Why Are Birth Order Effects
Dependent on Context?
by Judith Rich Harris
In 1983, after reviewing hundreds of studies on birth order --
the product of 35 years of research by psychologists all over the
world -- Swiss researchers Cécile Ernst and Jules Angst
concluded that birth order has little or no effect on adult
personality. "Whenever studies of representative adult samples were
carried out with an unobjectionable method," they said, "birth order
differences were nil" (1983, p. 186). In other words, firstborns and
laterborns were indistinguishable in personality. However, Ernst and
Angst observed that one kind of study did find significant
differences between firstborns and laterborns: the kind in which the
subjects' personalities were judged by their parents or their
Ernst and Angst's observations were usually correct and this was
no exception. The distinction they made between those two kinds of
studies has been confirmed by subsequent research. Studies using
standard methods for assessing personality find no effects of birth
order or occasional effects of negligible size that fail to hold up
from one study to the next (e.g., Beer & Horn, 2000; Freese et
al., 1999; Hauser et al., 1997; Jefferson et al., 1998). On the
other hand, "all-in-the-family" studies -- in which subjects might
be asked to compare themselves to their siblings or are asked
questions that tap their feelings about their parents -- generally
produce significant and consistent birth order effects (e.g.,
Paulhus et al., 1999; Salmon & Daly, 1998).
In other words, birth order effects are dependent on context. If
the subject is asked questions that evoke thoughts, feelings, and
memories associated with the childhood home and family, birth order
effects -- personality differences between firstborns and
laterborns, or between firstborns, lastborns, and middle children --
are usually found. If the testing conditions do not evoke the family
context, birth order effects are seldom found. What is the
explanation of this discrepancy and what are its implications for
theories of birth order?
All theories of birth order rest on an unstated assumption: that
the experiences children have at home during childhood have
permanent effects on their personality and behavior. Some birth
order theorists stress the fact that firstborns and laterborns have
different relationships with their parents; others point to the
sibling relationship itself. In either case, the assumption is made
that children develop patterns of behavior in interacting with their
parents and/or siblings that subsequently affect the way they behave
outside of the family, not just in childhood but in adulthood as
That assumption rests, in turn, on a more basic assumption: that
learned behaviors transfer or generalize from one context or
situation to another, at least during childhood. Moreover, this
transfer or generalization is assumed to occur unconsciously and
involuntarily. It's not that children think, "This person reminds me
of my mother so I'll behave with her the way I behave with my
mother" -- it's that they behave that way automatically.
Examining assumptions is always a good idea and that's what I'm
going to do here. I'll begin by looking at the evidence for the
general proposition that young children generalize: that what they
learn in one context or situation affects their behavior in other
contexts or situations. Then I'll get more specific and examine the
evidence that the particular strategies used by firstborns and
laterborns in dealing with their parents and siblings are used in
other social contexts. At the end I'll discuss what appears to be an
exception to the rule but turns out not to be.
Birth order effects are presumed to have their roots in
childhood. No one, as far as I know, has proposed that these effects
suddenly appear in adulthood -- a time of life when people see much
less of their parents and siblings. The important evidence,
therefore, must come from studies of children. The evidence that I
will describe here points clearly to one conclusion: that children
are not born with a tendency to transfer behavior learned in one
context to other contexts (Harris, 2000a). There is no built-in bias
in favor of repeating previously learned behaviors simply because
they have already been learned. If there is a bias, it is in the
other direction: to proceed on the expectation that, because the
circumstances are different, the rules for how to behave are going
to be different, too.
The theory of development I proposed in The Nurture
Assumption can account for the difference in results produced by
the two different kinds of birth order studies. To put it more
strongly, the discrepant results are a confirmation of the theory's
predictions. According to the theory, children acquire certain
patterns of behavior in dealing with their parents and siblings. But
these patterns of behavior, and the feelings that accompany them,
are linked to the context of the childhood home and family. Children
learn other ways of behaving outside the home, in the presence of
people who are not their close relatives.
When Does Learning Generalize?
Textbooks in introductory psychology usually define
generalization something like this: "The tendency for a
response learned in one situation to be elicited by other, similar
situations" (Hall, 1983, p. 692). John B. Watson's classic
experiment with Little Albert is often used to illustrate the
concept. As Watson and his research assistant Rosalie Rayner --
later his second wife -- described the experiment in 1920, they
first conditioned the 11-month-old infant to fear a white rat by
making a loud noise behind his back whenever he reached toward the
rat. After seven pairings of the rat and noise, Watson and Rayner
reported, the child became afraid of the rat and also showed
transfer (generalization) of his fear to other animals and objects
-- a rabbit, a dog, a fur coat, and a wad of cotton wool.
