1. family and parenting
2. children

IN REVIEW
Why Are Children in the Same Family So Different? Nonshared
Environment a Decade Later
Robert Plomin, PhD', Kathryn Asbury, MA, PG Dip2, Judith Dunn,
Objective: To review recent developments in the study ofnonshared environment; that is, the environmental influences that make
children growing up in the same family different, rather than similar.
Method: We review several recent influential books andpapers on the subject of nonshared environmentfrom the decadefoUowing the 1987 paper that highlighted its importance in psychological development.
Results: Modest progress has been made toward identifying the specific aspects ofthe environment responsiblefor nonshared environment. A Ithough parents treat their multiple children differently, such differential treatment accountsfor only a small amount
of nonshared environmental influence, once geneticfactors are controlled. It has been suggested that some degree of nonshared
environment may be due to the fact that siblings react differently to ostensibly shared environmental influences. Peer influence
and other experiences outside the family may be more important sources of systematic nonshared environment.
Conclusions: Despite the difficulties encountered in identifying specific sources of nonshared environment, thefact remains that
most environmental variance affecting the development of psychological dimensions and psychiatric disorders is not shared by
children growing up in the same family. More research and theory are needed to explain why such siblings are so different.
Chance, in the sense of idiosyncratic experiences, also needs to be considered.
(Can J Psychiatry 2001;46:225-233)
Key Words: nonshared environment, families, development, behavioural genetics, twins, adoption
O
ne of the most important discoveries arising from
behavioural-genetic research concerns nurture rather
than nature. Behavioural-genetic research provides the best
available evidence for the importance of environmental influences, but it shows that the environment works in a surprising
way. Although socialization theories assume that the environment of the family unit influences psychological development, with the result that shared experiences lead to
similarities among siblings, behavioural-genetics research
consistently indicates that children growing up in the same
family do not share effective environmental influences. It is
not shared experiences, but shared genetics, that make siblings resemble one another. Indeed, with regard to psychological development, environment makes siblings no more
similar to one another than to children picked at random from
the general population. This phenomenon—the fact that effective environments are not shared—has been termed "nonshared environment,"
Our goal is to review recent developments in this field and
point to new directions for research. We begin with a brief
overview of the phenomenon of nonshared environment.
Manuscript received and accepted December 2000.
'Research Professor, Social, Genetic and Developmental Psychiatty Research Centre, Institute of Psychiatry, King's College London, London,
UK.
^Research Assistant, Social, Genetic and Developmental Psyehiatry Research Centre, Institute of Psychiatry, King's College London, London,
UK,
•'Research Professor, Social, Genetic and Developmental Psychiatry Research Centre, Institute of Psyehiatry, King's College London, London,
UK,
Addressfor correspondence: Dr R Plomin, Research Professor, Social, Genetie and Developmental Psychiatry Research Centre, Institute of Psyehiatry, King's College London, DeCrespigny Park, Denmark Hill, London
SE5 8AF, UK,
e-mail: r,[email protected],ac,uk
Can J Psychiatry, Voi 46, April 2001
The Phenomenon
Nonshared environment lay hidden within behaviouralgenetic studies since such studies began 80 years ago. Twin
studies and adoption studies were designed to disentangle the
elements of nature and nurture in familial resemblance. For
example, schizophrenia runs in families, and environmentalists had assumed that it does so for environmental reasons:
family members share the same schizophrenogenic environment. Twin studies and adoption studies test this assumption
by studying family members who share nature or nurture to
varying degrees. Although most relevant evidence has come
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The Canadian Journal of Psychiatry
from twin studies, it is easiest to see the logic of behaviouralgenetic designs by using adoption studies. Studying genetically related individuals reared apart in uncorrelated (although not necessarily different) environments directly
estimates the extent to which familial resemblance is mediated genetically. These are the sort of data that convinced psychiatrists in the late 1960s of the importance of genetic
influence. For example, Heston's classic adoption study
showed that children adopted away at birth from biological
parents with schizophrenia were nonetheless at risk for
schizophrenia, suggesting genetic influence on parent-offspring resemblance for schizophrenia (1), Indeed these children, adopted away at birth, were just as likely to suffer from
schizophrenia as are children reared by parents with schizophrenia, implying that growing up in a schizophrenogenic environment does not add to the genetic risk for schizophrenia,
A more direct test of the nurture assumption, however, lies in
the other prong of the 2-pronged adoption design—studying
genetically unrelated individuals reared together in the same
adoptive family, which directly estimates the extent to which
familial resemblance is mediated by shared environment. For
schizophrenia, there is no risk from growing up in a family
with a parent or sibling who suffers from the disorder (2),
