how flexible the brain is. A brain hardwired for certain behaviors ought to be, in fact, hardwired. But the human brain actually exhibits remarkable flexibility (the technical term is plasticity) for adapting its tendencies in the wake of experience.

“One of the surprises of the last few years is the fact that we’re learning that the brain is hardwired for change,” says Ira Black, of the Robert Wood Johnson Medical School in New Jersey. “We’ve learned that the environment is capable of accessing genes and altering their activity within the brain.”18

Heredity does wire some predispositions into the brain, to be sure, but it’s a mistake to believe that experience must somehow defy the brain’s genetic hardwiring. It is actually the brain’s genetic wiring that creates the capacity to change with experience. “You are flexible because of your genes, not in spite of them,” declare neuroscientists Terrence Sejnowski and Steven Quartz in their book Liars, Lovers, and Heroes. “Your experiences with the world alter your brain’s structure, chemistry, and genetic expression, often profoundly, throughout your life.”19

So most experts would agree that genes are important, and genetic variation can influence propensities toward different kinds of behavior. On the other hand, genes are not so all-powerfully important as some gene-power dogmatists contend. Even animals, often portrayed as mere “gene machines” responding to stimuli with programmed responses, actually exhibit a lot of variability in their behavior that cannot be ascribed to genetic variations.

A few years back I ran across a study that put this issue in particularly sharp perspective, having to do with an especially simple behavioral response in mice. For years, scientists have annoyed mice by dipping their tails into a cup of hot water (typically about 120 degrees Fahrenheit). The idea is to test a mouse’s reaction to pain. Sure enough, the mice do not like having their tails dipped into hot water; as soon as you put the tail in, the mouse will jerk it out.

But not all mice behave in exactly the same way—at least, not all pull their tails out as rapidly as others. Experimenters have found that some mice react, on average, in a second or less; others might

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