But a later researcher, Ben Harris (no relation), looked at the
various reports Watson and Rayner gave of the Little Albert
experiment and decided that psychologists had been misled:
A major source of confusion about the Albert story is Watson
himself, who altered and deleted important aspects of the study in
his many descriptions of it. For example, in the Scientific
Monthly description of the study (Watson & Watson, 1921),
there is no mention of the conditioning of Albert to the dog, the
rabbit, and the rat that occurred at 11 months 20 days; thus
Albert's subsequent responses to these stimuli can be mistaken for a
strong generalization effect (for which there is little evidence).
(B. Harris, 1979, p. 154)
Even directly conditioned fears showed little or no transfer when
Albert was confronted with the same animals in a different room:
After an additional 5 days, Watson reconditioned Albert to the
rat (one trial, rat paired with noise) and also attempted to
condition Albert directly to fear the previously presented rabbit
(one trial) and dog (one trial). When the effects of this procedure
were tested in a different, larger room, it was found that Albert
showed only a slight reaction to the rat, the dog, and the rabbit.
Albert's "slight reaction" to the rabbit, for example, consisted
merely of his turning his face away from the animal.
I am not going to claim that generalization doesn't exist or
never occurs. My point is that generalization to different
situations isn't automatic. Generalization has to be learned.
As demonstrated by the experiments of Carolyn Rovee-Collier and
her colleagues, a young baby's initial inclination is not to
generalize at all. Rovee-Collier (1993) begins her experiments by
teaching a baby a simple trick. She ties one end of a ribbon to the
baby's ankle and the other end to a mobile suspended over his head.
When the baby kicks his foot, the mobile jiggles. Most babies are
delighted to discover they can make the mobile jiggle; they show
their enthusiasm by greatly increasing their rate of kicking the
beribboned foot. Moreover, they will remember this trick several
days later: if they're shown the mobile again, they will again kick
the appropriate foot. But they will do this only if nothing has been
changed. If the blue doodads hanging from the mobile have been
replaced with red doodads, or a different liner has been draped
around the crib, or the crib has been rolled from the bedroom to the
den, the baby will gaze cluelessly at the mobile, as though he had
never seen such a thing in his life.
But it's very easy to teach the baby to generalize: all you have
to do is to train him with a "series of different mobiles or in a
variety of contexts" (Rovee-Collier, 1993, p. 133). When the
learning setup is varied in this way, the baby learns that the rule
"kick your foot to make the mobile jiggle" holds true under a
variety of conditions. Babies learn this easily but they don't come
into the world already knowing it, even though it seems obvious to
us that a trick that made a mobile jiggle in the bedroom should also
make it jiggle in the den.
Teaching generalization isn't always so easy, however. It is very
difficult to teach school-age children to generalize problem-solving
skills they learn in the classroom. If they learn a method that
works on one problem they almost invariably fail to use it to solve
another problem, even if the two problems are very similar.
According to learning theorist Douglas Detterman,
The amazing thing about all these studies is not that they don't
produce transfer. The surprise is the extent of the similarity it is
possible to have between two problems without subjects realizing
that the two situations are identical and require the same solution.
(1993, p. 13)
Detterman's conclusion, after reviewing 90 years of research on
transfer of training, was: "People do not spontaneously transfer
even for situations that seem very similar" (1993, p. 18).
Very young children do not spontaneously transfer even when the
situations seem identical. Karen Adolph (1997) studied the
development of infant locomotion. She discovered that when babies
first learn to crawl they will attempt to crawl down an inclined
plane (a carpet-covered walkway) that is much too steep for them to
manage: "In their first week of crawling, most infants plunged
headlong down impossibly steep hills" (p. 65), landing ignominiously
on a cushioned surface at the bottom. Over a
period of several months, repeated experiences with the experimental
setup taught them which slopes they could negotiate and which were
too steep. But then Adolph's babies learned to walk. Incredibly,
there was no transfer at all from crawling to walking -- in the
first weeks of walking, the babies attempted to toddle down the same
steep slopes that had caused them to plunge headlong as crawlers,
with the same results!
In a later experiment, Adolph (2000) discovered that infants also
fail to use spatial knowledge acquired while sitting up (in the
pre-crawling stage) when they begin to crawl. The knowledge acquired
while sitting up is linked to the sitting-up position; babies do not
use it when they are in the crawling position. Even though these
results fit my predictions, I must confess that I find them
The studies of Rovee-Collier and Adolph show that babies come
into the world with a built-in bias against generalizing.
Generalization is something babies have to learn how to do, not
something they do automatically. The way they learn to do it is by
finding out that a response that worked in one situation will also
work in other situations. We don't have to learn how to work the
light switch in every new room we enter because we've encountered a
variety of light switches in a variety of different rooms and
they've all worked pretty much the same way. As a result of our past
experience with light switches, we feel pretty confident that the
switch on the wall next to the door will make the lights go on.
