1990). All of these studies are remarkably consistent. They suggest that
about half the variance in personality traits scores is attributable togenes and that almost none is attributable to a shared family environ -
ment. The “shared environment” consists of all the influences that chil -
dren share by growing up in the same family , including parental models,
family values, child-rearing practices, local schools, diet, religion, andso on. According to the behavior genetics literature, none of that seemsto matter in shaping the personality traits of adults. That conclusion issupported by studies of fraternal and identical twins, but it is seen per -
haps most clearly in adoption studies. When biologically unrelated chil -
dren are raised in the same family , they show no resemblance whatso -
ever in personality as adults. They may follow the same religion or
speak with the same accent or prefer the same kinds of food, but theyare no more alike in levels of Extraversion or Conscientiousness thantwo people plucked at random from the population.
The nonimportance of the shared environment had become clear to
behavior geneticists by the mid-1980s (Plomin & Daniels, 1987), andmany efforts have been made to understand it (Pike, Manke, Reiss, &Plomin, 2000), so far with limited success. If genes account for half thevariance and the shared environment none of it, what accounts for theother half? Technically , the remainder is called the “nonshared environ-ment,” and some researchers interpret it as all those shaping events thatdistinguish two children from the same family . For example, both Billyand Judy may attend the same local elementary school, but Billy istaught by Ms. Smith and Judy by Ms. Jones. Or perhaps Billy takes pi -
ano lessons while Judy studies guitar. Or Billy sits on the right side of
the dinner table, whereas Judy sits on the left. In principle it is possiblethat such differences might have important influences on personalitydevelopment, but it does not seem very plausible. Why should thechoice of instrument make a difference when it does not seem to matterthat Billy and Judy are both given a musical education?
Judith Rich Harris (1998) created something of a sensation when
she suggested that family influences count for much less than peer in -
fluences. We know that many children of immigrants grow up hearing
their ancestral language at home and English at school, and when theygrow up, they speak English. Perhaps in the same way , children acquirepersonality traits from their peers rather than their parents. Becausechildren from the same family generally have somewhat different peergroups, they may show limited personality resemblance as adults.194 PERSONALITY IN ADULTHOOD