Testosterone supplements can actually make people more sensitive to moral norms, say researchers, adding that the male hormone's influence on behaviour is more complicated than previously thought.

Some studies have linked high levels of testosterone to immoral behaviour.

The new study by researchers at The University of Texas at Austin took a deeper look at the hormonal underpinnings of moral reasoning. 

"There's been an increasing interest in how hormones influence moral judgments in a fundamental way by regulating brain activity," said Bertram Gawronski, a psychology professor at UT Austin. 

"To the extent that moral reasoning is at least partly rooted in deep-seated biological factors, some moral conflicts might be difficult to resolve with arguments".

Prior studies on how hormones influence moral judgment suggest that higher levels of testosterone are associated with stronger utilitarian preferences. 

The researchers in the new study published in the journal Nature Human Behaviour put the hypothesis to the test in a double-blind study that administered testosterone to a group of 100 participants and a placebo to another 100 participants.

"The study was designed to test whether testosterone directly influences moral judgments and how," said Skylar Brannon, a psychology graduate student at UT Austin. 

The researchers were surprised to find that those who received testosterone supplements were less likely to act for the greater good, and instead became more sensitive to moral norms. 

However, participants with high levels of naturally-occurring testosterone showed the opposite, making judgments that were less sensitive to moral norms.

The authors think naturally-occurring testosterone may be associated with certain moral judgments because people with particular personality traits tend to have different levels of testosterone. 

For example, people with high levels of psychopathy tend to have high levels of naturally-occurring testosterone and exhibit lower sensitivity to moral norms. 

"This does not mean that testosterone is the cause of psychopaths' insensitivity to moral norms. If anything, testosterone seems to have the opposite effect, increasing people's sensitivity to moral norms," the findings showed.