are often believed to be exaggerated. Identical twins
are genetically predisposed to a great deal of similari-
ties, and, through a process known as reactive correla-
tion, people around them tend to treat them
similarly, which may help lead the twins to be similar
beyond what their genetic profiles may warrant. The
correlation of .86 between the IQ scores of identical
twins, for example, may be contaminated with this re-
active correlation. Identical twins encounter environ-
mental experiences that are extremely similar to each
other’s, as the environment tends to react similarly to
those who are genetically similar. As a result, for in-
stance, adults and peers may treat identical twins sim-
ilarly, and teachers may also develop similar
expectations about these twins in terms of their emo-
tional, behavioral, and cognitive functions. This simi-
larity in environmental influences and expectations,
therefore, may cause heritability estimates and con-
cordance rates to be exaggerated.
Furthermore, the process of active correlation (or
niche-picking) suggests the possibility that children’s
genetic predispositions cause them to seek particular
environments, causing the differences in hereditary
predispositions to be enhanced by the subsequent
environmental exposure. If a child has the genetic
predisposition to enjoy cognitive challenges, for ex-
ample, that may prompt the child to seek situations,
friends, and activities that suit this particular predis-
position—provided that such choices are offered to
the child. This child, therefore, may start out with a
small genetically prompted inclination to want to use
his or her ‘‘brains,’’ but such a tendency would subse-
quently be magnified through environmental influ-
ences.
Given the varying degrees of genetic similarities
between identical and fraternal twins, these sources of
confusion may theoretically become more consequen-
tial when twins grow up in the same family. This is be-
cause twins reared in the same family are typically
subject to the same resources, parenting philosophy,
living environments, and so on. Their genetic predis-
positions, therefore, are most likely promoted—or in-
hibited—in similar ways. For example, if a pair of
twins share the hereditary predispositions for musi-
cality and their upper-middle-class parents own a
piano and are interested in fostering musicality in
these children, their musical potential will perhaps be
cultivated in very similar ways. Specifically, their par-
ents will probably get the same or similar piano te-
acher(s) for them, and they will probably be
encouraged to practice equally. Therefore, the genet-
ic similarities between the twins are magnified by vir-
tue of them growing up in the same household. How
does one address these concerns? Adoption studies
provide some answers.
Adoption StudiesCompared to traditional twin studies, adoption
studies are theorized to offer better alternatives for
separating hereditary influences from genetic ones.
There are typically two variations in adoption studies:
ones involving comparisons of identical twins reared
apart and ones comparing the degree of similarity be-
tween adopted children and their biological and
adoptive parents. Identical twins reared apart share
genetic patterns with each other, yet they do not share
the same environmental experiences. Adopted chil-
dren, by contrast, typically share with the rest of the
adoptive family similar environmental experiences
but do not share any genes with them. The advantage
of adoption studies is that researchers can reasonably
estimate the heritability by comparing the herit-
ability estimates and concordance rates of pairs of in-
dividuals varying in genetic relatedness and in envi-
ronmental distance. A typical adoption study may
involve, for instance, comparing the concordance
rates for the following two pairs: a child and her bio-
logical parent (shared genes but not environments)
versus the same child and her adoptive parents
(shared environments but not genes). Though the es-
timates of hereditary influences are generally lower in
adoption studies than in twin studies, adoption
studies provide results that are largely consistent with
twin studies. In a 1983 study, Sandra Scarr and Rich-
ard Weinberg found that the IQ scores of adopted
children showed higher correlations with the IQ
scores of their biological parents than with those of
their adopted parents. Similarly, John Loehlin, Lee
Willlerman, and Joseph Horn demonstrated through
a 1988 study that in the area of clinical depression,
adopted children tended to have much higher con-
cordance rates with their biological relatives than with
their adoptive relatives.Still, many scholars argue that heritability may be
overestimated in these studies. First, the reactive and
active correlations discussed earlier would occur, to a
degree, even if the twins were reared separately, as
the twins share all of the hereditary predispositions.
Second, one must also examine the possibility that
parents may systematically treat their adoptive chil-
dren differently than they do their biological chil-
dren, which may explain the less-than-expected
resemblance between children and their adoptive
parents. Given that biologically related individuals
tend to share greater hereditary similarities, it is fair
to state that heritability estimates may be thrown off
by environmental effects induced by particular genet-
ic predispositions.184 HEREDITY VERSUS ENVIRONMENT