Сьюзан Кейн - Quiet [The Power of Introverts in a World That Can't Stop Talking]

In truth, many people Aron considers sensitive have some of the twenty-seven attributes associated with the trait, but not all of them. Maybe they’re sensitive to light and noise, but not to coffee or pain; maybe they’re not sensitive to anything sensory, but they’re deep thinkers with a rich inner life. Maybe they’re not even introverts—only 70 percent of sensitive people are, according to Aron, while the other 30 percent are extroverts (although this group tends to report craving more downtime and solitude than your typical extrovert). This, speculates Aron, is because sensitivity arose as a by-product of survival strategy, and you need only some, not all, of the traits to pull off the strategy effectively.

There’s a great deal of evidence for Aron’s point of view. Evolutionary biologists once believed that every animal species evolved to fit an ecological niche, that there was one ideal set of behaviors for that niche, and that species members whose behavior deviated from that ideal would die off. But it turns out that it’s not only humans that divide into those who “watch and wait” and others who “just do it.” More than a hundred species in the animal kingdom are organized in roughly this way.

From fruit flies to house cats to mountain goats, from sunfish to bushbaby primates to Eurasian tit birds, scientists have discovered that approximately 20 percent of the members of many species are “slow to warm up,” while the other 80 percent are “fast” types who venture forth boldly without noticing much of what’s going on around them. (Intriguingly, the percentage of infants in Kagan’s lab who were born high-reactive was also, you’ll recall, about twenty.)

If “fast” and “slow” animals had parties, writes the evolutionary biologist David Sloan Wilson, “some of the fasts would bore everyone with their loud conversation, while others would mutter into their beer that they don’t get any respect. Slow animals are best described as shy, sensitive types. They don’t assert themselves, but they are observant and notice things that are invisible to the bullies. They are the writers and artists at the party who have interesting conversations out of earshot of the bullies. They are the inventors who figure out new ways to behave, while the bullies steal their patents by copying their behavior.”

Once in a while, a newspaper or TV program runs a story about animal personalities, casting shy behavior as unseemly and bold behavior as attractive and admirable. ( That’s our kind of fruit fly! ) But Wilson, like Aron, believes that both types of animals exist because they have radically different survival strategies, each of which pays off differently and at different times. This is what’s known as the trade-off theory of evolution, in which a particular trait is neither all good nor all bad, but a mix of pros and cons whose survival value varies according to circumstance.

“Shy” animals forage less often and widely for food, conserving energy, sticking to the sidelines, and surviving when predators come calling. Bolder animals sally forth, swallowed regularly by those farther up the food chain but surviving when food is scarce and they need to assume more risk. When Wilson dropped metal traps into a pond full of pumpkinseed fish, an event he says must have seemed to the fish as unsettling as a flying saucer landing on Earth, the bold fish couldn’t help but investigate—and rushed headlong into Wilson’s traps. The shy fish hovered judiciously at the edge of the pond, making it impossible for Wilson to catch them.

On the other hand, after Wilson succeeded in trapping both types of fish with an elaborate netting system and carrying them back to his lab, the bold fish acclimated quickly to their new environment and started eating a full five days earlier than did their shy brethren. “There is no single best … [animal] personality,” writes Wilson, “but rather a diversity of personalities maintained by natural selection.”

Another example of the trade-off theory of evolution is a species known as Trinidadian guppies. These guppies develop personalities—with astonishing speed, in evolutionary terms—to suit the microclimates in which they live. Their natural predators are pike. But some guppy neighborhoods, upstream of a waterfall for example, are pike-free. If you’re a guppy who grew up in such a charmed locale, then chances are you have a bold and carefree personality well suited to la dolce vita . In contrast, if your guppy family came from a “bad neighborhood” downstream from the waterfall, where pike cruise the waterways menacingly, then you probably have a much more circumspect style, just right for avoiding the bad guys.

The interesting thing is that these differences are heritable, not learned, so that the offspring of bold guppies who move into bad neighborhoods inherit their parents’ boldness—even though this puts them at a severe disadvantage compared to their vigilant peers. It doesn’t take long for their genes to mutate, though, and descendants who manage to survive tend to be careful types. The same thing happens to vigilant guppies when the pike suddenly disappear; it takes about twenty years for their descendants to evolve into fish who act as if they haven’t a care in the world.

The trade-off theory seems to apply equally to humans. Scientists have found that nomads who inherited the form of a particular gene linked to extroversion (specifically, to novelty-seeking) are better nourished than those without this version of the gene. But in settled populations, people with this same gene form have poorer nutrition. The same traits that make a nomad fierce enough to hunt and to defend livestock against raiders may hinder more sedentary activities like farming, selling goods at the market, or focusing at school.

Or consider this trade-off: human extroverts have more sex partners than introverts do—a boon to any species wanting to reproduce itself—but they commit more adultery and divorce more frequently, which is not a good thing for the children of all those couplings. Extroverts exercise more, but introverts suffer fewer accidents and traumatic injuries. Extroverts enjoy wider networks of social support, but commit more crimes. As Jung speculated almost a century ago about the two types, “the one [extroversion] consists in a high rate of fertility, with low powers of defense and short duration of life for the single individual; the other [introversion] consists in equipping the individual with numerous means of self-preservation plus a low fertility rate.”

The trade-off theory may even apply to entire species. Among evolutionary biologists, who tend to subscribe to the vision of lone individuals hell-bent on reproducing their own DNA, the idea that species include individuals whose traits promote group survival is hotly debated and, not long ago, could practically get you kicked out of the academy. But this view is slowly gaining acceptance. Some scientists even speculate that the evolutionary basis for traits like sensitivity is heightened compassion for the suffering of other members of one’s species, especially one’s family.

But you don’t have to go that far. As Aron explains, it makes sense that animal groups depend on their sensitive members for survival. “Suppose a herd of antelope … has a few members who are constantly stopping their grazing to use their keen senses to watch for predators,” she writes. “Herds with such sensitive, watchful individuals would survive better, and so continue to breed, and so continue to have some sensitive individuals born in the group.”

And why should it be any different for humans? We need our Eleanor Roosevelts as surely as grazing herds depend on their sensitive antelopes.