amental and behavioral range as well.

Berman saw that when rhesus troops are small, the mothers can let their young play freely, because strangers rarely approach. But as a troop grows and the number of family groups rises, strangers or semi-strangers more often come near. The adult females become more vigilant, defensive, and aggressive. The kids and adult males follow suit. More and more monkeys receive upbringings that draw out the less sociable sides of their behavioral potentials; fights grow more common; rivalries grow more tense. Things finally get so bad that the troop must split. “And that’s what happened here,” Suomi said. “It’s a very extensive feedback system. What happens at the dyadic level, between mother and infant, ultimately affects the very nature and survival of the larger social group.”

Studies by Suomi and others show that such differences in early experience can wildly alter how genes express themselves—that is, whether, when, and how strongly the genes switch themselves on and off. Suomi suspects that early experiences may affect later patterns of gene expression and behavior as well, including how flexible and reactive an animal is, by helping to set the sensitivity level of key alleles. A tense upbringing, he says, will produce watchful caution or vigilant aggression in any monkey (the parents’ way of preparing the offspring for tough times)—but this effect may be especially pronounced in monkeys with particularly plastic behavioral alleles.

That’s what Suomi thinks may have happened in the run-up to what he calls the Palace Revolt. Fiona’s injudicious aggression proved disastrous for her and Family 1. But Family 3, a group that had been diplomatically deferring to Family 1 for years, dramatically improved its fortunes by mounting an uncharacteristically aggressive and sustained counterattack. Suomi speculates that in the tenser, more crowded conditions of the large colony, gene-environment interactions had made some of the monkeys in Family 3, particularly those with more-reactive “orchid” alleles, not more aggressive but more potentially aggressive. During the period when they could not afford to challenge the hierarchy—the period before Pearl’s departure—aggressiveness would have led them into unwinnable, possibly fatal conflicts. But in Pearl’s absence the odds changed—and the Family 3 monkeys exploited a rare and decisive opportunity by unleashing their aggressive potential.

The coup also showed something more straightforward: that a genetic trait tremendously maladaptive in one situation can prove highly adaptive in another. We needn’t look far to see this in human behavior. To survive and evolve, every society needs some individuals who are more aggressive, restless, stubborn, submissive, social, hyperactive, flexible, solitary, anxious, introspective, vigilant—and even more morose, irritable, or outrigh

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