Thus, when a response has been made in several different
situations and has produced the desired results in all of them, the
response is likely to be repeated in new situations. That's the
first condition under which generalization occurs. The second
condition is when the "new" situation is so similar to the one in
which the response was learned that the subject doesn't perceive it
to be a new situation -- it's just the same situation again. In
Rovee-Collier's experiments, the babies who were shown a different
mobile -- a mobile with red doodads instead of blue ones -- stared
at it cluelessly, not realizing that the foot-kicking trick would
work with this mobile, too. But what if you replaced the doodads,
not with red ones, but with doodads in a slightly different shade of
blue? If you varied the shade of blue, you'd find that the closer it
was to the original shade, the more likely it was that the baby
would kick his foot. When the shade of blue is very similar to the
original, he can't tell the difference. As the color becomes less
and less similar to the original, he becomes less and less sure that
it's the same mobile.
When two stimuli or two situations are very, very similar, a
response learned in one will be replayed in the other. But do we
really want to call this "generalization"? Isn't it simply a
repetition of previously learned behavior?
A final complication is that similarity is in the eye of the
beholder. As the author of an up-to-date psychology textbook put it,
"You can't predict to what degree an individual will generalize from
one stimulus to the next unless you understand something about the
individual's mental concepts" (Gray, 1999, p. 138). If the two
stimuli evoke different mental concepts, generalization may not
occur even if the stimuli are objectively very similar. On the other
hand, if the two stimuli evoke the same mental concept --
which might mean only that the individual can't tell them apart --
the individual is likely to respond in the same way to both.
Two stimuli will evoke the same mental concept when they've been
found, through experience, to be equivalent. A baby soon learns that
Mommy is Mommy, even if she is wearing different clothes or has
changed her hairstyle. But if Mommy happens to have an identical
twin who looks just like her but isn't as nice, he will quickly
learn to tell them apart and will assign different mental concepts
Behaviors Learned in One Setting Don't Necessarily Work in
I've been talking about simple responses such as kicking a foot,
but complex patterns of social behavior also fail to generalize.
Even in toddlerhood, children tend to behave differently in
different social contexts -- so differently that, in parent-teacher
conferences, parents often find it hard to believe that the teacher
is talking about their child (see
cartoon). One researcher,
baffled by the fact that parents' judgments of their child's
temperament agreed so poorly with reports from the child's teacher
at the day-care center, arrived with obvious reluctance at this
conclusion: "There exists the possibility that the toddler's actual
behavior differs systematically in the home and day-care settings"
(Goldsmith, 1996, p. 230).
Behavior that differs in different contexts is called
"context-specific." The clearest example of context-specific
behavior is offered by language. In the United States, many children
-- the offspring of immigrants, for the most part -- speak one
language at home and a different language outside the home. I
described such a child in The Nurture Assumption (pp. 64-70,
253-257); I called him Joseph. Joseph and his parents moved from
Poland to rural Missouri when he was seven and a half. When he came
to this country he spoke hardly a word of English but he picked up
the language rapidly (Winitz et al., 1995). Within a year, Joseph
was speaking English almost as well as his American classmates;
within another year (like most children who immigrate before
puberty) he was speaking it without an accent -- even though he
continued to speak Polish at home with his parents. He could switch
back and forth between his two languages as easily as I switch
between two programs on my computer. Joseph's language behavior
outside his home -- his unaccented English -- was not a blend or a
compromise between what he learned at home and what he later learned
outside the home: it consisted entirely of what he learned outside
Psycholinguists call the ability to toggle between two languages
"code-switching." According to my theory of development (Harris,
1995, 1998), code-switching is a characteristic of learned behavior
in general, not just language. Many of the social behaviors children
learn at home are inappropriate outside of the home. Children are
alert to signals that their social context has changed and that
different behaviors are called for; they are quick to acquire new
patterns of behavior appropriate to their new setting. They can
switch between behavioral programs as easily as Joseph switched
between language programs, and for the same reason. An experiment
done many years ago showed that children who follow rules such as
"Do not lie" or "Do not cheat" in one context may ignore them in
another. Children who conscientiously follow family rules at home,
for example, may cheat in games on the playground or on tests in the
classroom; children who are honest in school may be dishonest at
home (Hartshorne & May, 1930/1971).
But sometimes a previously learned pattern of behavior does turn
out to be useful in a new setting. To use the example of language
again, most American children speak English both at home and outside
the home. Those who learned the language at home don't have to learn
it all over again in the nursery school or day-care center: they
only have to be given convincing evidence that speaking English is
the appropriate thing to do in their new setting. Some are convinced
more easily than others. Many toddlers are reluctant, when they
first begin to talk, to use their new words with people outside
their immediate family.
