What You Can Change _. And What You Can't - Martin E. Seligman [136]
Another quibble concerns how long the twins were reared together before separation. If they are not put up for adoption until late in childhood, the whole argument falls, and similarity could result either from genes or rearing. Most good twins-reared-apart studies, therefore, look only at twins separated early in the first year of life.
What is not a quibble is the possibility that personality is only indirectly heritable—that “nature works via nurture.” So we will see below that optimism is heritable: Identical twins are more concordant for it than fraternal twins. But optimism is produced by lots of success (and pessimism by lots of failure) in life. Success and failure, in turn, are caused by characteristics like looks, strength, and motor coordination, all of which are physical and heritable. Identical twins are more concordant for these characteristics (both twins are similar on massiveness or runtiness) than are fraternal twins. So what might be directly heritable are physical characteristics that produce the personality trait, with the personality trait actually wholly caused by the environment. This argument applies to all molar personality characteristics.
T. Bouchard, D. Lykken, M. McGue, N. Segal, A. Tellegen, “Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart,” Science 250 (1990): 223–28, apart from being a classic experiment, is also articulate on this point. While these findings are sometimes dismissed as “gene-environment covariation,” such dismissal misses the point. It is the environment that is primarily causal here, not the genes—and intervening to break the gene-environment covariation shows this.
Much of the future of environmental treatments for biologically loaded problems may be discovering ways to break the gene-environment covariation.
20. The best source for the details of this monumental study is Bouchard, et al., “Sources of Human Psychological Differences.” I highly recommend it to the curious reader. It is of high scope and quality.
Some people still have the preconception that IQ is not at all genetic. They are wrong. If someone tells you this, they are either scientifically illiterate or ideologically blinded. Don’t buy a used car from them. The identical-twin and adoptive data on IQ are massive and compelling: At least half (and in the Bouchard study, 75 percent) of the variance in IQ is genetic. What “intelligence” means, however, and what it predicts about achievement in life is much murkier.
The television-viewing study is R. Plomin, R. Corley, J. DeFries, and D. Fulker, “Individual Differences in Television Viewing in Early Childhood: Nature as Well as Nurture,” Psychological Science 1 (1990): 371–77. The religiosity study is N. Waller, B. Kojetin, T. Bouchard, D. Lykken, and A. Tellegen, “Genetic and Environmental Influences on Religious Interests, Attitudes, and Values,” Psychological Science 1 (1990): 138–42. The last four personality factors are from an analysis of the California Personality Inventory: T. Bouchard and M. McGue, “Genetic and Rearing Environmental Influences on Adult Personality: An Analysis of Adopted Twins Reared Apart,” Journal of Personality 58 (1990): 263–92.
21. See N. Pedersen, G. McClearn, R. Plomin, J. Nesselroade, J. Berg, and U. DeFaire, “The Swedish Adoption/Twin Study of Aging: An Update,” Acta Geneticae Medicae et Gemellologiae 40 (1991): 7–20. R. Plomin, M. Scheier, C. Bergeman, N. Pedersen, J. Nesselroade, and G. McClearn, “Optimism, Pessimism, and Mental Health: A Twin/Adoption Analysis,” Personality and Individual Differences 13 (1992): 921–30, presents the optimism piece of it.
22. See B. Hutchings and S. Mednick, “Criminality in Adoptees and Their Adoptive and Biological Parents: A Pilot Study,” in S. Mednick and K. Christiansen, eds., Biosocial Bases of Criminal Behavior (New York: Gardner, 1977), 127–42.
For similar logic applied to divorce, see M. McGue and D. Lykken,