Why Can't Birth Order Account
for the Differences Between Siblings?
by Judith Rich Harris
It's one of the things that convinces people that my theory of
development might not be as absurd as it appears at first glance.
Here's how a behavioral neuroscientist expressed it recently in an
online discussion group:
I think [Harris's] book got attention because its ideas were
startling and yet somehow resonant with many people's experiences.
For me, that resonance . . . had to do with my being one of three
siblings who, despite seemingly similar parenting, are each very
different (though each generally happy and successful). (Epstein,
Similar parenting but different kids -- a familiar story. How do
we account for the differences between siblings? If children's
personalities are formed by "nature" (their genes) and "nurture"
(the way their parents bring them up), and if siblings have a
similar genetic makeup (they inherited their genes from the same
parents) and a similar home environment -- the same home
environment, in most important respects -- how come they're not more
alike? If being reared in poverty, or in luxury, or by a single
parent, or by two parents who are constantly at each other's
throats, or by two parents who aren't home much, or by strict
parents, or by permissive parents -- if these things have effects on
children, how come they don't have the same effects on all the
children in the family?
It's not an illusion: siblings really do differ more than
conventional views of "nature and nurture" would lead you to expect.
The results started appearing in the 1970s, reported by researchers
in the field of behavioral genetics. The methods devised by these
researchers enabled them, for the first time, to tease apart the
contributions made by heredity and environment to the variations in
personality that are so noticeable in our species. Though the
methods were new, the questions these researchers were asking were
familiar ones: Why is Justin more aggressive than Jared? Why is
Claire shy and Alison outgoing? Why is Jamie cheerful and Taylor
The behavioral genetic research produced two outcomes. The
first surprised no one (or should have surprised no one): about half
of the variation in personality is due to variations in genes. One
of the reasons that siblings differ is that they have different
genes. Just as one sibling might be born with straight hair and the
other with curly hair, one might be born with a cheerful temperament
and the other with a tendency to be irritable or gloomy.
But the second outcome produced by the behavioral genetic
research was more like a question than an answer. The other half of
the variation in personality -- the part that wasn't genetic so it
had to be environmental -- couldn't be pinned on any of the usual
suspects. Everyone had assumed that the nongenetic variation in
adult personality was the result of differences in the circumstances
in which individuals were reared. This would mean that two children
reared in different homes by different parents should differ more in
personality than two reared in the same home by the same parents.
But that's not what the studies found. What they found was that two
people reared in the same home by the same parents were not
noticeably more alike than two people picked at random from the
population, once you deducted the similarities due to shared genes.
Full siblings, on average, share 50 percent of their genes. (For an
explanation of this statement, click here.)
Siblings are alike in personality only to the extent that they
share genes; if they do not share genes (if they are adoptive
siblings or stepsiblings) they aren't alike at all. Growing up
together -- going on the
same trips to the museum or the ballpark, coming home to the same
city apartment or house in the suburbs, living for 18 years with the
same parent or parents -- does not make them more alike.
Why Are the Differences Between Siblings a Problem for
Why does it matter that being reared in the same home and the
same family doesn't make siblings more alike? Why should we expect
that being reared in the same home would make people more alike?
The answer is that people who do research in developmental
psychology (my field) would like to think that the home environment
has predictable effects on children, and hence predictable effects
on what they will be like as adults. Consider, for example, the
personality trait psychologists call "conscientiousness" -- the
tendency to be responsible and well-organized. Wouldn't you think
that someone who was reared in a well-run home by conscientious
parents would tend to be more conscientious as an adult than someone
who was reared by happy-go-lucky slobs? Well, if this were the case,
then two people reared by conscientious parents should, on average,
both be more conscientious than two reared by slobs, which means
that two people reared by conscientious parents (or two reared by
slobs) should, on average, be more alike in conscientiousness than
one reared by a conscientious parent and one reared by a slob. But
that is exactly what the behavioral geneticists did not find. What
they found was that two adoptive siblings reared in the same family
are no more alike in conscientiousness than two adoptees chosen at
random from different families, and that a pair of identical twins
reared in the same family are no more alike in conscientiousness
than a pair separated at birth and reared apart. These results show
that being reared by conscientious parents does not, on average,
make people more conscientious; nor does it make them, on average,
less conscientious (Bouchard, 1994; Bouchard et al., 1990;
The implication was unsettling. If being reared by conscientious
parents has any effect at all on children, then it must be a random
one. It must make some children more conscientious and others less
so, unpredictably. And randomness and unpredictability are not what
developmental psychologists expect to find. They believe that how a
child is reared has predictable effects -- that there are good ways
and bad ways to rear kids, good home environments and bad ones. They
believe that good home environments, on the whole, produce better
Many aspects of personality and behavior have now been studied
with behavioral genetic techniques. In stark contrast to the
here-today-gone-tomorrow results so common in psychological
research, the behavioral genetic results have been resoundingly
consistent. Despite differences in the ways personality is measured
(standard personality tests, judgments by parents or teachers,
direct observations of behavior, etc.), and differences in the kinds
of subject pairs who participate (adoptive siblings, biological
siblings or half-siblings, identical and fraternal twins, reared
together or apart), the conclusions are almost invariably the same.
