development in infancy and childhood. There is every reason to believe
that children as young as 5 years of age can be meaningfully character -
ized in terms of the FFM (Kohnstamm, Halverson, Mervielde, & Havill,
1998), but whether 5-year-olds are more extraverted or less conscien -
tious than 12-year-olds is a difficult question. And does it make any
sense to even ask about levels of Conscientiousness in infants?
Some readers may be puzzled about how we reconcile the (largely)
genetic basis of traits with their lifespan development. One’s genes, afterall, do not change. But genetic influences are dynamic. They determinedevelopment not only in the embryo but in the growing infant and, tosome extent, in the aging adult. Parts of the brain continue to grow wellinto the 20s (Pujol, Vendrell, Junque, Martí-Vilalta, & Capdevila,1993). Menopause and middle-aged weight gain are programmed intobiological clocks. FFT merely adds that personality trait development isalso tied to biological clocks: Personality changes can be considered in-
trinsic maturation .
There are three lines of evidence that support this claim, although
research is limited so far: First and best established is the cross-culturalevidence we reviewed in Chapter 5. Adolescents differ from adults inthe same ways in almost all cultures, despite differences in history ,child-rearing practices, customs, and so on. There are other possible ex-planations for this phenomenon—for example, adolescents all over theworld watch television today , whereas their grandparents did not—butthese data are certainly consistent with FFT. Development is every-where the same because it is built into the human species.
Second, McGue et al. (1993) provided a direct test of the above hy -
pothesis by examining the heritability of changes. They followed a sam -
ple of twins from age 18 to age 30 and calculated the changes over that
period in a series of traits. On average, they saw changes that are consis -
tent with our findings—for example, twins decreased in Stress Reaction
(a measure of Neuroticism) and increased in Achievement (a measureof Conscientiousness). But there were also individual differences: Somepeople decreased markedly in Stress Reaction, whereas others onlyshowed small decreases. McGue and colleagues showed that thesechanges were heritable: When one identical twin showed big changes,the other twin usually showed big changes as well. These data are con -
sistent with the hypothesis that a genetically programmed biological
clock governs trait development. It would be helpful to have replica -
tions of this finding, but there are very few longitudinal studies of twins198 PERSONALITY IN ADULTHOOD