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Reuters/Issei Kato
Sex and violence are closely connected in the brain circuitry.
ANIMAL BEHAVIOR

There’s a neurological explanation for the link between sex and violence

By Olivia Goldhill

Sex and violence are far closer than we’d like to believe. And from a neurological perspective, the two seem to be more connected than even the most hardened cynic would expect.

David Anderson, a neurobiologist from California Institute of Technology, has published several papers showing that in mice and in fruit flies, triggers for sex and aggression come down to the same clusters of neurons. He’s due to present an overview of his work on the subject at this year’s Brain Forum in Switzerland. Anderson believes that there’s a “high likelihood” that the some of the brain circuit details he’s discovered in other animals will hold true for humans as well, suggesting that there’s an innate neurological basis for the overlap between sex and aggression.

Anderson has found that a tiny cluster of neurons (relative to the size of each animal’s brain—so 10 neurons on either side of the fruit fly’s brain, and 2,000 on either side of the mouse’s brain) have a role in controlling both sexual and aggressive behaviors of males. He’s made such discoveries by genetically targeting each neuron; with that level of precision, he can turn the neurons on and off. Now he’s working to establish whether it’d be possible for these clusters to prompt both sexual and aggressive behavior simultaneously, or whether it’s an either/or mechanism.

The close relationship between sex and violence is not particularly surprising, he says, given similarities in the two behaviors across the animal world. “They both involve an initial approach and close investigation, a lot of sniffing and sensing, and in some animals you see that sexual behavior can be accompanied by aggression, for example biting,” he says.

Why might the two be so closely connected? Anderson believes that sexual behavior may have led to a need for aggressive tendencies. The inter-male aggression he studies is typically used to fight for access to resources such as food, territory, or females. And aggression would allow animals to compete sexually. “Neurons that control sexual behavior might have given rise in evolution and development to neurons that control aggressive behavior,” says Anderson. If this were the case, it would explain the neurons’ physical proximity.

Though Anderson hasn’t directly studied human brains, he says that his research has bearing on understanding our own minds. “We really can learn something about the human brain, or the principles by which the mammalian brain processes information and converts it into behavior, from studying the brains of simpler insect systems,” he says. “What these studies are showing is that, at an abstract level, there’s a functional organization of circuits and how they process information that is very analogous between fly and mouse and human brains.”

After all, the parts of the brains where the neurons are located in mice—the hypothalamus and the amygdala—are some of the most “evolutionarily ancient” parts of the human brain, he says. These areas are structurally similar in mice and humans. Though there’s no absolute certainty that human sex and aggression neurons will function in similar ways, Anderson says that it seems highly likely—not least because, socially and culturally, sex and violence among humans overlaps with alarming frequency.

There’s a further, uncomfortable element to the sex and violence neurons in the animals Anderson has studied: In the long term, each behavior reinforces the other. In other words, “animals with sexual experience are more aggressive than those without sexual experience,” says Anderson. He suggests that this tension could relate to maladaptive aggression in humans.

This is a further area for study, as is the question of whether the neurons trigger the motivation to conduct sex or violence, or the actions themselves. In the long-term, Anderson says, “our ability to understand the circuits that underlie natural emotional behaviors” can help shed light on our understanding of human psychiatric disorders. He hopes to “try to arrive at some general principles and fundamental insights that will help us better understand ourselves.”

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