Of the many proposed triggers for autism, one of the most controversial is the “extreme male brain” hypothesis. The idea posits that exposure to excess testosterone in the womb wires both men and women to have a hypermasculine view of the world, prioritizing stereotypically male behaviors like building machines over stereotypically female behaviors like empathizing with a friend. Now, a study is raising new doubts about this theory, finding no effect of testosterone on empathy in adult men.
The extreme male brain hypothesis was first proposed by psychologist Simon Baron-Cohen at the University of Cambridge in the United Kingdom. In 2011, he and colleagues found that women given a single hefty dose of testosterone fared significantly worse at the Reading the Mind in the Eyes test (RMET), which asked them to gauge the emotional states of others based on their facial expressions. The women’s performance seemed to track with a controversial metric called the 2D:4D ratio, the relative lengths of the second and fourth fingers. Men—and people with autism—tend to have a longer ring finger than index finger, and some researchers believe that is due to higher prenatal exposure to testosterone.
To Melissa Hines, a neuroscientist at Cambridge, the team’s inability to replicate earlier findings is “unsurprising.” The small size of past studies and the unreliability of the 2D:4D measure make it unlikely that the results would replicate in women either, she says.
Still, Baron-Cohen finds it “strange” that Nadler’s team conducted the study exclusively in men “because to expect to see an effect of testosterone, one needs to study people who at baseline have relatively low” testosterone, he says—in other words, women.
But Nadler says Baron-Cohen’s objection is “incongruous” with the original paper’s hypothesis, which predicted that people with higher prenatal testosterone levels would be more sensitive to the empathy-blunting effects of testosterone. That means men should show “more pronounced effects [than women], not less,” he says.
Nadler cautions, however, that the study can’t disprove any theories about testosterone’s impact on the developing brain. “What we’re saying is that giving people testosterone as adults has no influence on their ability to understand people’s emotions.”
He hopes the study will dispel any misconceptions that by controlling testosterone—for example, using medications to block it in pregnant women—it might be possible to prevent autism. “If there’s no relationship, then we shouldn’t give people false hope.”