What You Can Change _. And What You Can't - Martin E. Seligman [167]
12. I thank my good friend Barry Schwartz for these colorful metaphors, which he uses in Psychology 1 at Swarthmore College. But I wish he would remember new minor forcing. See M. Rutter, “Pathways from Childhood to Adult Life,” Journal of Child Psychology and Psychiatry 30 (1989): 23–51, for an elaborate discussion of many possible models and paths.
13. My discussion of the influence of the childhood environment on adult personality has glossed over an important distinction for human-behavior geneticists: the distinction between “shared” and “unshared” environments. Joan and Sarah share certain experiences—being upper-middle-class, gardening with Daddy, and Catholicism, for example. They also have an unshared environment: Joan was molested at age ten, Sarah had appendicitis, and Sarah secretly hates gardening. Shared and unshared environments turn out to have very different influences on adult personality. The shared childhood environment—church, school, rearing techniques, socioeconomic status—has virtually no effect on adult personality. Identical twins reared apart are just as similar, maybe more similar, in adult personality than identical twins reared together. Conversely, adopted sibs raised in the same household are no more similar than if they had been raised separately. This means that shared environment in childhood adds nothing.
The whole kit and caboodle that conventional American developmental psychology bet on came up with a bust.
The unshared childhood environment is more promising. It probably accounts for between 15 and 50 percent of the variance in adult personality—probably not as much as genes, but a substantial amount. Before we environmentalists get too excited once again, however, let me say some of what “unshared environment” includes: big events like sexual abuse (which warms the hearts of environmentalists); small events like missing one ballet class; how you interpret, perceive, or remember big or small events; differing bodily reaction to events; parents’ loving you more or less than they did your sib; fetal hormones; childhood illnesses; and good old error of measurement—anything at all that you and your sib are not identical for. It is, unfortunately, a wastebasket category that contains three-quarters of psychology.
No specific piece of unshared environment in childhood has yet to be shown to have any effect at all on adult personality once genes are controlled. In the correct kind of design, J. Loehlin and R. Nichols, Heredity, Environment, and Personality (Austin: University of Texas Press, 1976), looked at over seven hundred pairs of identical twins. They isolated about fifty pairs in which one had saliently different events from the other during childhood—one had a serious illness but the other did not, one got spanked a lot, but the other did not, and so on. None of these differences produced any detectable differences in later personality.
To sum up: Genes have a big effect on adult personality. Surprisingly, childrearing techniques, schools, socioeconomic status, and religion do not have a detectable effect. Idiosyncratic experience—traumatic or nontraumatic events, differential treatment by parents, peculiar turn of mind—should in theory have a noticeable effect, but it has yet to be demonstrated.
This is a difficult but illuminating literature. I recommend that the serious student start with R. Plomin and D. Daniel’s classic “Why are Children in the Same Family So Different from One Another?” Behavioral and Brain Sciences 10 (1987): 1–16, along with the very high quality “peer commentary” that follows. It also has a first-rate bibliography. Then read J. Dunn and R. Plomin, Separate Lives: Why Siblings Are So Different (New York: Basic Books, 1990).
More recent papers include A. Tellegen, D. Lykken, T. Bouchard, et al., “Personality Similarity in Twins Reared Apart and Together,” Journal of Personality and Social Psychology 54 (1988): 1031–39; L. Baker and D. Daniels, “Nonshared Environmental Influences and Personality