In summary, researchers have used a variety of clever procedures to
separate the influence of genetics on personality from the influence of the
environment. Although the precise extent of the genetic influence is still un-
certain, the consistency of the findings from so many sources suggests that
adult personalities clearly are affected by heredity. However, the book is
far from closed on this issue. As discussed in the next section, there are rea-
sons to question some of the conclusions behavior geneticists draw from
their data.
Problems with Genetics Research
The strongest and most consistent evidence in favor of genetic influence onpersonality comes from twin-study research. However, researchers using thismethod make two key assumptions. The first is that twin pairs can be accu-
rately identified as MZ or DZ twins. Many “identical ”twins may in fact be DZ
twins who look very much alike. Fortunately, biological advances have made
this less of a problem than it once was. Today, zygosity can be determined in
almost all cases through blood tests.
The second assumption presents a bigger problem. Researchers assume
that MZ and DZ twins have equally similar environments. However, there isevidence that MZ twins may share more of their environment than DZ twins
(Hoffman, 1991; Lytton, 1977; Scarr & Carter-Saltzman, 1979). That is,identical twins may be treated more alike than are DZ twins. Identical twins
are often thought of as one unit —they are dressed alike, given identical pre-
sents, and so on. DZ twins grow up in similar environments, but they are usu-
ally allowed to dress differently, join different clubs, and have different friends.
DZ twins may even experience environments that are less similar than those
typical for siblings (Hoffman, 1985) because parents may look for and em-phasize their differences (for example, “Terry is the studious one ”;“Larry is
the troublemaker ”).
If this is the case, we would have to modify Figure 10.1. The environmen-
tal influence on personality traits may not be as similar for DZ twins as it is
for MZ twins. This possibility creates a problem when interpreting the twin-
study findings. We can ’t be certain if the higher correlations between MZ
twins are caused by greater genetic similarities or greater environmental simi-
larities. This interpretation problem may explain why data from twin-study
research suggest a larger role for genetic influences than is found with other
procedures.
However, some of these other procedures also rely on questionable as-
sumptions (Hoffman, 1985, 1991; Stoolmiller, 1999). Adoptions are not ran-dom events. Families who adopt children are typically older, more affluent,
more stable, and without many of the problems found in families that do not
adopt. Although separated twins may be placed in different homes, the homestypically selected for placement are very similar. Perhaps more misleading is
the assumption that parents treat an adopted child the same way they do their
biological offspring. It is likely parents have different expectations for adoptedchildren. Because they don’ t know the biological parents, adopting parents
may have few preconceived ideas about how the child ’s personality will unfold.Heritability of Personality Traits 257