Testosterone Supplements May Impair Decision-Making

Testosterone helps increase muscle mass, but its effects on cognition and behavior are more complicated to unravel. Amir Cohen / REUTERS

Here's a riddle: Taken together, a bat and a ball cost $1.10, but the bat costs $1 more than the ball. What's the price of the bat?

An immediate answer probably jumps into your mind: $1. But that's wrong. The correct answer is that the bat costs $1.05, and the ball $0.05. This query is part of what psychologists call the cognitive reflection test, which measures one's ability to suppress a seemingly intuitive but incorrect answer. More generally, it's a good gauge for one's ability to reflect on their own mental processes, and to avoid making mental errors.

A study to be published in the journal Psychological Science found that after being given a single dose of testosterone, men were more likely to get this and similar questions wrong, and showed a lower score on measures of cognitive reflection compared to those who took a placebo. The study is significant for several reasons. For one, it's "the only study [suggesting] that a single administration of testosterone affects cognition," says study first author Gideon Nave, a marketing assistant professor at The Wharton School, University of Pennsylvania. The study also included 243 male participants, the largest sample size of any such study of testosterone.

Past research has shown that in animals, testosterone is heightened when males fight over mates. (Work in humans also shows testosterone levels rise in competitive situations.) In these instances, it might be favorable to act more quickly, rather than taking the time to deliberate, says Nave, who completed the study while at CalTech, along with his then-advisor Colin Camerer, and Amos Nadler at Western University. The result fits with some past work linking higher levels of testosterone with more impulsive behavior.

However, the hormone's exact effects are quite complicated, and the data in the study don't "tell a clear story," says Shane Frederick, a research at Yale who developed the cognitive reflection test. For example, people who score higher on the cognitive reflection test tend to have a higher appetite for risk. However, higher levels of testosterone also have been linked to more risk-taking. But this paper suggests higher testosterone lowers scores on the test, which should be linked with less risk-taking; the correlations aren't in agreement. Frederick says further study is needed to unravel exactly what's going on.

Pranjal Mehta, an assistant professor in the department of psychology at the University of Oregon, who also wasn't involved in the paper, says he and colleagues just finished conducting a very similar study (which isn't yet published), and found that testosterone had no effect on cognitive reflection. The primary difference between the two studies is that Nave's group paid the participants, whereas Mehta's group didn't, he says (and his sample size was also slightly smaller). But neither he nor Frederick can say why payment would change the result.

Obviously more work needs to be done, and the effects of testosterone are quite complicated. Generally speaking, higher levels of testosterone are correlated with a higher concern for status. But whether or not one attains a certain status depends on much more. While the hormone has been linked to aggression in animals, what works in the animal kingdom doesn't also fly in human society.

Testosterone's "relation to aggressive behavior is nuanced—aggression can get you status in some contexts but not in others," such as "in a group of academic professors," Mehta says.

Nave maintains that it's hard to make a judgment about testosterone's effect on cognitive reflection. It's "difficult to make normative judgements about whether it's better to go with intuition or deliberation." Each can be preferable in different situations. "It's just something people should be aware of," he says.

Don't expect this result to have an impact on the $3 billion supplemental testosterone industry just yet. "Lists of side effects now don't include things like 'reduces cognitive reflection,'" Mehta says, "but maybe in 20 years they will."