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Even more impressive is how early younger siblings develop what's known as the theory of mind. Very small children have a hard time distinguishing the things they know from the things they assume other people know. A toddler who watches an adult hide a toy will expect that anyone who walks into the room afterward will also know where to find it, reckoning that all knowledge is universal knowledge. It usually takes a child until age 3 to learn that that's not so. For children who have at least one elder sibling, however, the realization typically comes earlier. "When you're less powerful, it's advantageous to be able to anticipate what's going on in someone else's mind," says Sulloway.
Later-borns, however, don't try merely to please other people; they also try to provoke them. Richard Zweigenhaft, a professor of psychology at Guilford College in Greensboro, N.C., who revealed the overrepresentation of firstborns in Congress, conducted a similar study of picketers at labor demonstrations. On the occasions that the events grew unruly enough to lead to arrests, he would interview the people the police rounded up. Again and again, he found, the majority were later- or last-borns. "It was a statistically significant pattern," says Zweigenhaft. "A disproportionate number of them were choosing to be arrested."
Later-borns are similarly willing take risks with their physical safety. All sibs are equally likely to be involved in sports, but younger ones are likelier to choose the kinds that could cause injury. "They don't go out for tennis," Sulloway says. "They go out for rugby, ice hockey." Even when siblings play the same sport, they play it differently. Sulloway is currently collaborating on a study of 300 brothers who were major league ballplayers. Though the work is not complete, he is so far finding that the elder brothers excel at skills that involve less physical danger. Younger siblings are the ones who put themselves in harm's way--crouching down in catcher's gear to block an incoming runner, say. "It doesn't just hold up in this study but a dozen studies," Sulloway says.
It's not clear whether such behavior extends to career choice, but Sandra Black, an associate professor of economics at UCLA, is intrigued by findings that firstborns tend to earn more than later-borns, with income dropping about 1% for every step down the birth-order ladder. Most researchers assume this is due to the educational advantages eldest siblings get, but Black thinks there may be more to it. "I'd be interested in whether it's because the second child is taking the riskier jobs," she says.
Black's forthcoming studies will be designed to answer that question, but research by Ben Dattner, a business consultant and professor of industrial and organizational psychology at New York University, is showing that even when later-borns take conservative jobs in the corporate world, they approach their work in a high-wire way. Firstborn CEOs, for example, do best when they're making incremental improvements in their companies: shedding underperforming products, maximizing profits from existing lines and generally making sure the trains run on time. Later-born CEOs are more inclined to blow up the trains and lay new track. "Later-borns are better at transformational change," says Dattner. "They pursue riskier, more innovative, more creative approaches."
If eldest sibs are the dogged achievers and youngest sibs are the gamblers and visionaries, where does this leave those in between? That it's so hard to define what middle-borns become is largely due to the fact that it's so hard to define who they are growing up. The youngest in the family, but only until someone else comes along, they are both teacher and student, babysitter and babysat, too young for the privileges of the firstborn but too old for the latitude given the last. Middle children are expected to step up to the plate when the eldest child goes off to school or in some other way drops out of the picture--and generally serve when called. The Norwegian intelligence study showed that when firstborns die, the IQ of second-borns actually rises a bit, a sign that they're performing the hard mentoring work that goes along with the new job.
Stuck for life in a center seat, middle children get shortchanged even on family resources. Unlike the firstborn, who spends at least some time as the only-child eldest, and the last-born, who hangs around long enough to become the only-child youngest, middlings are never alone and thus never get 100% of the parents' investment of time and money. "There is a U-shaped distribution in which the oldest and youngest get the most," says Sulloway. That may take an emotional toll. Sulloway cites other studies in which the self-esteem of first-, middle- and last-borns is plotted on a graph and follows the same curvilinear trajectory.
The phenomenon known as de-identification may also work against a middle-born. Siblings who hope to stand out in a family often do so by observing what the elder child does and then doing the opposite. If the firstborn gets good grades and takes a job after school, the second-born may go the slacker route. The third-born may then de-de-identify, opting for industriousness, even if in the more unconventional ways of the last-born. A Chinese study in the 1990s showed just this kind of zigzag pattern, with the first child generally scoring high as a "good son or daughter," the second scoring low, the third scoring high again and so on. In a three-child family, the very act of trying to be unique may instead leave the middling lost, a pattern that may continue into adulthood.
The holes in the theories
The birth-order effect, for all its seeming robustness, is not indestructible. There's a lot that can throw it out of balance--particularly family dysfunction. In a 2005 study, investigators at the University of Birmingham in Britain examined the case histories of 400 abused children and the 795 siblings of those so-called index kids. In general, they found that when only one child in the family was abused, the scapegoat was usually the eldest. When a younger child was abused, some or all of the other kids usually were as well. Mistreatment of any of the children usually breaks the bond the parents have with the firstborn, turning that child from parental ally to protector of the brood. At the same time, the eldest may pick up some of the younger kids' agreeableness skills--the better to deal with irrational parents--while the youngest learn some of the firstborn's self-sufficiency. Abusiveness is going to "totally disrupt the birth-order effects we would expect," says Sulloway.