A landmark review by Bouchard and McGue
(1981) summarizes many of these behavioral genetics
studies of intelligence. Table 7-3 presents some of
the data. As can be seen, similarity in intelligence
appears to be a function of the amount of genetic
material shared (monozygotic twins are more similar
in intelligence than dizygotic twins or siblings). It is
notable that this pattern also holds true for biological
relatives reared apart. McGue et al. (1993) conclude:
When taken in aggregate, twin, family, and
adoption studies of IQ provide a demon-
stration of the existence of genetic influ-
ences on IQ as good as can be achieved in
the behavioral sciences with nonexperi-
mental methods. Without positing the
existence of genetic influences, it simply is
not possible to give a credible account for
the consistently greater IQ similarity among
monozygotic (MZ) twins than among like-
sex dizygotic (DZ) twins, the significant IQ
correlations among biological relatives even
when they are reared apart, and the strongassociation between the magnitude of
familial IQ correlation and the degree of
genetic relatedness. (p. 60)
In general, estimates of the percentage of IQ
variance associated with genetic factors range from
30% to 80% (Deary et al., 2010). In general, it
appears that IQ heritability estimates vary as a func-
tion of the age of the sample; these estimates are
maximal in older age groups (Deary et al., 2010;
Hunt, 2011; McGue et al., 1993).
Given the evidence that intelligence scores are
influenced by genetic factors, does this mean that
IQ is not malleable? No. This is the source of much
confusion and controversy. Recall that heritability
estimates are not 100%. This suggests that the envi-
ronment plays some role in the development of
intelligence. As McGue et al. (1993) state, behav-
ioral genetics studies of IQ“strongly implicate the
existence of environmental influences: The correla-
tion among reared-together MZ twins is less than
unity; biological relatives who were reared together
are more similar than biological relatives who wereT A B L E 7-3 Average Familial IQ Correlations (R)
Relationship Weighted Average Correlation Number of Pairs
Reared-together biological relativesMZ twins 0.86 4,672
DZ twins 0.60 5,546
Siblings 0.47 26,473
Parent/offspring 0.42 8,433
Half-siblings 0.31 200
Cousins 0.15 1,176
Reared-apart biological relativesMZ twins 0.72 65
Siblings 0.24 203
Parent/offspring 0.22 814
Reared-together nonbiological relatives
Siblings 0.32 714
Adoptive parent/offspring 0.19 1,397
NOTE: MZ monozygotic; DZ dizygotic. Weighted average correlation was determined using sample-size-weighted average ofztransformations.
SOURCE: Adapted with permission from“Familiar Studies of Intelligence: A Review”by T. J. Bouchard, Jr., and M. McGue, 1981,ScienceVol. 212, p. 1056.
Copyright © 1981 by the American Association for the Advancement of Science.
THE ASSESSMENT OF INTELLIGENCE 203