Other behaviors, too, can be learned at home, tried out
tentatively in a new setting, and retained if they work there. A
child who learns to read at home, or to play a musical instrument,
doesn't have to learn these things all over again at school. Both at
home and at school, it is conventional to say "please" and "thank
you," to eat with a spoon or fork, to refrain from grabbing things
that are being used by someone else, to protest when one's own
belongings are grabbed, and so on. Furthermore, individual children
might find, through experience, that something they learned to do in
one context is also useful elsewhere. A child who amuses her parents
with her clever quips and funny stories might find that cracking
jokes also makes her popular among her peers. When children
generalize a learned social behavior from one context to another,
they aren't repeating it blindly, like a wind-up toy. They have
learned through experience that certain behaviors yield comparable
results in a variety of contexts.
Genetic Influences Can Be Mistaken for Generalization
I like to use the example of language for two reasons: first,
because the evidence is readily available and so clear that no
statistical tests are needed; and second, because language behavior
-- whether a child speaks English or Polish, for example -- is
purely environmental, entirely learned. Genes do not predispose a
child to speak one language rather than another.
But genes do influence most other aspects of a child's behavior,
and these genetic influences are the source of a good deal of the
confusion about generalization, because genetic influences on
behavior cut across contexts. Your genes influence your behavior
wherever you go. Behavioral genetic studies have shown that about
half the variance in personality characteristics such as
agreeableness, aggressiveness, and conscientiousness can be
attributed to genetic differences among individuals (see "Why Can't
Birth Order Account for the Differences Between Siblings?"
on this website). Children whose genetic makeup predisposes
them to have an agreeable personality will tend to be agreeable in
every social context. Children whose genetic makeup predisposes them
to be conscientious will be better at resisting the temptation to
cheat, both at home and in school. Inherited physical attributes
also play a role. Children who have the good luck to be born with a
pretty face or an appealing smile are likely to experience social
success in every social context and to behave accordingly.
These similarities of behavior across contexts are easy to
mistake for generalization. Developmental psychologists often fall
into this trap. For example, researchers who study infant
attachments find correlations (weak but sometimes statistically
significant) between the security of a toddler's attachment to his
mother and his later success in other social arenas -- friendships
with peers, for instance. Ignoring the precept that correlation
doesn't imply causation, some attachment researchers (e.g., Sroufe
et al., 1999) interpret the later social successes as consequences
of what the child learned earlier with his mother: because he got
along well with his mother, the child expects that he will also get
along well with his peers.
But the attachment theorists' assumption that the child's peer
relationships are affected by what he learned from his interactions
with his mother is contradicted by other evidence. For example,
babies whose mothers suffer from postpartum depression tend to
exhibit a distinctive pattern of behavior in interactions with their
depressed mothers: they behave in a subdued manner, with somber face
and muted movements. Researchers have found, however, that the
babies' subdued behavior is "specific to their interactions
with their depressed mothers" (Pelaez-Nogueras et al., 1994, p.
358). With caregivers who are not depressed, these infants behave
normally. Human babies who are smart enough to realize that, just
because Mommy is depressed, it doesn't mean that everyone is
depressed, are also smart enough to realize that, just because Mommy
loves me, it doesn't mean that everyone will love me.
I predict that if behavioral genetic methods are used to control
for genetic differences among children, the correlations reported by
attachment researchers will disappear. It will be found that the
weak correlations between secure attachments to mother and the
success of the child's other relationships are due entirely to
genetic factors. Through their effects on the child's physical and
psychological characteristics, genes have an indirect influence on
all the child's relationships.
Very few aspects of social behavior are unaffected by inherited
characteristics -- that's why I like to look at language and accent,
which are free of genetic complications. When you eliminate the
contribution made by the genes, you get a clearer picture of how the
environment affects children's behavior. Outside his home, Joseph
spoke English without a Polish accent. The language he used with his
teachers and friends was unaffected by what he had previously
learned from his parents.
Much of the theorizing in developmental psychology is based on
the idea that children transfer learned behavior from one social
setting to another. Those who question this idea fight an uphill
battle. When Greta Fein and Mary Fryer (1995a) reported that
children who are taught sophisticated games of make-believe by their
mothers do not use these games when playing with their peers -- in
fact, they are no more sophisticated in their play with peers than
children whose mothers are devoid of imagination -- other
developmentalists attacked their conclusions (see Fein & Fryer,
1995b). When Michael Lamb and Alison Nash (1989) pointed out that
there was "little empirical support" for the notion that children
who are securely attached to their mothers have better relationships
with their peers, they were ignored (e.g., by Sroufe et al., 1999).
When I stood up in public and said that behaviors learned at home do
not transfer to outside-the-home contexts (Harris, 1999), a chorus
of developmentalists stood up and said "Yes they do!" and they
pointed to evidence from intervention studies.
The intervention studies the developmentalists had in mind are
those in which parents are taught more effective methods for
managing their obstreperous children -- methods that employ
reinforcement for good behavior and time-outs for bad behavior,
instead of yelling and hitting. In many of these studies, the
interventionists actually succeed in changing the parents' behavior,
at least temporarily. When this happens, the children's behavior at
home often improves as well. But the developmentalists claimed that
teaching parents better ways of managing their obstreperous children
also improves the children's behavior in school. If this were
the case, it would be a clear contradiction of the predictions
generated by my theory.
