Story of Psychology - Morton Hunt [223]
From 1979 to 1990, Bouchard and his researchers tracked down nearly eighty pairs of identicals and thirty-three pairs of fraternals reared apart (out of some eight thousand pairs in their files), and put each twin through about fifty hours of intensive tests and interviews. For comparative purposes, they did the same with a number of identical and fraternal twins reared together. Statistical analysis of all the correlations within the twin pairs and among these various groups led the team to conclude that about 50 percent of variance in personality is due to heredity.79 (They reported similarly remarkable findings for many other psychological variables, including general intelligence, language ability, social attitudes, homosexuality, substance abuse, and even religious interests.)
Some other studies in behavior genetics, however, have yielded more modest estimates. John C. Loehlin, of the University of Texas, Austin, recently reviewed a large number of twin studies and found that on the whole the evidence indicated that heredity accounts for about 40 percent of the variance in personality.80 Several studies comparing adopted children to their adoptive mothers and to their biological mothers found only 25 percent of the variance attributable to heredity (although, interestingly, the adopted children resemble their biological mothers more than those who reared them in personality).81
Clarifying the matter, in 2003 Bouchard and a colleague, Matt McGue, performed a comprehensive review of Bouchard’s and other researchers’ twin, family, and adoption research. Sophisticated mathematical analysis produced cumulative evidence that “genetic influence on personality trait variation is in the 40%–55% range” and that “common (shared) family influence on personality traits is very close to zero.” Nonshared—that is, different—environmental influences account for much or most of the other personality variations, but have been extremely difficult to identify.82
The figures do not mean that 40 to 55 percent of any individual’s personality results from hereditary influences. Variance refers to the range of differences among people in any trait or set of traits. Data from Bouchard’s center show, for example, that if a group of adults range in height from, say, four feet to seven feet, 90 percent of that span of differences is due to heredity, 10 percent to environment. Similarly, the twin studies mean that 40 to 55 percent of the range of personality differences among any group of people are of hereditary origin. This may explain why Americans have so many more variations in personality than the members of a more genetically homogeneous population, like the Japanese.
The findings of behavior genetics yield new understanding on a theoretical level—a level very different from that which interests most personality psychologists, namely, insight into the emotions and social relations involved in personality, and ways of testing and influencing them. It is even possible to see behavior genetics as diminishing the hope that psychology can improve the quality of human life, since to the degree that personality is hereditary in origin, it is not amenable to parental or social influence, therapy, or any other potentially controllable environmental factor. Many psychologists, therefore, including those in the field of personality, consider the findings of behavior genetics valuable as science but of no benefit in practice. What matters to them is the rest of variance in personality—the extent to which it can be influenced for better or worse.
Late Word from the Personality Front
Personality is no longer the most prominent field of psychology, not because it has shrunk in size but because by a generation ago certain newer fields had expanded and become the foci of attention. Also, as in any mature field of science, many personality researchers now churn out overspecialized