Siblings are alike only to the extent that they share genes. Genes
make biological siblings more alike in personality; growing up
together does not.
The results are still coming in, but the handwriting was on the
wall by 1987. That was the year when behavioral geneticists Robert
Plomin and Denise Daniels published an article titled "Why Are Children
in the Same Family So Different from One Another?" The overview
Plomin and Daniels gave of the behavioral genetic findings, and
their explanation of why these findings are a problem for
developmental psychologists, have stood up well. The results they
summarized have been backed up by later research, and they are just
as much a problem for developmental psychologists today as they were
How Did the Developmental Psychologists React?
The first real attempt by a developmental psychologist to face
up to the implications of the behavioral genetic findings was made
in 1991 by Lois Hoffman. Hoffman began by questioning whether
siblings are really as different as the behavioral genetic evidence
suggested; then she said that sibling differences are what you would
expect, given the way that families work.
Now, at last, we get to birth order. Birth order (which she
called "ordinal position") is one of the reasons, according to
Hoffman, why siblings are different:
First, family roles and sibling interactions are very much
affected by ordinal position. Firstborn children are treated
differently by parents throughout life than are subsequent
children. . . . In addition, it is a different experience to be an
older sibling than it is to have an older sibling. (Hoffman, 1991,
Hoffman cited studies showing that parents treat firstborns and
laterborns differently: parents show "a special involvement" with
firstborns, they give firstborns more responsibility and attention,
they have higher expectations for the firstborn, and so on.
Unfortunately, as Hoffman admitted, Plomin and Daniels (1987)
had already considered and rejected the possibility that birth order
could account for the differences between siblings. Their rejection
was based on plenty of evidence, because psychologists had been
studying birth order for decades. The evidence (much of which had
been summarized in 1983 by the Swiss researchers Cécile Ernst and
Jules Angst) indicated that birth order has little or no effect on
personality: well-done studies generally find that firstborns and
laterborns are indistinguishable in personality. A 1990 book on
sibling differences written by Plomin and his wife, developmental
psychologist Judy Dunn, dismissed the idea that birth order could
get developmental psychology out of its pickle. Dunn and Plomin
agreed that parents do treat firstborns and laterborns differently,
but they carefully explained why this differential treatment can't
answer the question of why siblings differ in personality:
If there are no systematic differences in personality according
to birth order, then any differences in parental behavior that are
associated with birth order cannot be very significant for later
developmental outcome. (Dunn & Plomin, 1990, p. 85)
Hoffman acknowledged that the behavioral geneticists had
dismissed birth order as "unimportant," but conjectured that
"personality outcomes are affected by a multiplicity of interacting
environmental influences, and any given one is unlikely to explain
much variance" (1991, p. 194).
Explaining the Variance
When Hoffman spoke of explaining the variance, she was talking
about something developmentalists do -- or try to do -- all the
time. Modern developmental psychology is all about explaining the
differences among children (or differences among the adults the
children become). The aim is to account for the variance -- the
variation from one individual to another -- in whatever
characteristic or behavior the developmentalists are interested in.
The term variance has a precise mathematical meaning, and
accounting for it involves a good deal of mathematical
calculation, but the important point is that the researchers are
trying to explain why some children score high and others score low
on whatever it is they are measuring.
Let's say, for instance, that the researchers are interested in
aggressive behavior. They get a numerical estimate of the
aggressiveness of each child in the study, perhaps by asking the
child's teachers or peers to indicate, on a scale of 1 to 10, how
aggressive this child is compared to others of the same age. Because
the researchers believe that children's aggressiveness is related to
their parents' child-rearing style, they also interview the parents
and get an estimate of how often each child is spanked at home. Then
the researchers put the two sets of numbers together. They find that
there is a correlation between the measure of the children's
aggressiveness and the measure of how often they are spanked --
children who receive more spankings are, on average, judged to be
more aggressive -- and they interpret this correlation as evidence
that some of the variation in aggressiveness can be blamed on the
spankings. The square of the correlation gives the proportion of the
variance that has been "accounted for." If the correlation is .30
the amount of variance accounted for is .09, which means that
spankings account for 9 percent of the variance in aggressiveness.
Though it doesn't sound like much, developmentalists break open the
champagne if they can do that well.
