 9. Behavioral Genetics and Personality  227
ences in the actual environment experienced 
by each sibling (due, e.g., to varying ages of 
siblings at the time of parental divorce) as 
well as in siblings’ perspectives on that envi -
ronment (e.g., one sibling blames herself for 
conflict between her parents and becomes 
depressed and isolated, whereas another 
throws himself into activities away from 
home to escape) will be picked up as non -
shared environmental influences. Moreover, 
chance events and variations in specific com -
binations of environmental circumstances 
that act in concert from individual to indi -
vidual all contribute to nonshared rather 
than shared environmental influences. This 
is one reason that perceptions of the envi -
ronment may be more useful constructs for 
personality research than “objective” envi -
ronmental circumstances.
Family Dynamics and Meaning of Environmental 
Components of  Variance
Family members may also act to establish 
individuality by differentiating from each 
other (Ansbacher & Ansbacher, 1956; Fein -
berg & Hetherington, 2000), creating cor -
relations between genetic and nonshared 
environmental influences, to the extent that 
this differentiation process is at least initial -
ly influenced genetically. Schachter, Shore, 
Feldman-  Rotman, Marquis, and Camp -
bell (1976) suggested that this sibling de- 
identification process relieves competitive 
pressures between siblings and is most pro -
nounced for siblings who are more similar in 
age or sex. The small volume of research in 
this area has tended to focus on systematic 
associations between family circumstances 
and extent of differentiation between pairs 
of siblings. For example, Grotevant (1978) 
found that girls with sisters reported fewer 
feminine occupational interests than did 
girls with brothers. Feinberg and Hethering -
ton (2000) examined relations between age 
differences between siblings and similarity 
of sibling outcomes, with the general obser -
vation that siblings who were closer in age 
tended to be less similar. Feinberg, Reiss, 
Neiderhiser, and Hetherington (2005) found 
that shared environmental influences, indi -
cating greater sibling similarity, tended to be 
higher in families in which parents displayed 
greater negativity to their children, with the parental negativity assumed to restrict chil -
dren’s opportunities to express individuality. 
In contrast, shared environmental influences 
tended to be lower, indicating greater sibling 
differentiation, in the presence of higher lev -
els of conflict between parents, perhaps in -
dicating that children sought opportunities 
wherever they could find them to get away 
from unpleasant home environments.
Although these kinds of family dynam -
ics may very well affect degrees of sibling 
similarity and thus estimates of shared en -
vironmental influences, motivation to dif -
ferentiate from siblings may also be related 
to personality. For example, the well-known 
twin researcher David Lykken once men -
tioned that his work with twins had led him 
to wish that he had had an MZ twin himself, 
so that he would have experienced the kind 
of psychological closeness he had observed 
in many pairs. In contrast, others might be 
glad to have been spared what would feel 
like psychological intrusion, growing up 
constantly presented with someone so simi -
lar. Individual differences of this kind should 
“wash out” from many univariate estimates 
of shared environmental influences in sam -
ples representative of the population. This 
will be the case, however, only when this 
response to the physical proximity of psy -
chologically similar others is independent of 
the traits of interest. In addition, a sibling’s 
particular characteristics may inspire admi -
ration and emulation or scorn and desire to 
differentiate. Some evidence, at least for the 
admiration and emulation process, has been 
provided by McGue and Iacono (2001), who 
found that older siblings’ antisocial and sub -
stance use behavior contributed directly to 
similar behavior in younger siblings.
Propositions Based on Major Directions 
in Current Research
Genetic and  Environmental Influences 
on Personality  Interact
We have summarized in previous sections 
the relatively well-  established findings in 
personality genetics. The evidence for strong 
genetic and weak shared environmental 
influences on personality is robust across 
measurement approaches and study designs. 
When environments are measured directly, 