For example, if you inject extra testosterone into a female rat either at birth or during pregnancy and then investigate her behaviour postnatally, one finds that her behavioural patterns resemble that of a male rat. One way to show this is to let the rats run through a maze. Generally speaking, male rats find their way out of the maze much faster than female rats due to their ability to learn special routes quicker. Female rats that were injected with testosterone found their way through the maze much quicker than “normal” female rats. This supports the hypothesis that testosterone “masculinizes” the brain and behaviour.
These findings were coherent with other research and the effects of changes in testosterone levels were very clear at that point in other species. But until then no one had found a way to test it in humans. Cohan and his team eventually found a way to use on women in pregnancy who were having a procedure known as amniocentesis. This is where a needle is entered into the womb of a pregnant women, extracting the amniotic fluid that surrounds the womb. This process of extracting the amniotic fluid that surrounds the baby is called amniocentesis.
Normally, this process is used when there is a suspicion that the baby may suffer from Down syndrome, and the doctor analyses the amniotic fluid for chromosomal irregularities. Cohan and his team got the approval of women who were anyway having amniocentesis, to take some of the fluid and analyse it for levels of testosterone. The study would consist of deep freezing the amniotic fluid and, when the baby was born, trying to find a correlation between the levels of testosterone in the amniotic fluids and the infants behaviour.
Specifically, the researchers wanted to see whether the scale of testosterone levels (low, average, high) had anything to do with individual differences postnatally – for example, in rates of language development, in how sociable children are and on other dimensions.
Cohan and his team invited the babies every year since their birth and had a total of 500 babies in their experimental population.
On their second birthdays the researchers looked at language development. They did this by asking the parents to fill in a checklist of how many words their child knows and how many words their child could produce.
At the age of two, there were children who had a total vocabulary of 10 or 20 words and there were some children who were extremely chatty and had up to 600 words in their vocabulary. Looking back at the prenatal testosterone level, one could see that there was a significant negative correlation between prenatal testosterone and the size of the vocabulary – the higher the child’s prenatal testosterone level in the amniotic fluid the smaller their vocabulary was at two years old.
Another study was conducted with the same children when they were 4 years old. Now the researchers where looking at the empathy of the infants. This was measured by the judgement of their parents, the children taking empathy tests and information on how easily the infant mixes socially at school.
Again, a similar result emerged, the more prenatal testosterone level was measured, the less empathy, on average, the infant displayed at the age of 4.
These behavioural studies indicate that hormone levels that are determined prenatally still have significant influence on the development of a child’s behaviour and their brain. This could raise the question regarding determinism: Are we really free in our development if our development is influenced by prenatal hormone levels that we have no control over. This is a question that I will be writing about more in future.
Stay tuned!