But the study I've just described -- a typical study in
developmental psychology -- has a serious flaw: it didn't provide
any controls for genetic effects. Whenever children are found to
resemble their parents in some respect -- in this case, in
aggressiveness (parents who spank are behaving aggressively) -- it's
necessary to consider the possibility that some or all of the
resemblance is due to the fact that children are genetically similar
to their biological parents. That interpretation is supported by the
results of studies that do provide controls for genetic effects. A
study of adopted and nonadopted children by Kirby Deater-Deckard and
Robert Plomin (1999) showed that genetic influences on behavior
accounted for about half of the variance in the children's
aggressiveness, when their aggressiveness was judged by their
teachers (each child was assessed by five different teachers over a
period of several years). In contrast, being reared in a given home
accounted for zero percent of the variance. According to the
teachers' judgments, adoptive siblings who were being reared in the
same home by the same parents showed no resemblance at all in how
aggressive they were in the classroom or the playground. Biological
siblings did show some resemblance, but the resemblance was entirely
due to their shared genes.
Deater-Deckard and Plomin's results, which are typical of
behavioral genetic studies, leave half of the variance unaccounted
for. Only half of the variation in aggressiveness was accounted for
by the effects of genes. What about the other half? The other half
of the variance -- the nongenetic differences in aggressiveness
among the children in this study -- could not be attributed to any
aspect of the home environment that siblings share. Many aspects of
the home environment are shared by siblings: whether the parent or
parents are harsh and irritable or warm and mellow; whether they do
or do not believe in the use of physical
punishment. The results showed that factors of this sort, which are
experienced by all the children in the family, have no consistent
effects on the children's aggressiveness. In other words, being
reared by harsh, irritable parents who use spanking as a routine way
of enforcing obedience does not, on average, make children more (or
less) aggressive. And yet something other than genes makes them more
(or less) aggressive.
Since aspects of the home environment shared by siblings
couldn't explain the variation, the next question was obvious: What
about aspects of the home environment not shared by siblings?
For example, what if the parents were nicer to one sibling than to
the other? What if one sibling got more affection, the other got
more spankings? Couldn't that explain why they differ in
As everybody knows, parents don't treat their kids all alike.
And one of the things that makes parents treat their children
differently -- this is something that Lois Hoffman and Dunn
and Plomin agreed on -- is birth order.
Firstborns and laterborns have different experiences in the home,
right from the start. Firstborns are born to inexperienced and
anxious parents, laterborns to veterans. Firstborns have their
parents all to themselves for a while and then are abruptly
dethroned by a rival; laterborns always have to compete for their
parents' attention. Parents give firstborns more responsibility;
they give laterborns more affection.
That's right: it's the younger child who gets more affection.
Two studies (Dunn & Plomin, 1990; McHale et al., 1995) have
shown that at least half of parents with two children admit to
loving one better than the other, and a large majority of these
parents -- more than 80 percent -- say they love their younger child
best. These are big differences in parental affection. If
being loved more by their parents made children less aggressive (or
more aggressive), then we should see birth order effects on
aggressiveness. But the teachers in Deater-Deckard and Plomin's
(1999) study did not judge younger siblings to be any less
aggressive (or more aggressive) than older siblings.
The goal here is to account for 50 percent of the variance in
personality -- the 50 percent that is not genetic. If the
differences between siblings are due in part to the effects of birth
order, then birth order should account for some non-negligible
portion of that variance. But it doesn't. In studies of adults,
personality is usually measured with self-report personality tests,
the kind in which subjects check off their agreement or disagreement
with a list of self-descriptive statements. Birth order accounts for
little or none of the variance in scores on these tests (Ernst &
Angst, 1983; Sulloway, 1999; Jefferson et al., 1998).
Reluctant to give up their belief in birth order, some theorists
have instead given up their faith in standard self-report
personality tests (see "Why Did Sulloway's Results Differ From Those
of Ernst and Angst?" forthcoming on this website). They've claimed
that these tests are invalid -- that they are inaccurate or
insensitive measures of personality (Kagan, 1998; Sulloway, 1998,
1999). This is like blaming the yardstick when one's theory of why
some kids are taller than others fails to be confirmed. Standard
self-report personality tests are the yardstick that produced
the results we are trying to account for!
This is an important point so I will take the risk of belaboring
it. We are trying to account for the unexplained differences in
personality between siblings. How did researchers discover these
perplexing differences between siblings? By giving them standard
self-report personality tests. The variance we are trying to account
for is, for the most part, variance in scores on standard
self-report personality tests. So if birth order can't account for a
visible portion of the variance in those scores, then birth order
isn't the answer -- isn't even one of the answers -- we are
The validity of self-report personality tests has been
demonstrated over the years in two different ways: by their
agreement with other ways of measuring personality, such as
judgments by other people (Jefferson et al., 1998); and by their
agreement with other ways of measuring the effects of genetic and
environmental influences. Behavioral genetic studies using
self-report personality tests produce results that are consistent
with those of studies that measure other things: aggressive behavior
on the playground, the likelihood of getting divorced, and so on.
