This is a plausible hypothesis, but there is as yet little evidence by
which to evaluate it. If we could identify cliques of children who stayedtogether through most of childhood, we might hypothesize that, asadults, they would show more resemblance in personality traits thanchildren from different cliques. We do know that people who grow upin the same culture tend to show similar personality profiles relative topeople from other cultures (McCrae, 2000a), but it is not clear how tointerpret that finding. After all, people from the same culture also tendto be genetically more similar than people from different cultures.
There is another interpretation of the nonshared environment, con -
sistent with FFT. Perhaps it is mere error of measurement. We know
that all personality assessments include some degree of random error,reflected in the reliability (or unreliability) of the test. In addition, how -
ever, personality assessments include systematic error that can lead to
misinterpretations of data (McCrae, Jang, Livesley , Riemann, & Angleit-ner, 2001). Systematic error or bias cannot be distinguished from validvariance when only one source of information is used. Heinz may over-estimate his Conscientiousness, but if he does so consistently , it mayseem that his estimate is correct. It is only by asking his wife and hisfriends, or by observing him ourselves, that we can figure out whetherand by how much he is biased. When Riemann and colleagues (1997)based their estimates of heritability on a combination of self-reports andpeer ratings, they found substantially higher levels of heritability thanare usually reported, ranging from .66 to .79 for the five factors.
Other Biological Influences
These unbiased estimates suggest that genetics, not the environment, isresponsible for most of the variance in adult personality traits—but notall. That does not mean that FFT is wrong, though, because there ismore to Biological Bases than genetics. Finch and Kirkwood (2000)point out that chance plays a large role in human biology . Genes pro -
vide a blueprint, but in the actual course of development—especially
embryonic development—that blueprint is not followed precisely . Mu -
tations, intrauterine hormones (Resnick, Gottesman, & McGue, 1993),
and other factors subtly affect the finished product. That is why “identi -
cal” twins are never quite identical, even in such purely physical attrib -
utes as height or fingerprints. There is no reason to expect that person -
ality traits would be different in this regard.A Five-Factor Theory of Personality 195