Walk into any preschool classroom and you’ll see that some 4-year-olds are always getting into fights—while others seldom do, no matter the provocation. Even siblings can differ dramatically—remember Cain and Abel. Is it nature or nurture that causes these deep differences in aggression?

The new techniques of genomics—mapping an organism’s DNA and analyzing how it works—initially led people to think that we might find a gene for undesirable individual traits like aggression. But from an evolutionary point of view, the very idea that a gene can explain traits that vary so dramatically is paradoxical: If aggression is advantageous, why didn’t the gene for aggression spread more widely? If it’s harmful, why would the gene have survived at all?

Two new studies suggest that the relationship between genes and aggression is more complicated than a mere question of nature vs. nurture. And those complications may help to resolve the evolutionary paradox.

In earlier studies, researchers looked at variation in a gene involved in making brain chemicals. Children with a version of the gene called VAL were more likely to become aggressive than those with a variation called MET. But this only happened if the VAL children also experienced stressful events like abuse, violence or illness. So it seemed that the VAL version of the gene made the children more vulnerable to stress, while the MET version made them more resilient.

A study published last month in the journal Developmental Psychology, by Beate Hygen and colleagues from the Norway University of Science and Technology and Jay Belsky of U.C. Davis, found that the story was even more complicated. They analyzed the genes of hundreds of Norwegian 4-year-olds. They also got teachers to rate how aggressive the children were and parents to record whether the children had experienced stressful life events.

As in the earlier studies, the researchers found that children with the VAL variant were more aggressive when they were subjected to stress. But they also found something else: When not subjected to stress, these children were actually less aggressive than the MET children.

Dr. Belsky has previously used the metaphor of orchids and dandelions to describe types of children. Children with the VAL gene seem to be more sensitive to the environment, for good and bad, like orchids that can be magnificent in some environments but wither in others. The MET children are more like dandelions, coming out somewhere in the middle no matter the conditions.

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