Relaxed and exploratory, the rovers have fun, make friends and will take risks, both rewarding and dangerous ones, as they grow. According to Daniel Nettle, a Newcastle University evolutionary psychologist, extroverts are more likely than introverts to be hospitalized as a result of an injury, have affairs (men) and change relationships (women). One study of bus drivers even found that accidents are more likely to occur when extroverts are at the wheel.

In contrast, sitter children are careful and astute, and tend to learn by observing instead of by acting. They notice scary things more than other children do, but they also notice more things in general. Studies dating all the way back to the 1960’s by the psychologists Jerome Kagan and Ellen Siegelman found that cautious, solitary children playing matching games spent more time considering all the alternatives than impulsive children did, actually using more eye movements to make decisions. Recent studies by a group of scientists at Stony Brook University and at Chinese universities using functional M.R.I. technology echoed this research, finding that adults with sitter-like temperaments looked longer at pairs of photos with subtle differences and showed more activity in brain regions that make associations between the photos and other stored information in the brain.

Once they reach school age, many sitter children use such traits to great effect. Introverts, who tend to digest information thoroughly, stay on task, and work accurately, earn disproportionate numbers of National Merit Scholarship finalist positions and Phi Beta Kappa keys, according to the Center for Applications of Psychological Type, a research arm for the Myers-Briggs personality type indicator — even though their I.Q. scores are no higher than those of extroverts. Another study, by the psychologists Eric Rolfhus and Philip Ackerman, tested 141 college students’ knowledge of 20 different subjects, from art to astronomy to statistics, and found that the introverts knew more than the extroverts about 19 subjects — presumably, the researchers concluded, because the more time people spend socializing, the less time they have for learning.

THE psychologist Gregory Feist found that many of the most creative people in a range of fields are introverts who are comfortable working in solitary conditions in which they can focus attention inward. Steve Wozniak, the engineer who founded Apple with Steve Jobs, is a prime example: Mr. Wozniak describes his creative process as an exercise in solitude. “Most inventors and engineers I’ve met are like me,” he writes in “iWoz,” his autobiography. “They’re shy and they live in their heads. They’re almost like artists. In fact, the very best of them are artists. And artists work best alone ... Not on a committee. Not on a team.”

Sitters’ temperaments also confer more subtle advantages. Anxiety, it seems, can serve an important social purpose; for example, it plays a key role in the development of some children’s consciences. When caregivers rebuke them for acting up, they become anxious, and since anxiety is unpleasant, they tend to develop pro-social behaviors. Shy children are often easier to socialize and more conscientious, according to the developmental psychologist Grazyna Kochanska. By 6 they’re less likely than their peers to cheat or break rules, even when they think they can’t be caught, according to one study. By 7 they’re more likely to be described by their parents as having high levels of moral traits such as empathy.

When I shared this information with the mother of a “sitter” daughter, her reaction was mixed. “That is all very nice,” she said, “but how will it help her in the tough real world?” But sensitivity, if it is not excessive and is properly nurtured, can be a catalyst for empathy and even leadership. Eleanor Roosevelt, for example, was a courageous leader who was very likely a sitter. Painfully shy and serious as a child, she grew up to be a woman who could not look away from other people’s suffering — and who urged her husband, the constitutionally buoyant F.D.R., to do the same; the man who had nothing to fear but fear itself relied, paradoxically, on a woman deeply acquainted with it.

Another advantage sitters bring to leadership is a willingness to listen to and implement other people’s ideas. A groundbreaking study led by the Wharton management professor Adam Grant, to be published this month in The Academy of Management Journal, found that introverts outperform extroverts when leading teams of proactive workers — the kinds of employees who take initiative and are disposed to dream up better ways of doing things. Professor Grant notes that business self-help guides often suggest that introverted leaders practice their communication skills and smile more. But, he told me, it may be extrovert leaders who need to change, to listen more and say less.