Regardless of what is measured and how it is measured, the results
are almost invariably the same: genetic factors account for 40 to 60
percent of the variance; shared home environment accounts for little
The failure to find systematic differences between firstborns
and laterborns on standard self-report personality tests is like an
elephant in the living room: you can't ignore it and you can't get
A Spoonful of Evolutionary Psychology
Some theorists have proposed that the differences between
siblings result from the effects they have on each other, rather
than from the way they are treated by their parents. Here, for
example, is an explanation for sibling differences given by
developmentalist Jerome Kagan:
The child holds a set of standards regarding proper behavior and
has conceptualized his role in relation to authority. . . . Now let
us suppose that another enters his life space whom he cannot
ignore because the other is a competitor for some of the same
resources the child desires. If the other is a sibling, the
child perceives the other to share parental attention. The
child now is pushed to differentiate himself from the other,
toward the values of the parents, in order to retain the favored
position. . . . Hence, first-borns with younger siblings are more
likely to adopt the values of the parents and to be rigid about
adherence to those early standards than younger siblings. (Kagan,
1976, p. 170; italics in the original)
A similar view was expressed twenty years later by Frank
Sulloway in his book Born to Rebel (which he dedicated to
Siblings compete with one another in an effort to secure
physical, emotional, and intellectual resources from parents. . . .
Siblings create different roles for themselves within the family
system. These differing roles in turn lead to disparate ways of
currying parental favor. Eldest children, for example, are likely to
seek parental favor by acting as surrogate parents toward their
younger siblings. Younger siblings are not in a position to
ingratiate themseves with parents in the same manner. Their niche is
typically less parent identified, less driven by conscientious
behavior. . . . In particular, eldest children tend to identify more
closely with parents and authority. (Sulloway, 1996, p. 21)
What Sulloway added to Kagan's formulation was a spoonful of
evolutionary psychology. Siblings differ, according to Sulloway's
theory, because they are engaged in a Darwinian struggle for
survival -- a competition for parental favors that, in ancestral
times, could have proved fatal for the loser, either because the war
between siblings turned bloody (Cain and Abel are mentioned more
than once in Sulloway's book) or, more commonly, because the loser's
share of parental favors was inadequate to keep him or her alive.
The principles of evolutionary psychology that Sulloway makes
use of were first proposed by William Hamilton (1964) and Robert
Trivers (1985). Hamilton explained why it makes sense, in
evolutionary terms, for humans to share food or put themselves in
danger in order to help close relatives, such as their children.
Close relatives share genes, so a person who increases the survival
chances of her relative is also increasing the survival chances of
the genes that she and the relative have in common (see Dawkins,
1976; Pinker, 1997).
Trivers explained why it makes sense, in evolutionary terms, for
a person to put her own interests above the relative's interests:
because they don't share all of their genes, only some
of their genes. A parent and a child, for example, have 50 percent
of their genes in common, not 100 percent, which means that the
parent's interests don't always coincide with the child's. The
result is what Trivers called "parent-offspring conflict": for
example, the mother wants to wean the infant, the infant doesn't
want to be weaned.
Like parents and offspring, siblings share 50 percent of their
genes. Thus, siblings should help each other, just as parents help
their offspring. But Sulloway's theory puts more emphasis on the 50
percent of their genes that siblings don't share. Some see
the cup shared by siblings as half full; Sulloway sees it as half
empty. It's the fact that siblings don't share all their genes that
leads to "sibling conflict" (1996, p. 60). But the conflict is an
uneven match, because the firstborn got there first and is bigger
and stronger. Thus, firstborns and laterborns must devise different
strategies in their competition for family resources. Differences in
personality between siblings, according to Sulloway, are the outcome
of these different strategies. Firstborns develop patterns of
behavior in their attempts to monopolize their parents' attention
and to dominate their younger siblings. Laterborns develop other
patterns of behavior in their attempts to win some of their parents'
attention and to placate their older siblings.
I do not doubt that children vie for their parents' attention
and use different strategies to win it. But Sulloway is ignoring an
important part of Trivers' theory. Here's how the evolutionary
psychologist Steven Pinker explained it:
Trivers reasoned that, according to the theory of
parent-offspring conflict, parents should not necessarily have their
children's interests at heart when they try to socialize them. Just
as parents often act against a child's interests, they may try to
train the child to act against its own interests. . .
. So even if children acquiesce to a parent's rewards, punishments,
examples, and exhortations for the time being because they are
smaller and have no choice, they should not, according to the
theory, allow their personalities to be shaped by these tactics.