Taken together, these adoption data imply that familial resemblance for schizophrenia is mediated genetically, not environmentally. The environment is important, however. For
example, identical twins are only 50% concordant for schizophrenia—not 100% concordant, which would be the case if
schizophrenia were entirely due to genetic factors. Because
they are genetically identical, identical twins growing up in
the same family environment can only be 50% discordant for
schizophrenia for environmental reasons. Such environmental influences have been called nonshared because they
are not shared by children growing up in the same family. The
distinction between shared and nonshared has many other labels, such as "common" versus "unique," "between-family"
versus "within-family," and "E2" versus "Ei," The environment is important, but it does not contribute to familial resemblance. It should be emphasized that such studies only
address the contributions to resemblance among family members: their goal is to disentangle nature and nurture in familial
resemblance. There may be more to schizophrenogenic family environment than is indexed by schizophrenia in the rearing parents. What we know from research on nonshared
environment, however, is that the effect of such measures of
the family environment is to make children growing up in the
same family different from one another.
The importance of nonshared environment for personality, as
well as for psychopathology, was obvious from behaviouralgenetic data since such research began a century ago, but it
was the hidden background in a figure-background illusion
in which the figure was genetics. Only in 1976 was this illusion reversed to focus on the background. In a classic book on
Voi 46, No 3
a large twin study, Loehlin and Nichols reported the typical
finding of genetic infiuence for personality, but in their conclusions they noted
,,. a consistetit—though perplexing—pattern is emerging from
the data (and it is not purely idiosyncratic to our stud)'). Environment carries substantial weight in deterrnining personality— it appears to account for at least half the variance— but
that environment is one for which twin pairs are correlated close
to zero. , , , In short, in the personality domain we seem to see
environmental effects that operate almost randomly with respect to the sorts of variables that psychologists (and other people) have traditionally deemed important in personality
development (3, p 92),
This theme was highlighted and expanded in a 1981 paper (4),
The evidence for the importance of nonshared environment in
the development of psychopathology, personality and cognitive abilities was brought together in a Behavioural and Brain
Sciences (BBS) target article called "Why are children in the
same family so different from one another?" (5), which was
published with 32 commentaries and a response to the commentaries (6), Even though they were addressed in the target
BBS article and response to the commentaries, the following
issues resurfaced during the next decade: 1) nonshared environment needs to be distinguished from error of measurement
(yes); 2) genotype-environment interaction and correlation
could account for nonshared environment (no, these cannot
explain why identical twins are different); 3) shared environment may have more effect in extreme situations, such as
those found in abusive families (yes); 4) perceptions of environment may be an important source of nonshared experience
(yes); and 5) nonshared environment may involve chance, in
the sense of idiosyncratic experiences, including prenatal
events (yes). None of the responses then or since have challenged the basic finding that nonshared environment is important. The message of the BBS paper was upbeat: there is a
new way, and an empirical tool to study the environment.
Namely, it is to study more than one child per family to find
out why they are so different. Three steps were identified for
this research program: 1) document differential experiences,
which requires the construction of measures of the environment that are specific to each child in the family; 2) document
the association between such differential experiences and differential outcomes; and 3) investigate the extent to which associations between differential experiences and differential
outcomes are causal.
The 1987 paper also included a table listing possible sources
of nonshared environment (see Table 1), These are not limited to the psychosocial environments that psychologists typically mean when they use the word "environment,"
Behavioural genetics defines "genetic" very narrowly to
mean inheritance, which is what is typically meant when we
say, for example, that eye colour is genetic. In contrast, environment is defined extremely broadly to include anything that
is not genetic in this narrow sense. For example, many DNA
events (such as chromosomal anomalies or somatic mutations
April 2001
Why Are Children in the Same Family So Different?
that are not transmitted to offspring) are not genetic in that
they are not inherited. Thus, this broad definition of environmental as nongenetic includes all noninherited intra- and interorganismic infiuences. While we are discussing basic
issues, we should also reiterate the point that genetic research
describes what is rather than predicting what could be. For example, high heritability for height means that height differences among individuals are largely due to genetic
differences, given the genetic and environmental infiuences
that exist in a particular population at a particular time (what
is). Even a highly heritable trait like height can be affected by
an environmental intervention, such as improving children's
diet or preventing illness (what could be). Such environmental factors are thought to be responsible for the average
increase in height across generations, even though individual
differences in height are highly heritable in each generation.
