The Secret World of Siblings
Emotional ambivalence often marks the most enduring relationship in life
It is something every brother or sister eventually marvels at, a conundrum that novelists have played upon in a thousand different ways: There are two children. They grow up in the same house, share the same parents, experience many of the same events. Yet they are stubbornly, astonishingly different.
A growing number of studies in the relatively new field of behavioral genetics are finding confirmation for this popular observation. Children raised in the same family, such studies find, are only very slightly more similar to each other on a variety of personality dimensions than they are, say, to Bill Clinton or to the neighbor's son. In cognitive abilities, too, siblings appear more different than alike. And the extent to which siblings do resemble one another in these traits is largely the result of the genes they share--a conclusion drawn from twin studies, comparisons of biological siblings raised apart and biological children and adopted siblings raised together.
Contrasts. Heredity also contributes to the differences between siblings. About 30 percent of the dissimilarity between brothers and sisters on many personality dimensions can be accounted for by differing genetic endowments from parents. But that still leaves 70 percent that cannot be attributed to genetic causes, and it is this unexplained portion of contrasting traits that scientists find so intriguing. If two children who grow up in the same family are vastly different, and genetics accounts for only a minor part of these differences, what else is going on?
The answer may be that brothers and sisters don't really share the same family at all. Rather, each child grows up in a unique family, one shaped by the way he perceives other people and events, by the chance happenings he alone experiences, and by how other people--parents, siblings and teachers--perceive and act toward him. And while for decades experts in child development have focused on the things that children in the same family share--social class, child-rearing attitudes and parents' marital satisfaction, for example--what really seem to matter are those things that are not shared. As Judy Dunn and Pennsylvania State behavioral geneticist Robert Plomin write in Separate Lives: Why Siblings Are So Different, "Environmental factors important to development are those that two children in the same family experience differently."
Asked to account for children's disparate experiences, most people invoke the age-old logic of birth order. "I'm the middle child, so I'm cooler headed," they will say, or "Firstborns are high achievers." Scientists, too, beginning with Sir Francis Galton in the 19th century, have sought in birth order a way to characterize how children diverge in personality, IQ or life success. But in recent years, many researchers have backed away from this notion, asserting that when family size, number of siblings and social class are taken into account, the explanatory power of birth ranking becomes negligible. Says one psychologist: "You wouldn't want to make a decision about your child based on it."
At least one researcher, however, argues that birth order does exert a strong influence on development, particularly on attitudes toward authority. Massachusetts Institute of Technology historian Frank Sulloway, who has just completed a 20-year analysis of 4,000 scientists from Copernicus through the 20th century, finds that those with older siblings were significantly more likely to have contributed to or supported radical scientific revolutions, such as Darwin's theory of evolution. Firstborn scientists, in contrast, were more apt to champion conservative scientific ideas. "Later-borns are consistently more open-minded, more intellectually flexible and therefore more radical," says Sulloway, adding that later-borns also tend to be more agreeable and less competitive.