(Pinker, 1997, pp. 447-448)
In other words, children should accommodate themselves to their
situation in the family by developing temporary strategies --
strategies that will enable them to get by until the time comes when
they are no longer dependent on their parents. They shouldn't allow
their parents to have permanent effects on their behavior
because the behaviors desired by the parents might not serve them
well in their adult lives.
Trivers' reasoning is supported by the evidence, which shows
that the patterns of behavior children acquire at home -- bossing
around younger siblings, deferring to older ones, whining for their
mother's attention, and so on -- are not carried to the schoolroom
or the playground. Though these patterns of learned behavior may
affect relationships with parents and siblings even in adulthood,
they don't affect relationships with other people -- with peers, for
example -- even in early childhood (Abramovitch et al., 1986;
Stocker & Dunn, 1990). As I pointed out in my previous essay on
this website (see "Why Are Birth Order Effects Dependent on
Context?"), it wouldn't make evolutionary sense for patterns of
behavior acquired within the family to generalize to social contexts
outside the family, because these patterns of behaviors are likely
to be counterproductive in other contexts. Evolution designed
children wisely. Childhood is preparation for a later stage of life
-- a stage on which the most important characters will probably not
be labeled "Mom," "Dad," "little sister," or "big brother."
It is also inconsistent with the principles of evolutionary
psychology to expect people to behave the same way with people
outside the family as they do with their parents and siblings. Most
mammals, including humans, behave differently toward kin and nonkin.
The ability to make such distinctions is widespread in the animal
kingdom (Mock & Parker, 1998; Pfennig & Sherman, 1995).
The theory that sibling differences are the result of
competition between siblings is based on a particular pattern of
family life: the nuclear, child-centered family, common today in
industrialized societies. Observations of hunter-gatherer and tribal
societies (e.g., by Eibl-Eibesfeldt, 1989) indicate that the kind of
intense, prolonged parental care we give our children, and the
intense, prolonged sibling rivalry that often accompanies it, are
recent innovations in the history of our species. In traditional
societies, intense parental (that is, maternal) care lasts only
about three years. After that, children spend most of their time in
the local play group. They don't bother competing for their mother's
attention because it wouldn't work -- almost all of her attention is
given to the latest baby. Siblings look after each other in these
societies. Brothers are allies, not rivals.
The same is true of chimpanzee brothers (Goodall, 1986). Among
nonhuman animals, the kind of sibling rivalry that results in
physical aggression is found almost exclusively in species that rear
their young in litters, rather than one at a time (Mock &
What About Twins?
In all human societies, a distinction is made between relatives
and nonrelatives; as a rule, relatives are treated better. Humans
are nice to their kin even if they don't think their favors will
ever be reciprocated. Parents feed their children and older siblings
defend their younger brothers and sisters against bullies without
expecting to be repaid.
Among the psychological mechanisms that permit mammals to
distinguish relatives from nonrelatives and close relatives from
distant ones, an important one for humans is the recognition of
similarity. Because multiple marriages and sex outside of marriage
are common in most human societies, siblings often have different
biological fathers and thus share only 25 percent of their genes.
Similarly, a man is not necessarily the biological father of all his
wife's children; he may share no genes at all with her latest baby.
Consistent with the predictions of evolutionary psychology, people
form closer emotional ties with others who resemble them physically
or psychologically. For instance, parents feel more grief at the
death of a child who resembles them than at the death of one who
does not (Littlefield & Rushton, 1986; Segal, 1993).
Because twinning is relatively uncommon in the human species, it
is unlikely that humans evolved special psychological mechanisms to
deal with the twin relationship. But the same mechanism that causes
people to feel closer to those who are similar to themselves makes
identical twins -- who share 100 percent of their genes -- feel the
closest of all. The loss of an identical twin is more devastating
than the loss of a child or of a husband or wife. Compared to
fraternal twins (who, like ordinary siblings, share about 50 percent
of their genes), identical twins are less competitive and more
cooperative with each other (Segal, 1993, 1999).
And yet the nongenetic differences in personality between
identical twins, reared together from birth, are about as large as
the nongenetic differences in personality between ordinary siblings
(click here for an explanation of this statement).
Reared-together identical twins aren't nearly as alike as most
people think; in fact, they are no more alike in personality than
identical twins separated in infancy and reared in different homes.
Reared together or apart, the correlation between their scores on
personality tests is only about .50 (Bouchard et al., 1990). This
correlation is due entirely to genetic influences on personality;
their environments have made them neither more alike nor less alike.
