218 II. BIOLOGICAL  FOuNDATIONS
2005), and we hope to convey some of these 
important advances, as well as the challeng -
es that remain outstanding.
We have organized this chapter into three 
sections. In the first section, we describe 
some postulates based on well-  established 
research findings. In the second section, we 
describe current directions in research as a 
set of propositions, focused on understand -
ing and modeling the ways in which genetic 
and environmental forces correlate and in -
teract. In the third section, we move beyond 
what we currently know and delineate a set 
of questions for future research. In the pro -
cess, we hope the reader will come to share 
our enthusiasm for genetically informed 
personality research, and share our desire 
to relegate the nature–  nurture debate to the 
historical backdrop for the development of a 
scientifically compelling account of how ge -
nomes and environmental inputs combine to 
create individual personalities.
Postulates Based on Well -Established 
Research  Findings
Personality Results from  Both Genetic 
and Environmental  Influences
Evidence from  Twins
Although Postulate I seems self-  evident (i.e., 
how could personality come about, except 
through transactions between the blueprint 
for the construction of the organism [DNA] 
and the world outside the organism?), es -
tablishing this proposition was scientifically 
critical, because it countered the radical en -
vironmentalist view that “there is no such 
thing as an inheritance of capacity, talent, 
temperament, mental constitution, and char -
acteristics. These things again depend on 
training that goes on mainly in the cradle” 
(J. B. Watson, 1924, p. 94). Watson’s view is 
incompatible with the results of behavioral 
genetics studies of self-  reported personality 
variables, most of which were conducted in 
the decades since Watson was writing. Be -
havioral genetics studies parse individual 
differences (population variance) in traits 
into at least three distinct components: (1) 
genetic influences, (2) shared environmental 
influences, and (3) nonshared environmen -
tal influences. Genetic influences  index the extent to which observed or “phenotypic” 
variation in a trait arises from genetic differ -
ences among people. The well-known heri -
tability statistic is the ratio of variance from 
genetic sources to total variation in the trait, 
or the proportion of total trait variation 
linked to genetic variation. Importantly, it is 
a statistic indexing variance—  the extent of 
differences among persons within a group. 
As a concept, it only applies at the level of a 
specific group of persons (a sample or popu -
lation of persons, in statistical terms). When 
we speak of, for example, 50% of variance 
in extraversion being due to genetic influ -
ences, we are referring to differences among 
individuals . We do not  mean that 50% of 
any individual’s level of extraversion can be 
attributed to genetic influences.
Shared environmental influences  index 
the extent to which people are similar, in -
dependent of genetic influences, because 
they grew up in the same household, thereby 
sharing factors such as family-  level socio -
economic status and religious traditions that 
differ among families. These factors can—
but do not always—  make people growing 
up in the same family similar to each other. 
For example, siblings often experience the 
same religious upbringing, and many con -
tinue that religious observance in adulthood, 
though considerable numbers do so to rather 
different degrees.
Nonshared environmental influences  
index the extent to which family members 
are different, in spite of sharing genetic 
material and growing up together. Com -
monly used examples include having dif -
ferent teachers and friends, participating 
in different leisure activities such as sports, 
and receiving different parental treatment. 
For example, when one child displays more 
frequent temper tantrums than another, 
parents may convey sensitively but firmly 
to that particular child that such behavior 
is unacceptable. This differential parenting 
may make the child less similar to the other 
over the long term. Thus, the distinction be -
tween shared and nonshared experiences is 
subtle. People within a family commonly ex -
perience the same putatively objective event 
(e.g., a household move) differently, so that 
it does not make them similar. If the event 
acts to make persons within the same family 
different, then it will show up as nonshared 