FRIDAY, Sept. 16 (HealthDay News) -- When people are hungry, they are more likely to be angry or aggressive. And now researchers have found the reason why: serotonin levels -- a hormone that helps regulate behavior -- fluctuate when people are stressed out or haven't eaten, according to a new study.

Rising and falling serotonin levels affect parts of the brain that allow people to control their anger, researchers from the University of Cambridge explained in the report published in the Sept. 15 issue of the journal Biological Psychiatry.

"We've known for decades that serotonin plays a key role in aggression, but it's only very recently that we've had the technology to look into the brain and examine just how serotonin helps us regulate our emotional impulses. By combining a long tradition in behavioral research with new technology, we were finally able to uncover a mechanism for how serotonin might influence aggression," the study's co-first author, Molly Crockett, who worked on the research as a Ph.D. student at Cambridge's Behavioural and Clinical Neuroscience Institute, said in a university news release.

In conducting the study, the researchers controlled the diet of healthy volunteers to manipulate their serotonin levels. The participants' brains were then scanned using functional magnetic resonance imaging (fMRI) as they viewed faces with different expressions -- angry, sad and neutral -- to determine how various parts of their brains reacted and communicated with each other.

The study revealed that low levels of serotonin made communications between certain parts of the brain weaker than normal. The researchers concluded that when this happens it may be harder for the brain to control emotional responses to anger.

The participants also completed a personality questionnaire to assess whether or not they had a natural tendency towards aggression. Those that were predisposed to aggression had even weaker communication between certain regions of their brain when serotonin levels were low, the investigators found.

The findings could be applied to a range of psychiatric disorders in which violence is a common problem, such as intermittent explosive disorder, which is characterized by extreme and uncontrollable outbursts of violence, the authors suggested.

"We are hopeful that our research will lead to improved diagnostics as well as better treatments for this and other conditions," the study's co-first author, Dr. Luca Passamonti, who worked on the research while a visiting scientist at the Cognition and Brain Sciences Unit of the Medical Research Council in Cambridge, England, said in a news release.

More information

The U.S. National Institute of Neurological Disorders and Stroke has more about the brain.