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Sex and violence are linked in brain cells stimulated by male sex hormones, science tells us. The implications for both the continuation and the safety of human life are huge.

In 2011, the British novelist E. L. James released an astonishingly popular romantic novel entitled Fifty Shades of Grey. Selling over 125,000,000 copies worldwide in just four years, the novel chronicles the relationship between business tycoon Christian Grey and new college graduate Anastasia Steele, with explicit and, for some, intoxicating descriptions of bondage, discipline, dominance, submission, sadism, and masochism, sex with explicit violence.

Why would a novel on these themes become so popular with so many people so fast? Explaining literary success is beyond the scope of this site, but there are answers in science to the questions about the links of sex, violence, and the way our brains work.

Aggression Is a Fundamental Human Behavior

What is aggression? At its simplest level, aggression is attacking other humans, animals, inanimate objects, or oneself. In humans, aggression can be physical, verbal, or more subtle, sometimes a matter of what we don't do as well as what we do. Aggression can take the form of violence. In times of scarcity, aggression is directed toward defending territory, maintaining status, or obtaining food, shelter, or sex.

Aggression Is Activated Independently of Rational Thought

In human beings, aggression is so essential to maintaining life that it can be activated independently of rational thought. Aggressive emotions are "brain states," according to some researchers such as neurobiologist David Anderson from the California Institute of Technology, that are activated independently of memory or critical thinking. 

Since it's easier to study the brains of animals than it is to study the brains of humans, Anderson and his colleagues studied aggression and brain changes in animals. Almost all animals and humans respond to fear with either freezing in place or flight. The result of anger is usually to fight. The Cal Tech researchers bred genetically modified fruit flies that had specific brain cells that could be activated by light. They wanted to determine which cells in fruit fly brains can be stimulated to cause freezing, flying away, or aggression against other fruit flies.

Programmed Aggression in Fruit Fly, Rodent, and Human Males

The scientists found that in male fruit flies, activating certain brain cells could result in aggressive behavior toward potential mates. Blocking the activity of those brain cells stopped the aggressive behavior, although the male flies were still interested in sex. Anderson and coworkers concluded that the male-specific brain cells produce a specific protein that triggers aggressive behavior in fruit flies.

The scientists then identified a group of about 2,000 neurons in part of the brain known as the ventromedial hypothalamus in male laboratory mice that were closely associated with aggressive behavior. About 20 percent of these neurons were also associated with sexual behavior. As with the fruit flies, the researchers created genetically engineered mice with neurons that can be activated by exposure to light. They found that using a tiny fiber optic cable to deliver low-intensity light into this area of the mouse's brain could stimulate mounting and other sexual behavior, but high-intensity light triggered aggressive, fighting behavior.

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