Researchers from the Universities of Cambridge and Utrecht have published new findings which suggest that testosterone, the male sex hormone, makes people less empathetic and reduces their ability to read facial expressions. The research has important implications for understanding why men are more likely to be autistic than women.

The researchers gave female volunteers testosterone and then examined the change in their social intelligence. The volunteers were asked to look at photographs of people’s eyes and guess their emotions. This is a test of empathetic ability, and, on average, women tend to perform better than men. However, when the women were given testosterone, their aptitude for the test fell.

Simon Baron-Cohen, an expert on autism and the University of Cambridge’s Professor of Developmental Psychopathology was one of the leaders of the study. He has been a sometimes controversial figure for emphasising the importance of biology over social conditioning in explaining the differences between male and female behaviour. He has championed the idea of autism as an "extreme male brain", because, he claims, many autistic tendencies are exaggerations of typical masculine traits.

Baron-Cohen told Varsity, "The idea of autism being an extreme of the male brain is supported by evidence from psychological and neural measures where there are key sex differences, and where people with autism show an extreme of the typical male profile. But this hypothesis is formulated with a broad brush and our research aims to identify with greater precision where in the brain such atypical development occurs."

As well as finding that testosterone reduced women’s empathic abilities, the researchers discovered that women who experienced high levels of testosterone as foetuses are more likely to display this reduction. They concluded that testosterone in the womb ‘primes’ the developing foetus, leading to an adult brain which is more responsive to testosterone. To analyse how much testosterone the volunteers were exposed to prenatally, the scientists measured the length of the women’s index and ring fingers. Whilst this may sound unconventional, the ratio of these lengths is established as a good indicator of a person’s foetal testosterone exposure. Men tend to have a shorter index finger than ring finger; the reverse is true in women. The study found women with more masculine ratios were more sensitive to testosterone.

This finding has implications for the androgen theory of autism, which Baron-Cohen has also advocated. The theory claims that foetal testosterone (an androgen) is important in the development of autistic traits. Baron-Cohen told Varsity that, whilst this research relates to the androgen theory, his goal is to test it directly. "One can extrapolate this to autism but to directly test the androgen theory of autism one would need measures of testosterone in people with this diagnosis. We are currently collecting these." The recent study, he says, is important because it, "contributes to our knowledge of how small hormonal differences can have far-reaching effects on empathy".