During the past decade, researchers have often emphasized a
single source of nonshared environment, such as family composition (7), sibling interactions (8), peer infiuence (9), and
nonsystematic factors (10), However, the list shown in Table 1 has remained unchanged, not because of the researchers' prescience, but simply because it includes all possible
types of nongenetic infiuence.
In one sense, thinking about environmental influences that create differences between children in the same family represents a
dramatic reconceptualization of psychological environments.
On the other hand, this reconceptualization need not involve
mysterious elements in the environment: Any environmental
factor can be viewed in terms of its contribution to nonshared
envirotimental variance. For example, parental affection can be
easily construed as a source of differences among children in the
same family, because parents may be more affectionate toward
one child than another (5).
What was new was the focus on measures of experience specific to each child, including subjective perceptions of experience. As discussed later, the same event (for example,
parental divorce) may be experienced differently by 2 children in the same family, and we can assess the children's differential experience of a shared event. In trying to understand
why siblings are so different, however, it is reasonable first to
investigate experiences that differ within families.
In 1990, the BBS paper spawned a nonacademic book called
Separate Lives: Why Siblings Are So Different, which reinterpreted sibling socialization research along the lines of nonshared environment. It generally showed that siblings
growing up in the same families experience environments
that differ from those experienced by their parents and their
cosiblings as much as the environments experienced among
siblings growing up in different families (11), A more detailed review of the methods and evidence for nonshared environment for the development of psychopathology,
personality, and cognitive abilities was published in 1994
(12), Several nonshared environment studies of siblings were
launched in the late 1980s and early 1990s; the early ones
227
were described in 1994, in an edited book called Separate Social Worlds of Siblings: Impact of Nonshared Environment
on Development (]2). A recent metaanalysis, discussed later,
identified 43 papers that have addressed the second stage of
the nonshared environmental program of research—identifying associations between nonshared experiences and siblings' differential outcomes (10), The largest study is the
Nonshared Environment in Adolescent Development
(NEAD) project which sprang directly from discussions
based on the BBS paper, NEAD was a decade-long study that
involved a collaboration between a psychodynamic family
psychiatrist, a child psychologist specialising in families, and
a behavioural geneticist. The NEAD project aimed to address
all 3 steps in the program of research listed above, focusing on
measures of the family environment and their effect on adolescent psychopathology in a genetically sensitive design. Its
results are described below.
The Reaction
Despite the revolutionary implications of realizing the importance of nonshared environment, there was until recently little response from the developmental psychology community,
perhaps because the responses to the 1987 BBS paper were so
thorough. Nonetheless, an early-warning shot was fired in a
1991 paper in Psychological Bulletin (7), The critique began
with a general broadside criticizing behavioural-genetic
methodology, only some of which was relevant to the issue of
nonshared environment. The paper argued that, although
some behaviours studied (such as IQ) showed resemblance
for adoptive siblings, which suggests shared environmental
infiuence, other behaviours (such as self-concept) which
would show shared environmental infiuence, had not yet
been studied. Although we agree that behavioural-genetic
methods are only quasi-experimental designs, the convergence of results from the different designs of the twin study
and adoption study that point to the importance of nonshared
environment is impressive (14). As indicated above, the easiest way to see the importance of nonshared environment—one that sidesteps most of the complexities—lies in
observing the considerable differences seen within pairs of
identical twins reared together (even though studies of identical twins probably underestimate nonshared environment because there are special twin effects that contribute to their
similarity). Concerning the hypothesis that IQ and selfconcept show shared environmental infiuence, the general
rule that effective environments are nonshared does not depend on showing that there are no exceptions to it. Nonetheless, one of the most interesting results in this area involves
IQ: although younger adoptive siblings resemble each other,
studies of postadolescent adoptive siblings consistently show
no resemblance, suggesting that, in the long run, the environment operates as nonshared environment, even for IQ
(15,16), This finding suggests that in childhood we will find
shared environmental infiuences that relate to cognitive
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The Canadian Journal of Psychiatry
development but that what we need to look for in childhood
are nonshared environmental influences that predict cognitive abilities in the long run (17). Conceming the issue of
self-concept, the first behavioural-genetic study of selfconcept showed no evidence of shared environment (18), a
finding confirmed in subsequent studies (19,20). If the critique had considered psychopathology, however, it could
have pointed to at least 1 exception to the rule that environmental influences are nonshared: juvenile delinquency shows
some shared environmental influence (14,21), although even
this evidence for shared environment may be artificial in that
twins may be partners in crime (22).