It is a point that bears repeating: their environments have made
them neither more alike nor less alike. Some people who read about
the behavioral genetic results and see the question "Why are
siblings so different?" get the idea that something in the home
environment -- perhaps the rivalry between them, perhaps the way
their parents encourage them to follow separate paths -- actually
creates or widens differences between siblings. That is not the
case: the home environment simply fails to have the expected effect
of making them more alike. Adoptive siblings reared in the same
family are not less alike in personality than adoptees picked at
random from the population -- the correlation between them is
approximately zero. If their shared home environment were creating
or widening differences between them, the correlation would be a
negative number. The net effect of the home environment is
approximately zero. It was this baffling finding that caused me to
conclude that prevailing theories of development must be wrong and
that a new theory was needed (see The Nurture
Any theory that purports to account for the differences between
siblings -- the unexplained nongenetic variance in personality --
has to account for the fact that the same results are found for
identical twins, fraternal twins, ordinary biological siblings, and
adoptive siblings. The amount of variance that is left unexplained
by shared genes and a shared home environment is about the same --
roughly 50 percent -- for all these sibling pairs. A theory that
attributes the sibling differences to birth order effects or
competition between siblings cannot account for the fact that the
same differences in personality are found between identical twins,
who do not differ in birth order and who compete less and cooperate
more than ordinary siblings.
Recent Attempts to Account for the Unexplained Variance
Ever since the publication of Plomin and Daniel's 1987 article,
cutting-edge researchers have been searching for the source of the
unexplained variance -- measuring differences in the experiences of
pairs of siblings who live in the same home and trying to link the
direction and magnitude of these differences to the differences in
the siblings' personalities. The best of this research is summed up
in two recent reports.
The first, by Turkheimer and Waldron (2000), is a meta-analysis.
(A meta-analysis is a method for combining the results of a
collection of individual studies, taking into account the number of
subjects who participated in each study and the size of the obtained
effect.) The studies included in this meta-analysis investigated a
variety of factors -- including birth order -- that might (in the
opinion of the researchers who did the work) account for the
differences between siblings. None of the studies managed to account
for much of the variance, but the least successful studies were
those that looked at "family constellation variables" such as birth
order and age differences between siblings. Family constellation
variables managed to account, on average, for only 1 percent of the
variance -- a negligible fraction of the 50 percent of the variance
that needs to be accounted for.
The other report is a single large, exceptionally well-done
study that directly measured sibling differences, as well as many of
the environmental factors that the researchers thought might be
responsible for the differences. David Reiss and his colleagues
(2000) studied 720 adolescent sibling pairs living in stable
two-parent families. The siblings were the same sex and included
identical and fraternal twins, ordinary siblings, half-siblings, and
stepsiblings. Each pair was examined twice over a three-year period;
the researchers took hundreds of measurements of their home
environments and of the siblings themselves. The measures of the
home environment included judgments of parent-offspring closeness,
rapport, monitoring, disagreement, and control (made separately for
each sibling by both parents and by the sibling in question);
judgments of parental rejection, conflict, closeness, warmth,
support, and communication (made separately for each sibling by the
researchers); judgments of sibling conflict, negativity, warmth, and
support (made by both parents and both siblings); and judgments of
parental conflict about the adolescents (made by both parents). The
measures of the adolescents included judgments of their antisocial
behavior, depressive symptoms, industriousness, autonomy, school
performance, sociability, social success, social responsibility, and
self-esteem. These judgments were again made for each sibling
separately, by the siblings themselves, their parents, and the
researchers. Because the measures obtained in this study were based
on the combined judgments of several different people, they were
exceptionally accurate and reliable.
The study took twelve years to complete. When it was done the
researchers were more than disappointed: Reiss has confessed to a
journalist that he was "shocked" by the results (Paul, 1998, p. 46).
I, on the other hand, was neither surprised nor disappointed: the
results were exactly what my theory (Harris, 1995, 1998) predicted.
But Reiss has my sympathy. Though he made a valiant effort to
find something to say that would justify his unshakable faith in the
importance of the family environment, the data speak louder than the
words (Reiss, 2000). What he and his colleagues got out of all their
hard work was pretty close to zilch. Yes, they confirmed that there
are sizable differences between siblings that can't be attributed
either to genes or to aspects of the home environment that the
siblings share; and yes, they confirmed that parents behave
differently toward different offspring. But the researchers' attempt
to find the sources of the differences between the siblings was a
total flop. The differences in parental behavior were almost
entirely a response to innate differences in the offspring
themselves, rather than a cause of the differences between
them. Nor was it possible to pin the differences between siblings on
disagreements between the parents on how to deal with them. Nor was
it possible to pin the differences on the sibling relationship
itself -- the hostility or camaraderie between them, the domination
of one by the other. Reiss summed up his negative results this
We can say with confidence that, on the basis of the data we
collected, the following family characteristics do not reflect
nongenetic, nonshared influences on the adolescent: differential
marital conflict about the adolescent versus the sib, differential
parenting toward siblings, and asymmetrical relationships the sibs
construct with each other. . . . Given that our very large
twelve-year study was designed to identify nongenetic, nonshared
factors, this dearth of findings is not only disappointing but
galvanizing. (Reiss, 2000, pp. 406-407)
One reason why "asymmetrical relationships the sibs construct
with each other" could not explain the differences between them was
that usually the sibling relationships were not asymmetrical.