But it is not the case. I came to that conclusion after a long,
hard look at the published research. Rex Forehand, a leading
researcher in the field of parent-training interventions, came to
the same conclusion. In a review of twenty years of research in this
field, Wierson and Forehand (1994) reported that training can
improve the parents' behavior toward the child and the child's
responsiveness to the parents. "However," Wierson and Forehand
admitted, "research has been unable to show that the child's
behavior is modified at school" (p. 148). Likewise, a school-based
intervention can improve the child's behavior in school but will
have no effect on troublesome behavior at home (Barkley et al.,
2000; Grossman et al., 1997). Although some researchers claim to
have shown transfer of the effects of an intervention from one
context to another (e.g., Cowan & Cowan, 2002; Dishion &
Andrews, 1995; Forgatch & DeGarmo, 1999), I have examined these
studies and found them to have serious flaws (see Harris, 2000b;
The developmentalists believed that the evidence from
intervention studies was the best evidence against my theory -- the
best evidence that what children learn at home transfers to
outside-the-home settings. Instead, intervention studies provide
strong evidence in favor of my theory. Improving children's behavior
at home, by modifying their parents' child-rearing style, does not
improve their behavior at school. Likewise, improving behavior at
school does not improve it at home. It is possible to improve
children's behavior in both places, but to do that we need
home-based interventions and school-based interventions.
Correlations Between Behaviors in Different Contexts
Much of the work in the intervention field is based on the
theories of Gerald Patterson (1982). Patterson postulated that
antisocial behavior outside the home has its origins in the home.
Some parents, he said, use ineffective methods of discipline that
have the effect of reinforcing defiant or aggressive behavior. As a
result, their children behave even more defiantly or aggressively --
not just at home, but in the classroom, the playground, and the
streets. The bad behavior acquired at home spills out, and pretty
soon these children are a thorn in the side of society.
Developmentalists find this story so persuasive that they are
undeterred when their studies fail to support it. Thomas Dishion,
for example, is one of Patterson's followers. In 1994, Dishion and
his colleagues reported the results of a study of children's
"coercive" behavior -- the type of behavior (hostile, uncooperative,
or bossy) that is supposed to result from inadvertent reinforcement
by the parents. The researchers recorded instances of coercive
behavior under two conditions: when children were interacting with
their parents in the laboratory, and when they were interacting with
their peers on the playground. Behavior in the two social contexts,
the researchers found, was "only weakly correlated": the correlation
was .19 (Dishion et al., 1994, p. 260). If you tried to predict how
children would act with their peers on the basis of how they acted
with their parents, you would be wrong most of the time. Contrary to
Patterson's predictions, there were children who behaved in an
obnoxious manner with their peers even though they were reasonably
well-behaved with their parents.
But the correlation between behavior in the two contexts wasn't
zero -- it was .19 -- and that does require an explanation. I
provided one in The Nurture Assumption (p. 62): I explained
that the weak correlation between behavior with parents and behavior
with peers could be due entirely to genetic influences on behavior,
which were not measured in Dishion's study. A genetic predisposition
to be disagreeable or aggressive would presumably increase the
chances that a child will be coercive both with parents and with
peers. That explanation raises another question: if there are
sizable genetic influences on disagreeableness and aggressiveness,
and if these genetic influences cut across contexts, why is the
correlation between coercive behavior with peers and with parents so
There are two reasons. First, since children have different
experiences in different social contexts, even those who are
predisposed to be disagreeable or aggressive might not behave that
way everywhere they go. Consider, for example, a child who was born
with an aggressive temperament but who is small for his age and puny
in build. This child has learned from experience that he cannot
dominate his peers, so he has given up trying. His parents, on the
other hand, are intimidated by his outbursts of temper, so he
continues to behave in a coercive manner with them. Now consider
another child, also born with an aggressive temperament, but who is
big and strong for his age. He has been successful in his attempts
to dominate his peers and behaves in a bullying manner towards them.
But his parents also happen to be big and strong and aggressive
(heritable traits run in families), and they have little tolerance
for disobedience. He doesn't dare to defy them.
