Autism: Is It Simply The Result Of A Manlier Brain?

It has been known for a long time that autism is more common among males than females. In 2018 the CDC established that the condition affects approximately one in 37 boys and one in 151 girls. What is not known is why.

"Of course, the primary suspect when we have something that is sharply differentiated by sex is testosterone," says Gideon Nave, an assistant professor of marketing in Penn's Wharton School.

Testosterone is the primary male sex hormone and is responsible for the development of male reproductive tissues such as the testes and prostate. It also promotes secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair.

Yet, in the largest study of its kind led by Nave and his colleagues involving nearly 650 men, no proof of an association between testosterone and cognitive empathy was found. Cognitive empathy is the capacity to discern how another person feels and what they might be thinking, which in kind makes us more tactful.

"Several earlier studies have suggested a connection between testosterone and reduced cognitive empathy, but samples were very small, and it's very difficult to determine a direct link," says Amos Nadler of Western University, the first author of the study.

Preceding this, the strongest evidence for an association between testosterone exposure and reduced cognitive empathy came in 2011 in a study that found administering testosterone to healthy women reduced their ability to read emotions during a test. The results suggested that testosterone impaired their performance. Moreover, the work used the 2D:4D ratio as a sensitivity indicator to testosterone. This is the belief that the additional length of the ring finger in comparison to the index finger reflected greater in utero testosterone exposure. Evidence for this connection remains mixed.

The authors of this 2011 study argued that their findings supported a theory that prenatal testosterone exposure created a manlier brain that was less open to perceiving the emotional state of others.

The study was used as support for the "extreme male brain" hypothesis of autism, which contends that autism is an exaggeration of "male" tendencies toward a cognitive style characterized by systemizing over empathizing. However, its sample size was only 16. Additionally, the majority of past research on the link between testosterone and reduced cognitive empathy relied on correlative rather than causative evidence and thus, resulted in inconclusive findings.

To obtain more accurate and substantiated evidence on this link, Nave, Nadler, and their colleagues recruited 642 healthy men in their study and gave them either an application of testosterone gel or placebo before they completed questionnaires and behavioral tasks that measured cognitive empathy. Participants were then shown a photo of an actor's eyes and asked to select the emotional state that best described their expression. All participants also had their 2D:4D ratio measured.

While the testosterone gel did increase participants' levels of the hormone, the researchers found neither their performance on tests of cognitive empathy to be affected nor a relationship between participants' performance on the tests and the lengths of their fingers.

"The results are plain," says Nave. "However, it's important to note that the absence of evidence is not evidence of absence. We found that there is no evidence to support this effect of testosterone, but that doesn't rule out any possible effects. From what we know, though, it seems that if testosterone does have an influence, the effect is complex, not linear. Reality is typically not that simple."

The extreme male brain theory of autism has received a lot of attention but, Nave notes, "if you look at the literature carefully, there is still not really strong support for it."

"For now, I think we have to embrace our ignorance on this."