Whether they were twins, full siblings, half-siblings, or
stepsiblings, most relationships between siblings were found to be
remarkably symmetrical -- what evolutionary psychologists call "tit
for tat." If Sibling A was nice to Sibling B, then B was nice to A.
If A said that B was a jerk, B was likely to say the same about A.
Relationships that work the same in both directions cannot explain
the differences between siblings.
Occasionally asymmetries were found in the sibling relationship.
In some cases, Sibling A showed more kindness or more hostility to
Sibling B than B did to A. In such cases, other differences were
usually found between A and B -- differences, for example, in their
autonomy, sociability, and antisocial behavior. But the data
indicated that the asymmetries in the sibling relationship and the
other measured differences between the siblings could both be
attributed to genetic factors. A kid who was hard to get along with
from birth was likely to be nasty to his sibling and to be
troublesome in other areas of his life as well. Like parent-child
relationships, sibling relationships reflect the genetically
influenced characteristics of both parties. And like differential
parenting toward children, asymmetric sibling relationships are a
result, rather than a cause, of differences between the
Because Reiss was trying to explain the differences between
twins, as well as those between other kinds of siblings, he didn't
look for birth order effects. But he did look at the two factors
that are believed to be the basis of birth order effects:
differences in how parents treat their children, and asymmetric
relationships between the children themselves. Reiss's findings are
consistent with those of Turkheimer and Waldron (2000). Both studies
demonstrated that birth order cannot account for the unexplained
variance in personality.
Getting Along at Home and Elsewhere
It's easy to misunderstand the results of the behavioral genetic
research. Many people assume that because the home doesn't make
siblings more alike, it must make them more different. Not so.
Siblings are not more different than a bunch of children picked at
random from the population. The unexplained variance represents both
differences between siblings and differences between children who
are not siblings. Researchers expected that the environment shared
by children who grow up in the same home would account for some of
that variance, but it didn't. They thought that knowing that Justin
is aggressive would give them a clue about what to expect from his
brother Jared, but (apart from the similarities due to their shared
genes) it didn't. The variance left to account for is the same
variance they had to begin with, before they entered into their
calculations the fact that Justin and Jared were being reared in
the same home by the same parents. The
question "Why are siblings so different?" really means "Why aren't
siblings more alike?" Prevailing theories of development led people
to expect that siblings would be more alike. "Why are they so
different?" is the mournful cry of unfulfilled expectations.
Those unfulfilled expectations have produced a lot of useless
research. Looking for things that make siblings different -- instead
of looking for things that produce differences among all children,
whether or not they sleep under the same roof -- implies that we
need a special explanation for sibling differences. It implies that
shared experiences in the home must have similar effects on
the children who live there, and that therefore we must look for
other factors in the home that undo the similarity-producing
effects of the shared experiences.
But maybe the puzzle of the unexplained variance has a simpler
solution: maybe what happens to children at home doesn't contribute
anything at all to the variance, one way or the other. Maybe what
happens to children at home has -- as Trivers predicted -- no
long-term effects on their personalities. Maybe the strategies that
children work out for getting along with their parents and siblings
affect their behavior only when they're with their parents and
siblings; they don't use these strategies in other social contexts
because they are useless, or worse than useless, for dealing with
people outside the family. (See "Why Are Birth Order Effects
Dependent on Context?" on this website.)
Birth order is important at home because, unless they're twins,
siblings differ in age. Differences between them in size, strength,
and know-how persist for years, and children have to accommodate
themselves to these differences. But outside the home, birth order
loses its importance. This is true both in modern societies, where
children's companions outside the home are mainly others of the same
age, and in traditional societies, where children play in mixed-age
groups. In the same-age groups of modern societies, children who are
a bit bigger, stronger, or more knowledgeable than their agemates
have higher status; it doesn't matter whether they are the biggest
or the smallest at home. In the mixed-age play groups of our
ancestors, children's status increased as they got older. Each child
-- whether he was his mother's firstborn child or her fifth -- began
as the smallest child in the play group and worked his way up,
eventually becoming one of the largest, as those ahead of him
graduated out of the group.
Birth order cannot explain the personality differences between
siblings, though it can shed light on their relationships with each
other and with their parents. Because family relationships are
important to us, birth order seems important too (see "Why Do People
Believe in Birth Order Effects?" forthcoming on this website). But,
as important as they are in other ways, family relationships do not
For the answer to the mystery of environmental influences on
personality, we must look at the experiences children have in their
other environment -- the world outside their home. This is the world
in which they will spend their adult lives.