Despite its general attack on behavioural genetics, the critique in the end agrees that environmental influences for personality are nonshared:
There is basic consensus across behavioural genetics and developmental psycholog)' that both genetics and environment play a
role in personality developtnent and that environmental influences do not result in sibling similarity (7, p 190).
The author suggests that developmental psychologists already knew that the way in which the environment worked
was nonshared:
The behavioural geneticists assume that such variables, if operating, would make siblings alike in personality. Environmentally
oriented developmental psychologists, on the other hand, hold
that these influences operate differenfly for different children,
and thus the absence of sibling similarity' does not mean these
variables are not having an effect (7, p 190).
If developmentalists had known about the importance of nonshared environment, why is it that far less than 1% of all developmental studies of socialization studied more than a
single child per family—the only way to investigate nonshared environment?
A presidential address for the Society for Research in Child
Development (23) and a double-barrelled blast of popular
books by David Rowe (24) and Judith Harris (9) elicited a
decade-delayed reaction from developmental psychologists
(8,25,26). These publications provocatively spelled out the
implications of nonshared environment for theories of socialization; they also tackled larger issues about the nature of
nurture. The title of the Harris book is The Nurture Assumption, and one of the subtitles on the dust jacket is: "Parents
matter less than you think and peers matter more." This book
attracted much media attention and was the subject of a cover
story in Newsweek, entitled "Who needs parents?" (27). It
also received glowing publicity in a lengthy and influential
review ("Do parents matter?") that was published in The New
Yorker (28). (For the extraordinary media coverage of The
Nurture Assumption, see http://home.att.net/~xchar/tna/.)
The negative reaction to these publications is largely directed
toward nature-and-nurture issues other than nonshared environment per se. The Harris book, for example, is criticized for
arguing that peers are more important than parents (8), an
Vol 46, No 3
argument to which Harris has responded (29). Fueling this
controversy is the mistaken interpretation that, if parenting
does not shape children's personality, parents do not matter;
however, what makes siblings similar or different is a much
narrower issue (30).
One issue specific to nonshared environment that continues
to be raised is that estimates of nonshared environment are reduced when measurement error is taken into account (26,31).
The need to separate nonshared environment from measurement error was emphasized in the 1987 BBS paper, and the
point is this: environmental variance (whether a proportion of
total variance or variance corrected for measurement error) is
not shared—the effective environments are nonshared. Another criticism is that adoptive families have restricted variance and thus underestimate the effect of shared environment
on IQ (32). However, even though most families studied by
developmentalists do not fully represent the population, the
intemal logic of the adoption method is unaffected when
adoptive families and comparison nonadoptive families have
similar variances, as in the Colorado Adoption Project (33).
This is also the case for comparisons between identical and
fratemal twins. Moreover, the hypothesis of restricted variance ignores crucial adoption data showing that a correlation
of 0.26 was found for IQ for more than 200 pairs of adoptive
siblings at the average age of 8 years, but 10 years later these
same siblings showed a correlation near 0.0 (34). Because
shared environment is found for IQ in childhood but not after
adolescence in the same families, restriction of range cannot
be an important factor in finding that the long-term influence
of shared environment for IQ is negligible. A more useful
suggestion is that more shared environment may be found in
high-risk families, which implies that nonshared environment might be more likely to be found in low-risk families
(35,36). Other useful points are that nonshared environment
might involve perinatal factors, especially for psychopathology (37) and sibling social-comparison processes, such as
sibling rivalry and sibling de-identification (38,39). An evolutionary perspective on sibling competition for parental investment suggests that sibling interactions in relation to
parental attention might be a useful place to look for nonshared environment (40,41).