The second reason for the low correlation between behavior in
different social contexts is that different genes might be involved:
the genetic influences that predispose children to be coercive with
their peers might not be identical to the genetic influences that
predispose them to be coercive with their parents. Schmitz and her
colleagues (1996) studied activity level in 7-year-olds, obtaining
measurements of how physically active the children were in two
settings: at school (based on ratings by their teachers) and in the
laboratory (where they were tested individually by trained
observers). These settings differ in two important ways: the school
is a familiar environment, the laboratory is not; and at school, but
not in the laboratory, the children were observed in the company of
The teachers' and testers' ratings of the children's activity
level were only weakly correlated -- as it happens, the correlation
was .19 (just a coincidence -- don't read too much into it!). But
this study, unlike the study of coercive behavior, used behavioral
genetic techniques, and the researchers were able to estimate the
degree to which genetic and environmental influences contributed to
activity level in the two settings. They found that the correlation
between the two settings was due entirely to genetic influences on
the children's behavior. But genetic influences were also found that
didn't contribute to the correlation -- genetic influences that were
specific to the school or to the laboratory. One child might be very
active on the playground but sit still in the lab, whereas another
might fidget in the lab but avoid taking part in active games on the
The fact that activity level in a given setting is highly
heritable doesn't mean that a child will be equally active in every
setting. The differences between behavior in two social contexts --
like the similarities -- can be due to genetic, as well as
environmental, influences. Unless researchers use methods that
permit them to estimate the magnitude of these influences, their
results are uninterpretable -- to put it bluntly, worthless. The
explanations they give of their results are based on assumptions
that can neither be tested nor supported by their data.
Birth Order and Evolutionary Psychology
At school, children are surrounded by their agemates. At home,
children spend their time with people who differ from them in age,
with consequent disparities in size, strength, and knowledge.
Because age is a major determinant of power and influence among
children, birth order is important at home.
Birth order affects the way children behave with their siblings
and parents. The child with older siblings has to learn how to get
along with them because they are bigger and stronger than he is. The
child with younger siblings learns lessons in responsibility,
leadership, and, we hope, patience. The child in the middle is
landed with both jobs. These children will be treated differently by
their parents: firstborns are given more responsibility, laterborns
more affection (see "Why Can't Birth Order Explain the Differences
Between Siblings?" on this website).
At home, in the context of the family they grew up in, firstborns
and laterborns develop different patterns of behavior. But from the
standpoint of evolutionary psychology, it wouldn't make sense to
expect these patterns of behavior to generalize to social contexts
outside the family.
One of the principles of evolutionary psychology is that humans
and other animals behave differently toward kin and non-kin (Pinker,
1997). The ability to make this distinction has been reported for
species ranging from wasps to salamanders to woodpeckers to gorillas
(Pfennig & Sherman, 1995). An animal might be friendly toward a
member of its species that is perceived to be a relative, but attack
one -- perhaps even eat it -- if it is perceived to be a
nonrelative. It might mate with a nonrelative but reject the
overtures of a relative. Primates are good at making such
distinctions. Why should we expect human children to behave in the
same way toward family members and people outside the family?
To enable them to deal with a complex social life, evolution has
equipped children with massive brains. As Steven Pinker (1998) put
it in his Foreword to The Nurture Assumption, "Relationships
with parents, with siblings, with peers, and with strangers could
not be more different, and the trillion-synapse human brain is
hardly short of the computational power it would take to keep each
one in a separate mental account." According to my theory of
development, that's exactly what children do: keep each of their
relationships in a separate mental account. They don't generalize
from one to another because they don't have to -- they have the
brainpower to learn new rules and develop new expectations for each
new relationship -- and because it wouldn't be to their advantage to
Evolution designed childhood as preparation for the future.
Children have to prepare themselves to make their way in the larger
world, the world outside the family. In that world, family baggage
is more likely to be a hindrance than a help. Laterborns who have
learned to yield the right-of-way to their older siblings would be
handicapped if they went through life with the expectation that they
will always have to yield to others. Firstborns who have learned
that they can dominate their younger siblings could find themselves
in big trouble if they tried to act the same way with their peers.
But the evidence is clear: in the company of their peers, laterborns
are not more likely to yield, and firstborns are not
more likely to try to dominate, even at nursery-school age
(Abramovitch et al., 1986). Children who fight like cats and dogs
with their siblings are not more likely to have stormy
relationships with their friends (Stocker & Dunn, 1990). And the
peer relationships of only children are perfectly normal -- their
lack of experience with siblings does not disadvantage them in any
way in their dealings with their peers (Falbo & Polit, 1986).
Studying Birth Order by Making Direct Comparisons Between Siblings
As I said at the beginning, standard studies of adult personality
generally show no birth order effects at all (or effects of
negligible size that fail to be replicated in other studies),
whereas studies that involve judgments of or by family members
generally do show birth order effects. Ernst and Angst (1983) put
their faith in the studies that used standard methods and offered
several hypotheses (pp. 170-171) to explain why the
all-in-the-family method might produce spurious results. They
pointed out, for example, that judgments made by family members,
such as parents' descriptions of their children's personalities,
necessarily involve comparing people who differ in age: the
firstborn in the family is always going to be older than the
laterborn. People change as they grow older, and they change in ways
that correspond to the popular stereotypes associated with birth
order: they become more responsible, more conscientious, less
rebellious, less willing to take risks. If you compare an older
child to a younger one, or an adult to an adolescent, on the average
you will judge the older one to be more responsible, less
rebellious, and so on. Though the age difference between siblings
will eventually become unimportant, parents' ideas about their
children, and siblings' ideas about each other, may be based (at
least in part) on the days when they all lived under the same roof.