I thank William Dickens, Charles Harris, Steven Pinker, Robert
Plomin, and David Rowe for their comments on earlier versions of
Abramovitch, R., Corter, C., Pepler, D. J.,
& Stanhope, L. (1986). Sibling and peer interaction: A final
follow-up and a comparison. Child Development, 57,
Bouchard, T. J., Jr. (1994, June 17). Genes, environment, and
personality. Science, 264, 1700-1701.
Bouchard, T. J., Jr., Lykken, D. T., McGue, M., Segal, N. L.,
& Tellegen, A. (1990, October 12). Sources of human
psychological differences: The Minnesota study of twins reared
apart. Science, 250, 223-228.
Dawkins, R. (1976).
The selfish gene. New York: Oxford University Press.
Deater-Deckard, K., & Plomin, R. (1999). An adoption study
of the etiology of teacher and parent reports of externalizing
behavior problems in middle childhood. Child Development, 70,
Dunn, J., & Plomin, R. (1990).
Separate lives: Why
siblings are so different. New York: Basic Books.
Eibl-Eibesfeldt, I. (1989). Human ethology. Hawthorne,
NY: Aldine de Gruyter.
Epstein, D. H. (2001, February 1). Re: Cheesy Debate.
TIPS (Teaching in the Psychological Sciences) online
Ernst, C., & Angst, J. (1983). Birth order: Its influence
on personality. Berlin, Germany: Springer-Verlag.
Goodall, J. (1986). The chimpanzees of Gombe: Patterns of
behavior. Cambridge: Harvard University Press.
Hamilton, W. D. (1964). The genetical evolution of social
behaviour (I and II). Journal of Theoretical Biology, 7,
Harris, J. R. (1995).
Where is the child's environment? A group socialization theory of development.
Psychological Review, 102, 458-489.
Harris, J. R. (1998).
The nurture assumption: Why children turn out the way they do.
New York: Free Press.
Hoffman, L. W. (1991). The influence of the family environment
on personality: Accounting for sibling differences. Psychological
Bulletin, 110, 187-203.
Jefferson, T., Jr., Herbst, J. H., & McCrae, R. R. (1998).
Associations between birth order and personality traits: Evidence
from self-report and observer ratings. Journal of Research in
Personality, 32, 498-500.
Kagan, J. (1976). The role of the family during the first half
decade. In V. C. Vaughn, III, & T. B. Brazelton (Eds.), The
family--Can it be saved? Chicago, IL: Yearbook Medical
Kagan, J. (1998, September 13). A parent's influence is
peerless: "Nurture Assumption" ignores more theory than it proposes.
Boston Globe, p. E3.
Littlefield, C. H., & Rushton, J. P. (1986). When a child
dies: The sociobiology of bereavement. Journal of Personality
& Social Psychology, 51, 797-802.
McHale, S. M., Crouter, A. C., McGuire, S. A., & Updegraff,
K. A. (1995). Congruence between mothers' and fathers' differential
treatment of siblings: Links with family relations and children's
well-being. Child Development, 66, 116-128.
Mock, D. W., & Parker, G. A. (1998).
The evolution of sibling rivalry. Oxford, UK: Oxford University Press.
Pfennig, D. W., & Sherman, P. W. (1995, June). Kin
recognition. Scientific American, 272, 98-103.
Pinker, S. (1997).
How the mind works. New York: Norton.
Plomin, R., & Daniels, D. (1987). Why are children in the
same family so different from one another? Behavioral and Brain
Sciences, 10, 1-16.
Reiss, D., with Neiderhiser, J. M., Hetherington, E. M., &
Plomin, R. (2000).
The relationship code: Deciphering genetic and social influences on
adolescent development. Cambridge, MA: Harvard University Press.
Rowe, D. C. (1994).
The limits of family influence: Genes,
experience, and behavior. New York: Guilford Press.
Segal, N. L. (1993). Twin, sibling, and adoption methods: Tests
of evolutionary hypotheses. American Psychologist, 48,
Segal, N. L. (1999).
Entwined lives: Twins and what they tell
us about human behavior. New York: Penguin.
Stocker, C., & Dunn, J. (1990). Sibling relationships in
childhood: Links with friendships and peer relationships. British
Journal of Developmental Psychology, 8, 227-244.
Sulloway, F. J. (1996).
Born to rebel: Birth order, family dynamics, and creative lives.
New York: Pantheon.
Sulloway, F. J. (1998, November). Birth order and the nurture
misassumption: A reply to Judith Harris. Edge (
Sulloway, F. J. (1999). Birth order. In M. A. Runco & S.
Encyclopedia of creativity (vol. 1, pp. 189-202). San Diego,
CA: Academic Press.
Trivers, R. (1985). Social evolution. Menlo Park, CA:
Turkheimer, E., & Waldron, M. (2000). Nonshared environment:
A theoretical, methodological, and quantitative review.
Psychological Bulletin, 126, 78-108.