relations are never that high, even when corrected for unreliability of
measurement. Over very long intervals, like 50 years, only about 60% ofthe variance is stable (Costa & McCrae, 1992b).
Some writers take that fact as evidence of the effects of life experi -
ence on personality . If personality were entirely determined by biologi -
cal bases, wouldn’t it be perfectly stable? Of course, the answer is “no.”
Consider the twins studied by McGue et al. (1993). As we noted, someshowed big decreases in Stress Reaction, some showed small decreases.The effect of that difference would be to shift the rank-ordering of peo -
ple in the sample and to yield a retest correlation less than 1.0 (in fact, it
was .53 for Stress Reaction). We might suppose that the magnitude ofthe decreases was related to some life experience—perhaps some ofthem took up meditation or were happily married. But in this case wehappen to know that their identical twins showed changes of the samemagnitude, although they were unlikely to have experienced the samelife events. It is possible that all the changes in rank order are due to in-dividual differences in the hypothesized biological clocks.
That is not the only possibility . Indeed, McGue and colleagues
(1993, Table 7) characterized the heritability of changes as modest tomoderate in magnitude, leaving room for other causes. Environmentalinfluences are one possibility , but so are other biological causes, such asbrain diseases or injuries (even very subtle ones). Longitudinal studiesof the brain using magnetic resonance imaging show that there is appre-ciable atrophy: After age 55, people lose about a teaspoon of gray matterevery year (Resnick, Davatzikos, Kraut, & Zonderman, 2000). De -
pending on where that tissue comes from, it might affect personality
traits. With age, human cells deteriorate, and—experience quite aside—the brain of a 70-year-old is not the same as it was 30 years earlier. Whatis remarkable from a biological point of view is how stable personalitytraits are.
EVALUATING FIVE-FACTOR THEORY
How should we evaluate FFT as a theory of personality? It certainlysummarizes and, in our view , makes sense of a good deal of research onthe origin and development of personality traits. It also relates personal -
ity traits to many if not most of the other kinds of variables that person -
ality psychologists have studied—attitudes, relationships, identity , and200 PERSONALITY IN ADULTHOOD