A more general criticism concems the need to assess the environment in behavioural-genetic research:
Behaviour-genetic analyses, however, can establish that nonshared environment contributes to individual differences in a
domain but cannot document the connections between objectively measured nonshared environmental events and development (25, p 221)
To the contrary, as discussed in the BBS paper, it is a major
strength of quantitative genetics that its twin and adoption
study methods can describe the net effect of genetic and environmental influences without attempting the impossible task
of assessing specific genes and specific environments. The
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Why Are Children in the Same Family So Different?
next Step in the environmental program of research is to identify specific aspects of the environment responsible for nonshared environment, just as the next step in the genetic
program of research is to identify specific genes responsible
for heritability. As difficult as it is to identify specific genes
operating in multiple-gene systems (42), it may be even more
difficult to identify specific environmental infiuences responsible for nonshared environment. With genes, we at least
know the unit of transmission and how transmission occurs,
but for the environment, especially nonshared environment,
we know neither,
A constructive suggestion emerging from the past decade's
literature is that nonshared environmental effects can be
found in children's differential responses to ostensibly shared
events (26), That is, as they interact with children s characteristics, ostensibly shared events can result in nonshared environmental effects. This distinction can be seen most clearly
for variables that are usually measured on a family-wide basis, such as parental illness, education, poverty, unemployment, or neighbourhood. Depending on children's
characteristics, such as age, sex, and personality, the effect of
such factors could differ for children in the same family. An
example that has been used is research showing that the effect
of paternal unemployment during the Great Depression depended on the age and sex of the child: a shared experience
could have nonshared effects (43), Although it seems quite
likely that such interactions could be a source of nonshared
experience, it should be noted that the unemployment example is drawn from an analysis between families, rather than
within families. To test whether the association occurs within
families, and thus accounts for sibling differences in developmental outcomes, it would be necessary to show that the effects of paternal unemployment on sibling differential
outcomes are moderated by sibling age or sex differences.
Empirical research showing the importance of such interactions in explaining nonshared environment is needed. In trying to account for the differences among siblings in a family,
however, it seems reasonable to begin by looking for main effects of experiences that differ for siblings (for example, differential parental treatment), rather than looking for
interactions using experiences that do not differ for siblings.
Also, a better strategy might be to investigate main effects of
siblings' different perceptions of such shared experiences.
The hypothesis that shared events can have nonshared effects
sometimes leads to the conclusion that "there is nothing about
the findings of these traditional studies that is invalidated by
their having studied only one child" (26, p, 16), To the extent
that one is interested in finding environmental infiuences that
affect development, this conclusion is unwarranted, because
we now know that the effective environments are nonshared.
Traditional between-family associations between parenting
and children's outcomes can only explain nonshared environment to the extent that they can also be shown to operate
within families, to make siblings different. Other nonshared
229
Table 1. Categories of environmental influences that cause children
in the same family to differ (adapted from Plomin and Daniels, |5|)
Categories
Examples
Error of measurement
Test-retest reliability
Nonshared environment
Nonsystematie
Aeeidents, differential prenatal effeets,
illness, trauma
Systematie
Family eomposition
Birtb order, sex differenees
Sibling interaetion
Differential treatment or perceptions
Parent-ehild relations
Differential treatment or perceptions
Extrafamilial
Differential experienees with peers, friends,
teachers, sports, otber activities and
interests, education, occupations, spouses,
family life
environmental explanations of associations between parenting and child outcome arising from between-family studies
include the possibility that parenting is an effect, rather than a
cause, of differences in children's outcomes or that the associations are due to a third variable, genetics, as discussed below. The goal of nonshared environmental research is to
explain that siblings growing up in the same family are so different because this is how the environment works.
New Research
A consensus exists that children growing up in the same family experience different environments. This is the first step in
the 3-step program of research on nonshared environment.
