If there is an extraterrestrial college student orbiting Earth or floating invisibly among us while writing a thesis on human behavior, then current events have provided some good examples of one basic human trait: retaliation.

Retaliation was, for example, implicit in President George W. Bush as justification for the invasion of Iraq. And in Japan, the elementary school killer of her 12-year-old classmate Satomi Mitarai told police that her victim had said she was “heavy” and a “goody-goody.” On June 1, the 11-year-old stabbed Satomi in the neck with a box cutter.

If our ET college student made a detailed study, she/he/it would notice that retaliation occurs among nonhumans too. In birds (blue-footed boobies and European moorhens), elephant seals, side-striped jackals as well as in a host of primates, animals will use violence in response to violence. Animals will attack another animal that has injured them.

Our student alien might conclude that the impulse to avenge an injury is universal in humans, and common in other animals. ET might report that “an eye for an eye, and a tooth for a tooth” is a driving force in human violence. And a really thorough alien might cite a human study that appeared in the June issue of the Archives of Pediatrics and Adolescent Medicine. The work is especially relevant to the Satomi Mitarai case.

Researchers at The Children’s Hospital of Philadelphia in Pennsylvania surveyed 190 children aged 8 to 14 who had been brought to the hospital’s emergency department for injuries caused by interpersonal violence. (Injuries that were unintentional, self-inflicted or caused by child abuse were not included in the study.) They found that girls in middle and elementary school who are involved in violent incidents are more likely than boys of the same age to be retaliating for a previous event. Although girls are more likely to retaliate, for both genders the most common reason given for a fight was “being disrespected” or “teasing.” This is exactly what Satomi’s killer told investigators.

Of course, the incident in the Sasebo, Nagasaki Prefecture schoolroom is an extreme case, but it is an example nonetheless of female retaliation for a perceived wrong. “Although interpersonal violence is not uncommon among pre-adolescent and young adolescent girls, little research has focused specifically on females,” said Cynthia Mollen, an emergency-medicine physician at The Children’s Hospital of Philadelphia, and first author of the study. “If health-care providers know more about specific patterns of violence, they may be able to better prevent future incidents.”

Weapons were more often present in events involving at least one female. But why are girls more likely to retaliate and to use weapons? “These are all young girls and boys in this study between 8 and 14 years old,” said Mollen in an e-mail interview. “One could speculate that compared to boys of this age, girls may be more skilled at planning violent events in response to some prior insult or fight. In contrast, young boys may react more ‘in the moment’ and base their actions on intricate relationship issues.”

Mollen points out that her study provides information for parents and teachers. “For instance, because ‘disrespect’ appears so prominently as a trigger for violence, children and parents could benefit by learning techniques for responding to perceived insults in a nonviolent manner,” she said. “We know from previous research that a parent’s attitude about appropriate triggers for violence has an effect on children’s behavior.”

The effect of parents’ attitude notwithstanding, the impulse to retaliate is widespread in the animal kingdom. And even in cultures without the linguistic concept of revenge, the impulse to do so is still there, suggesting that revenge is not a human cultural construct.

James Boster, an anthropology professor at the University of Connecticut, studied the Waorani, a tribe in eastern Ecuador. Up until recent contact with the outside world, the Waorani engaged in a vicious cycle of revenge killings, but the Wao language lacks a term for the concept of “revenge killing.” They simply refer to the act as waenganta (killed) or taenongantapa (speared), Boster said.

This suggests that a social pattern of revenge killing is not dependent on the recognition of “revenge” as an abstract idea. The urge to retaliate for a perceived wrong is arguably, therefore, a basic human trait.

That’s not to say, of course, that retaliation is always, or even sometimes, justified. Sometimes it may be attributed to a false idea (that Saddam Hussein was responsible for the 9/11 attacks), or it can be grossly out of proportion to the offence (as in the Mitarai case).

Mollen had advice that might be relevant to the Mitarai case, in which the young killer had earlier appealed for help on her Web page. “Because girls were more likely to offer retaliation as a reason for violence, health-care providers could screen injured girls about their safety concerns and their plans for retaliation,” she said. “Understanding gender differences in violent behavior could help us in designing school-based and community-intervention programs for children in this age group.”

Understanding our nature helps because humans can learn to control their basic traits.

Are violent acts among children increasing? We don’t know. “Some measures of violence in the United States — like homicide — have declined over recent years, but there aren’t very good surveillance mechanisms for non-fatal injuries,” said Mollen. “It seems as though violence is getting more attention in the press, especially due to school shootings, but it is unclear if the overall incidence is increasing.”

What we do know is that retaliation is a deep part of human nature. Whether that knowledge can ever help us understand what happened in that Sasebo classroom is another matter.

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