 9. Behavioral Genetics and Personality  219
environmental variation. Measurement im -
precision (e.g., random and systematic error) 
also gets categorized with nonshared envi -
ronmental effects, since such imprecision 
acts to make every individual appear unique 
(e.g., a person responds randomly or exhibits 
socially desirable responding when complet -
ing a questionnaire). Like genetic influences, 
both shared and nonshared environmental 
effects are variance concepts, and they are 
commonly discussed as proportions of total 
variance. For example, akin to the heritabil -
ity, the extent of shared environmental con -
tributions can be conceptualized as the ratio 
of shared environmental variation to total 
variation in the trait, or the proportion of 
total trait variation linked to shared envi -
ronmental variation.
By far the majority of studies decompos -
ing population variance in personality have 
been conducted using samples of twins, on 
the presumption that except for the manner 
in which they were conceived and gestated, 
they are just “ordinary folks.” Though there 
are areas in which this is not entirely accurate 
(e.g., twin siblings may share a unique social 
bond; Tancredy & Fraley, 2006), adult per -
sonality does not appear to be one of them 
(Johnson, Krueger, Bouchard, & McGue, 
2002); perhaps relatedly, this study found 
that twins expressed slightly greater social 
closeness as a general personality trait than 
did singletons. In many countries, it is there -
fore feasible to study twin samples that are 
unusually representative of the general popu -
lation, particularly when twins are recruited 
using population records of twin births.
Twins are valuable research participants, 
because they can vary in genetic similarity. 
Monozygotic (MZ) twins share effectively 
all their genetic material, whereas dizygotic 
(DZ) twins share on average 50%, as is the 
case with full biological singleton siblings. 
Other family constellations, particularly 
adoptive and biological family sets, can also 
be used to develop variance decompositions. 
Studies using such samples enrich the litera -
ture, involving complementary design fea -
tures (e.g., we describe the contributions of 
adoption studies below) that offer different 
perspectives, but twin studies have clearly 
dominated.
The Big Five dimensions (John, Chapter 
2, this volume) have been major targets of research on the heritability of personality. 
After reviewing the literature, Bouchard 
and Loehlin (2001) concluded that, in adult 
samples, genetic influences accounted for 
almost half the variation in each of the Big 
Five domains. Nonshared environmental in -
fluences accounted for most of the remain -
ing variance, and shared environmental 
influences accounted for essentially no vari -
ation. This pattern of findings is well repli -
cated. It holds across sexes, with heritabili -
ties similar for men and women (Bouchard 
& Loehlin, 2001) when models of person -
ality structure other than the Big Five are 
considered, and no matter which behavioral 
trait is examined (e.g., Polderman et al., 
2015). Indeed, Turkheimer (2000) went so 
far as to describe this pattern as essentially 
applicable to all known human individual 
differences and enshrined it as a series of 
laws.
There are several remarkable aspects of 
the law-like pattern of genetic and environ -
mental influences typically observed in the 
literature. First, it is striking that each of the 
Big Five domains is similarly heritable, espe -
cially considering the breadth of human in -
dividual differences encompassed by the Big 
Five—from more temperamental features 
such as activity level and emotional tone, 
encompassed by domains such as extraver -
sion and neuroticism, to more attitudinal 
features such as interests in art and litera -
ture, encompassed by the domain of open -
ness. Across this breadth of human experi -
ence, genes play major roles in explaining 
why people differ.
Second, genetic influences on personality 
are not small; accounting for even 30% of 
the variance in any psychological variable is 
unusual. Third, the relatively small propor -
tion of variance attributable to shared en -
vironmental influences has been surprising 
(but see Matteson, McGue, & Iacono, 2013, 
for evidence that some personality traits may 
contain shared environmental variance that 
is more readily identified by combining twin 
and adoption study evidence). This observa -
tion has been a source of much generative 
controversy (Rowe, 1994), as many theories 
of personality development rest on the prem -
ise that family experiences are formative. 
Yet when one reflects on what the shared 
environmental component of variance rep -