Concerning the second step—relating differential experience
of siblings to differential outcomes—we predict that all the
factors listed in Table 1 make some contribution to nonshared
environment, A metaanalysis of 43 papers addressing this
second step concluded that "measured nonshared environmental variables do not account for a substantial portion of
the nonshared variability" (10, p 78), Looking at the same
studies, however, an optimist could conclude that this research is off to a good start. The proportion of total variance
accounted for in adjustment, personality, and cognitive outcomes was 0.01 for family constellation, 0,02 for differential
parental behaviour, 0,02 for differential sibling interaction,
and 0,05 for differential peer or teacher interaction. Moreover, these effects are largely independent, because aggregate
measures of differential environment account for 13% of the
total variance. These studies have not yet, however, taken the
third step in the research program: the attempt to disentangle
cause and effect. Earlier, we noted that it may be more difficult to identify specific environmental infiuences responsible
for nonshared environment than it is to identify specific genes
responsible for heritability. Nonetheless, one could argue that
at least as much progress has been made in identifying nonshared environment as has been made in identifying genes
(14),
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The Canadian Journal of Psychiatry
In this section, we focus on the results of the decade-long
NEAD project because it represents the leading edge of research on nonshared environment in attempting to address all
3 steps in the research program (44). During 2 visits of 2 hours
each, made at 3-year intervals to 720 families with 2 same-sex
sibling offspring ranging in age from 10 to 18 years, extensive
questionnaire and interview measures of the family environment were administered to both parents and offspring, and
parent-child interactions were videotaped during a session in
which problems in family relationships were discussed. Multiple measures across multiple sources of information were
combined to create highly reliable composite measures. The
first step was to identify differential experiences of siblings.
Sibling correlations for children's reports of their family interactions (for example, reports of their parents' negativity)
were moderate, as they were for observational ratings of
child-to-parent, and parent-to-ehild, interactions. Because
these measures were highly reliable, this finding suggests that
such experiences are largely nonshared. In contrast, parent reports yielded high sibling correlations (as, for example, when
parents reported on their own negativity toward each of the
children). Although this may be due to a rater effect in that the
parent rates both children, the high sibling correlations for
parent reports of children's environments indicate that parent
reports are not good candidates for investigating nonshared
environmental factors.
Once child-specific nonshared experiences are identified, the
second step is to ask whether these nonshared experiences relate to psychological outcomes. For example, to what extent
do differences in parental treatment account for the nonshared environmental variance known to be important for
personality and psyehopathology? Although research in this
area has only just begun, some success has been achieved in
predicting differences in adjustment from differences in sibling experiences (13). The NEAD project provides several
examples using different models and methods to investigate
associations between siblings' differential experiences and
differential outcomes. (A useful, simple, and general methodology has been described by Rodgers and others [45]; see also
Turkheimer and Waldron [10]). For instance, negative parental behaviour directed specifically to one adolescent sibling
(controlling for parental treatment of the other sibling) relates
strongly to that child's antisocial behaviour and, to a lesser
extent, to that child's depression (44). Most of these associations involve negative aspects of parenting, such as confliet,
and negative outcomes, such as antisocial behaviour. Associations are generally weaker for aspeets of positive parenting, such as affection.
At this point it would appear that NEAD supplied a happy
ending for the story of nonshared environment: parents treat
children differently and this differential treatment relates to
differential adjustment. The issue suddenly became more
complex, however, at the third step in this researeh
Vol 46, No 3
program—asking whether differential parenting is a cause or
an effect of children's adjustment. Longitudinal cross-lagged
analyses were conducted but were not very informative: the
parenting measures and adjustment measures were quite stable over the 3-year interval in whieh NEAD families were
studied, whereas the success of cross-lagged analyses depends on change. A unique feature of NEAD is that its extensive measures of family environment were embedded in a
genetieally sensitive design to assess possible genetic mediation of associations between nonshared environment and adolescent psyehopathology. The 720 NEAD families were
selected to include identical and fraternal twins, full siblings,
half-siblings, and genetically unrelated siblings. In these
families, the twins and siblings were the adolescent children,
not the parents, so that genetic mediation could be seen to the
extent that differential parenting is a response to children's
genetieally driven differential adjustment.
NEAD used multivariate genetic analysis of eovadance to address this issue (46). Unlike traditional univariate genetic
analysis of the variance of a single variable, multivariate genetic analysis decomposes the eovariance between variables
into genetic and environmental sources of eovarianee (47).