(See "Why Do People Believe in Birth Order Effects?", forthcoming on
This explanation might account for some of the birth order
effects obtained in all-in-the-family studies, but I think another
of Ernst and Angst's explanations is more important: they proposed
that the reported behavioral differences between firstborns and
laterborns might be specific to the family context. This explanation
is related to the first one, because the behavioral differences
between siblings are due in part to the age differences between
them. But it differs from the first explanation because it pertains
specifically to how people act in the presence of their parents and
siblings, and acknowledges that they act differently in other social
contexts. It doesn't rely on people's memories of how their children
or siblings behaved in childhood -- it doesn't have to, because they
still behave that way when they get together. The age differences
between siblings, differences in temperament and talents, and
differences in the way they are treated by their parents, all
contribute to the way people behave in the presence of these family
members. These patterns of behavior, though context-specific, are
persistent: with the right technique, they can be evoked and
assessed at any time in the lifespan.
Thus, personality judgments made by parents and siblings are not
wrong: it's just that they are necessarily based on how
people behave in the presence of their parents and siblings and
therefore do not give an accurate picture of how these people behave
in other contexts.
Consider, for example, a 1999 study by Delroy Paulhus and his
colleagues. The researchers asked their subjects to compare
themselves with their siblings, and their siblings with each other,
by indicating which member of their family was the academic achiever
and which was the most rebellious. The data revealed a significant
birth order effect: firstborns were most often nominated (or
nominated themselves) as the family achiever; laterborns were most
often named as the family rebel.
Undoubtedly, many people believe that their oldest brother or
sister is the family achiever and the youngest in their family is
the rebel. The question is, are these family stereotypes accurate?
This is an easy question to answer because there are voluminous
amounts of objective data on school grades, graduation rates, and
the like. The answer is no, these stereotypes are not accurate.
Studies that controlled for family size and social class have
demonstrated that firstborns do not, on average, make better grades
than laterborns; nor are firstborns more likely to go to college.
Laterborns are not more likely to rebel in childhood by
underachieving in elementary school, or to rebel in adolescence by
dropping out of high school (Blake, 1989; Ernst & Angst, 1983;
McCall, 1992). Ernst and Angst were right when they concluded that
the all-in-the-family method yields spurious results.
This doesn't mean, however, that we can't make direct comparisons
between siblings. In fact, if it's done right, making comparisons
between siblings is the most accurate and sensitive way to look for
birth order effects. In most birth order studies, each subject comes
from a different family, so the researchers have to compare
firstborns from one set of families with laterborns from a different
set. Differences between firstborns and laterborns due to unmeasured
differences between these two sets of families are hard to
distinguish from differences due to birth order. By comparing
siblings from the same family, researchers can focus on the
differences within families and filter out all the differences
This method was recently used to settle a long-standing question
in psychology: Are there birth order effects on intelligence? Joseph
Rodgers and his colleagues (2000) settled the matter by doing a
definitive study: they made direct comparisons between the IQ scores
of siblings in the same family. They found no birth order effects on
IQ -- none at all. Contrary to popular belief, firstborns are not
one whit smarter, on the average, than their younger brothers and
Both of these studies -- the IQ study and the one in which people
were asked to name the family achiever and family rebel -- made use
of within-family comparisons. But the methods used in these two
studies differed in important ways. To illustrate the differences,
I'll describe a study that used both methods at once.
The researchers, Kirby Deater-Deckard and Robert Plomin (1999),
were interested in aggressive behavior, and they studied pairs of
young siblings -- biological and adoptive -- from the same family.
The children's aggressiveness was rated both by their parents and by
their teachers; these ratings were made five times over a period of
several years, when the children were between the ages of 7 and 12.
On the average, parents judged the older of the two siblings to be
more aggressive than the younger one. According to the teachers'
ratings, however, there was no significant difference between the
These conflicting results are informative. Teachers' and parents'
ratings differ in two ways, as the researchers pointed out. First,
"Teachers are able to compare each child to a broader reference
group of children," whereas most parents lack this broader reference
group and tend instead to compare their children with each other.
Second, teachers can observe the behavior of a given child in "an
environment that provides opportunities for peer interaction,"
whereas "parents usually only observe children's behavior at home
and with other family members" (Deater-Deckard & Plomin, 1999,
Parents base their judgments largely on how their children behave
at home -- which means, in the case of aggressiveness, largely on
how they behave with each other. Because the older sibling can
inflict more damage than the younger one, the younger is more
motivated to avoid a physical confrontation and, if one occurs, more
likely to end up getting hurt. So the older sibling is seen by the
parents as being more aggressive. And, because most parents lack a
broader reference group to use as a standard of comparison and tend
to base their judgments on how a given child compares with his or
her siblings, parents tend to exaggerate the differences between
their children (Simonoff et al., 1998; Spinath & Angleitner,
1998). Thus, a child may be judged highly aggressive by her parents
if, in her behavior at home, she is more aggressive than her little
brother or sister.
