Though it has been previously established that testosterone - a steroid hormone present in both men and women - influences aggression in men, a new study published in Biological Psychiatry suggests there is a neural circuit in the brain through which testosterone imposes these effects.

Aggressive manShare on Pinterest
Normal levels of testosterone in men increase activity in brain areas involved in threat processing and aggressive behavior, according to the latest study.

Testosterone levels in men have been linked to risks for cardiovascular disease, Parkinson's and even rheumatoid arthritis.

And Medical News Today recently reported on a study focusing on ancient skulls that suggested early humans' breakthrough in tool-making 50,000 years ago coincided with a lowering of testosterone levels in our species.

The researchers from this latest study, led by Justin Carré of Nipissing University in Canada, say previous studies found that administering a single dose of testosterone in subjects influenced brain circuit function, though these studies were - surprisingly - carried out in women.

To investigate testosterone's effects on the brain's threat response in men, the team recruited 16 healthy young male volunteers. Focusing on brain structures involved in threat processing and aggressive behavior, such as the amygdala, hypothalamus and periaqueductal gray, the researchers had the men complete 2 test days during which they received either a placebo or testosterone.

"Understanding testosterone's effects on the brain activity patterns associated with threat and aggression may help us better understand the 'fight or flight' response in males that may be relevant to aggression and anxiety," says Dr. John Krystal, editor of Biological Psychiatry.

Testosterone has 'profound effect on brain circuits involved in human aggression'

During the testing days, the men received a drug that suppressed their own testosterone so as to ensure that all study participants had similar levels for the study. As such, any men who received testosterone only received enough to return their levels to the normal range.

Next, the men underwent a functional magnetic resonance imaging (fMRI) scan while completing a face-matching task.

After analyzing the data, the researchers found that the men who received testosterone had increased reactivity of the amygdala, hypothalamus and periaqueductal grey when viewing angry facial expressions, compared with the placebo group.

Commenting on their findings, Carré says:

"We were able to show for the first time that increasing levels of testosterone within the normal physiological range can have a profound effect on brain circuits that are involved in threat-processing and human aggression."

The team says that understanding how testosterone affects the male brain is important, since controlling or manipulating testosterone levels has become marketed as a solution to reduced virility in older men.

But despite their findings, Carré says further research is needed, adding:

"Our current work is examining the extent to which a single administration of testosterone influences aggressive and competitive behavior in men."

Medical News Today recently reported on a study that suggested female intuition comes from lower testosterone exposure in the womb.