The basic idea is that instead of eomparing identical and fraternal twin correlations for a single variable in order to decompose the variance of that variable, multivariate genetic
analysis eompares identical and fraternal twin crosscorrelations—that is, the correlation between variable X for
one twin and variable Y for the other twin—in order to decompose the covariance between X and Y. Genetic mediation
of the covariance is implied if the identical twin crosscorrelation exceeds the fraternal twin cross-correlation (see
14, for details). In addition to families with identical and fraternal twins, NEAD applied multivariate genetie analysis to
the covariance between family environment and adolescent
outcome for families with full siblings, half siblings and unrelated siblings. This approach estimates genetie mediation of
the eovarianee between family environment and adolescent
outcome, and, just as importantly, it estimates shared and
nonshared environmental contributions to the eovariance, independent of genetie effects. Moreover, if the standard assumption is made that error of measurement does not
correlate across measures, the nonshared environmental contribution to the eovarianee is free of measurement error.
Multivariate genetie analysis of the associations between parental negativity and adolescent adjustment yielded an unexpected finding: most of these associations were mediated by
genetie factors. Hardly any nonshared environmental mediating effeets remained independent of genetic mediation. The
finding that nonshared environment fails to mediate these assoeiations is supported by simpler analyses that found little
assoeiation between identical twin differences in family environment and identical twin differences in adolescent outcomes—an analytic approach that directly assesses
April 2001
Why Are Chitdren in the Same Family So Different?
nonshared environmental mediation of the association (48).
The finding of genetic mediation implies that, to a substantial
extent, differential parental treatment of siblings reflects genetically influenced differences between the siblings. As implausible as this finding might seem on first encounter, it is
part of the second great discovery of genetic research at the
interface between nature and nurture—genetics contributes
substantially to experience (49). The NEAD quest for nonshared environment led to genotype-environment correlation; that is, children select, modify, construct, and
reconstruct their experiences in part on the basis of their genetic propensities. Many of the recent reactions mentioned
earlier (8,25,26) focus on this larger issue of genotype-environment interplay, rather than on nonshared environment per
se. The NEAD book (44) is called The Relationship Code because genotype-environment correlation restores to the family some of the infiuence lost to nonshared environment
factors, in the sense that genetic propensities are expressed in
the family environment. Genotype-environment correlation
is neither a solely genetic nor a solely environment issue—it
is both. This, however, is another story for another time.
The Future
The relevance ofthe NEAD results in relation to the story of
nonshared environment is that we must begin again to identify nonshared environment. The traditional measures of family environment used in NEAD did not result in the
identification of nonshared environmental links with differential sibling outcomes. Perhaps other measures ofthe family
environment will be more successful. Perhaps some sources
of nonshared environment can still be found in NEAD, in age
and sex interactions with shared environmental factors, as
suggested by recent papers.
In retrospect, however, as Harris (9) has trenchantly pointed
out, it seems odd to have looked for differential experiences
of siblings solely in the family because siblings live in the
same family. In defense of NEAD, investigating the family
environment as a first step in the quest for nonshared environment allowed NEAD to capitalize on the huge research effort
that has gone into assessing family environment. Indeed, it
turns out that siblings experience very different family environments and that these differential experiences are strongly
associated with differential outcomes. The problem is that genetic mediation largely accounts for these associations.
As suggested by the results of the metaanalysis mentioned
earlier (10), extrafamilial factors seem to be good candidates
for nonshared environments—candidates that have not been
studied nearly as much as has family environment. Harris anticipates that research will confirm the impact of peers: although genetic influence is involved in choosing and being
chosen by friends and peers (50-53), a recent report involving both NEAD and an adoption study found that peer groups
largely involve nonshared environment independent of
231
genetics, as predicted by Harris (54). Nonetheless, nonshared
environment appears to be present early in life, long before
children experience peer infiuence, which implies that nonshared environmental factors may differ from age to age—a
topic to which we shall return.
No matter how difficult it may be to find specific nonshared
environmental factors, it should be emphasized that the effective environment operates in terms of nonshared experience.
More research is needed to understand these mechanisms.
The price of admission to research on nonshared environment
is to study more than a single child per family. This is not difficult because more than 80% of families have more than a
single child: siblings add synergistically to any research design. You can study whatever you were going to study in the
traditional between-family way and then look at the data from
a within-family perspective, which can in turn clarify the interpretation of any between-family findings (55). Any study
of siblings can be used to investigate the first 2 steps in the research program mentioned earlier: documenting differential
experiences and documenting the association between differential experiences and differential outcomes. Longitudinal
studies can begin to address the third step: interpreting the direction of effects for the association—between, for example,
parenting (X) and child outcomes (Y). Does X cause Y, or
does Y cause X, or is a third factor responsible for the association between X and Y? Behavioural-genetic studies of twins
or adoptees are useful to investigate genetics as a possible
third factor mediating such associations.
