226 II. BIOLOGICAL  FOuNDATIONS
contribute little to socialization of their off -
spring beyond the genes they pass along, be -
cause (1) most socialization takes place in the 
context of peer, not family, interactions and 
(2) learning is context-  dependent, so what is 
learned at home does not translate to those 
all- important situations outside the home. 
This is consistent with the observations that 
many variables considered “environmental” 
show genetic influences, and the interpreta -
tion that this occurs because genetic influ -
ences shape both perception and generation 
of situational circumstances.
Distinguishing between  Objective and  Effective 
Measurement of  Environments
To understand how specific environmen -
tal circumstances contribute to personality 
differences, it is important to distinguish 
between objective and effective measure -
ment of environments (Goldsmith, 1993; 
Turkheimer & Waldron, 2000). “Objective” 
measurement of environments refers to con -
strual of environmental circumstances as 
they appear to the researcher, regardless of 
the construal of these circumstances by the 
people who experience them. Many of the 
most commonly used variables that distin -
guish among families, such as parental so -
cioeconomic status and relationship quality, 
or residence neighborhood and school, are 
measured objectively this way. Environmen -
tal circumstances are objectively nonshared 
when each sibling within a family has a 
unique measured value for the circumstance, 
again, regardless of whether the circum -
stance acts to make the siblings similar or 
different. Peer relationships and classroom 
assignments are common examples of non -
shared environmental circumstances usually 
measured objectively. To emphasize, shared 
and nonshared characterizations of objec -
tively measured environments refer to their 
definitions of measurement, not to the kinds 
of influences the environments may have.
“Effective” measurement of environments 
refers to the natures of the influences of those 
environments on the people who experience 
them, regardless of how they were measured. 
Environmental circumstances are effectively 
shared when they act to make family mem -
bers more similar, and nonshared when they 
act to make family members different. The estimates of shared environmental variance 
resulting from behavioral genetics analyses 
refer, without actually specifying them, only 
to environmental circumstances that are ef -
fectively shared. This means that objectively 
shared environmental circumstances, such 
as socioeconomic status, that are so con -
sistently associated with outcomes such as 
health and intelligence actually contribute to 
shared environmental variance only if they 
act to make family members more similar 
than they would be based on their shared 
genetic endowments.
Consider the example of parental educa -
tion, often measured using the midparent 
average or the mother’s education. Two sib -
lings in an intact family may both grow up 
with, say, one parent who attended but did 
not graduate from college and another who 
did graduate from college. Measured objec -
tively in this way, the siblings’ parental edu -
cational environment is shared, and both ex -
perience the influences of growing up with 
parents with these levels of education. To the 
extent that matters uniformly for outcomes 
in question, their parents’ education is act -
ing to make them more similar to each other 
than to siblings with other patterns of pa -
rental education. But one sibling may share 
more interests with the parent who gradu -
ated from college than the other sibling, 
and may therefore be more influenced by a 
higher level of parental education. If this is 
the case, then, measured effectively, at least 
some of the influence of parental education 
is nonshared in this family. An estimate of 
variance due to shared environmental influ -
ence will pick up only the portion of influ -
ence of parental education acting to make 
the siblings more similar to each other than 
they are to siblings with other patterns of 
parental education.
Moreover, to the extent that social forces 
and sample recruitment procedures result in 
little variance in the environmental measure, 
ability to pick up greater similarities within 
families than between families is reduced 
(Stoolmiller, 1999). For example, if socio -
economic status matters and its range is 
restricted in a sample, then all the families 
in that sample will be relatively similar, and 
within-  family similarities can be observed 
only within the context of small between- 
family variation. In addition, subtle differ -