Scientists at the California Institute of Technology have found, in high-tech experiments in mice, that the brain neurons responsible for aggression appear to be located close to the neurons responsible for mating.

The researchers used electrodes to record the activity of single neurons in the brains of male mice while they interacted with other male mice or with female mice. The scientists also made video recordings of the mice while the electrophysiological data were being acquired. They then analyzed their findings to pair the activity of specific neurons in the brain with aggression or mating.

They found that the hypothalamus contains neurons that are activated during male aggressive encounters with other males and that it also contains neurons that are activated during male mating with females, with a level of overlap of about 20 percent.

The scientists were surprised by these findings, one of them—David Anderson, Ph.D., a professor of biology at the Howard Hughes Medical Institute of the California Institute of Technology—wrote January 4 in Biological Psychiatry. “Apparently the circuitry for sex and violence is intimately linked in the male brain,” he said.

Why has such a linkage evolved? “The observation that neurons involved in attack are apparently progressively inhibited during male-female mating suggests that part of the reason for the linkage might be to prevent inappropriate aggressive behavior during mating,” he proposed. “Perhaps the imperative to ‘make love, not war’ is hard-wired in our nervous system to a greater extent than we have realized.”

Anderson and his colleagues also believe that their findings have implications for psychiatry, and they raised the possibility that the roots of some forms of pathological aggression in men might reflect faulty neuronal wiring in the hypothalamus. For example, suppose the mechanisms that normally shut off hypothalamic aggression neurons during mating were faulty in a man. Might he then direct his aggression to his sexual partner and rape her? Functional imaging and other emerging new technologies could be used to see whether such neuron abnormalities exist in rapists, they contended.

Yet when Psychiatric News asked Anderson whether anyone is using functional brain imaging or other new technologies to see whether abnormalities in hypothalamic aggression neurons underlie human sociopathic behavior, he said that he didn’t know of anyone pursuing such research.

When he was asked whether his team’s identification of the neural substrates of aggression could point toward new therapies for pathological violence, he replied, “Someday maybe. But it will require much more knowledge of the system before such therapies could be contemplated.”

“This is a very sophisticated neurobiological analysis,” Richard Krueger, M.D., medical director of the Sexual Behavior Clinic of the New York State Psychiatric Institute, said during an interview. The major thrust of the findings—that the control centers for sex and aggression are closely located in the brain—is quite plausible, he believes, and might help explain why sexual and aggressive behavior come to be fused in many individuals—say, in those who engage in sexual sadism or in couples who, by mutual consent, engage in sadistic-masochistic behavior. And clearly within the animal kingdom, there are many instances of sexual aggression, he pointed out, which makes a strong case that the control systems of sex and aggression reside closely in the brain.

It is hoped that this study, Krueger continued, will lead to more research on aggression and especially on sexual aggression—subjects that the National Institute of Mental Health and private foundations have been loath to fund for many years.

“The last study of any consequence for aggression was … about 25 years ago, at the New York State Psychiatric Institute,” he said.

This present study, he added, might provide new instruments for measuring aggression or sexual aggression, since the only other instrument that has been available is the Overt Aggression Scale.

The research was funded by the Weston-Havens Foundation, the Paul G. Allen Family Foundation, and the Howard Hughes Medical Institute.

An abstract of “Optogenetics, Sex, and Violence in the Brain: Implications for Psychiatry” is posted at www.biologicalpsychiatryjournal.com/article/S0006-3223(11)01127-9/abstract.