We also need to consider the gloomy prospect that chance
contributes to nonshared environment in terms of random
noise, idiosyncratic experiences, or the subtle interplay of a
concatenation of events (5,10,11,56-58). Francis Galton, the
founder of behavioural genetics, suggested that nonshared
environment is largely due to chance, when he commented
that "the whimsical effects of chance in producing stable results are common enough." Sounding a bit like a fortune
cookie, he also observed that "tangled strings variously
twitched, soon get themselves into tight knots" (59, p 195).
Two overlooked findings from behavioural-genetic research
also point to the importance of chance. The first is multivariate genetic research showing that nonshared environmental
infiuences on one trait, such as depression, are independent of
nonshared environmental infiuences on other traits, such as
antisocial behaviour (44,60,61). The second finding involves
the application of multivariate genetic analysis to longitudinal data. This assesses the genetic and environmental origins
of age-to-age change and continuity (62). Longitudinal genetic analyses indicate that nonshared environmental infiuences are age-specific for psychopathology (63,64),
personality (65-67), and cognitive abilities (68). In NEAD,
although most nonshared environment was unstable, some
stability was found for depression and antisocial behaviour
over a 3-year interval in adolescence (44,69). That is.
The Canadian Journal of Psychiatry
232
nonshared environmental infiuences at one age are largely
different from nonshared environmental infiuences at another age.
What environmental processes other than chance could explain these 2 findings? Our view, nonetheless, is that chance
is the null hypothesis, although measures of life events can assess some of its aspects (70). Systematic sources of nonshared
environment need to be thoroughly examined before we dismiss it as chance. Chance might only be a label for our current
inability to identify the processes by which children—even
pairs of identical twins—growing up in the same family come
to be so different. For example, to some extent children might
make their own luck.
A larger implication of nonshared environment is that it provides an excellent indication that some ofthe most important
questions for genetic research involve the environment, and
some of the most important questions for environmental research involve genetics (71). Genetic research will profit if it
includes sophisticated measures ofthe environment, environmental research will benefit from the use of genetic designs,
and understanding of development will be advanced by collaboration between geneticists and environmentalists. In
these ways, behavioural scientists are putting the nature-nurture controversy behind them and bringing nature and nurture
together in the study of development to understand the processes by which genotypes become phenotypes.
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Resume— Pourquoi les enfants d'une meme famille sont-ils si differents? Le milieu non
partage, dix ans plus tard
Objectif: Examiner les recents developpements d'une etude du milieu non partage, c 'est-a-dire, les influences du milieu qui font
que les enfants grandlssant dans la meme famille sont differents plutot que semblables.
Methode : Nous passons en revue plusieurs livres et articles recents et influents sur le sujet du milieu non partage, dans les dix
annees suivant I 'article de 1987 qui en soulignait I 'importance sur le developpement psychologique.
Resultats: On a realise desprogres modestes dans I 'identification des aspects specifiques de I 'environnement qui sont responsables du milieu non partage. Bien que les parents traitent leurs multiples enfants differemment, cette difference de traitement ne represente qu 'une infime partie de I 'influence du milieu non partage, unefois que les facteurs genetiques sont pris en compte. On
croit qu 'un certain degre du milieu non partage peut etre attrtbuable aufait quefreres et soeurs reagissent differemment aux influences du milieu manifestement partagees. L 'influence des pairs etd'autres experiences a I exterieur de lafamille peuvent etre
des sources plus importantes de milieu non partage systematique.
Conclusions: Malgre les difftcultes Hies a I 'identification des sources precises du milieu non partage, it n en demeurepas moins
que la majorite de la variance de I 'environnement qui affecte le developpement des dimensions psychologiques et des troubles
psychiatriques n 'est pas partagee par les enfants grandissant dans la meme famille. Des recherches et theories additionnelles
sont necessaires pour expliquer pourquoifreres et soeurs sont si differents. Ilfaut egalement tenir compte du hasard, au sens des
experiences idiosyncrasiques.