Teachers, on the other hand, observe the child in the company of
others the same age, where the fact that she is the biggest one at
home gains her no advantage. Knowing how children of that age
typically behave, teachers have a more accurate standard of
comparison and can judge whether a given child is more or less
aggressive than average. Teachers' judgments are like IQ tests in
that respect: they are not affected by differences due to age. The
IQ score of a child or adolescent is computed by comparing his or
her performance on an IQ test with that of others of the same
age. The fact that older children have more knowledge and
cognitive sophistication than younger ones is corrected for by the
scoring system. That's why Rodgers and his colleagues were able to
compare the IQs of siblings in the same family: the measure they
used automatically controlled for the age differences between the
Implicitly or explicitly, parents' judgments of their children
involve making comparisons between them. In contrast, in the case of
the IQ scores and the teachers' ratings of aggressiveness, it was
the researchers doing the comparing. For each pair of siblings in
their study, Deater-Deckard and Plomin calculated the average
teachers' rating of aggressiveness for the older child and compared
it to the average teachers' rating for the younger one (each child
was rated five times by five different teachers). The averaged
ratings for older and younger siblings were virtually identical.
Though firstborns might be more aggressive at home, in the classroom
and on the playground they are no more aggressive than laterborns.
In order to find out how children behave in the classroom and the
playground, researchers either have to observe them in those contexts
or ask their teachers or their peers. Once they have this
information, they are free to make comparisons between siblings. As
long as it is only the researchers doing the comparing, and as long
as the measures they use aren't much affected by the age differences
between siblings, making direct comparisons between siblings is an
excellent way to study the effects of birth order.
This method has also been used with adults. Researchers have used
standard self-report questionnaires to look for birth order effects
on adult personality (Hauser et al., 1997) and social attitudes
(Freese et al., 1999). Comparing the responses of siblings from the
same family, they found no significant differences between
firstborns and laterborns.
An Exception to the Rule?
There is one type of birth order effect that can be detected in
the world outside the family; it involves "problem" teenage
behaviors such as tobacco and drug use and early initiation of
sexual activity. Ernst and Angst (1983) found that tobacco and drug
use were more prevalent among laterborns. Rodgers and Rowe (1988)
found that younger siblings become sexually active at an earlier age
than their older brothers and sisters.
Such findings can be attributed either to direct sibling
influence (the older sibling provides the younger one with drugs or
information, or the younger sibling imitates the older one's
behavior) or to the influence of the peer group to which both
siblings might belong (younger siblings often hang around with their
older siblings' friends). In either case, the younger sibling is
introduced at an earlier age to the temptations of teenage life.
Because earlier initiation of substance use is associated with a
higher risk of continued use (Grant and Dawson, 1997), the early
start can explain why substance use is more prevalent, all through
adolescence, among laterborns.
The hypothesis that early initiation of problem teenage behaviors
results from exposure to older peers is supported by evidence from
studies of kids who go through the physical changes of puberty at an
earlier age. The same problem behaviors -- early substance use and
early sexual activity -- are associated with early physical
maturation in girls (Stattin & Magnusson, 1990; Wilson et al.,
1994) and have been explained in the same way: early maturers are
more likely to hang around with kids who are older than themselves.
Thus, laterborns may get into trouble in adolescence if they hang
around with their older siblings and their older siblings' friends.
This birth order effect appears to be limited to problem teenage
behavior -- there's no evidence of lingering effects on personality.
Nor is there any evidence that behaviors acquired at home are being
transferred to outside-the-home settings -- the activities in
question almost invariably get started outside the home. Though
these activities no doubt contribute to the notion that laterborns
are rebels, they are rebels only in the sense that they do the same
things as their older siblings, but at an earlier age.
They don't, however, drop out of school at an earlier age (Blake,
1989). Dropping out of school isn't something that happens all at
once, in adolescence -- drop-outs have generally been performing
poorly in school for years. Since laterborns don't perform more
poorly in elementary school than firstborns (McCall, 1992), they
aren't at increased risk of dropping out. Why doesn't the influence
of their older siblings' peer group cause them to perform poorly in
elementary school? Because doing poorly in school isn't age-related,
the way problem teenage behavior is. Teenagers are much more likely
than younger kids to use drugs or have sex, but they're not much
more likely to do poorly in school. The older sibling may be a
better student or a worse one than the younger sibling, which means
that the influence of the older sibling's peer group may be either
good or bad.
Birth order affects the way we behave with our siblings and our
parents, and the way we feel about them. It may also affect the age
at which adolescents begin to do things their parents do not want
them to do. But birth order has little or no effect on adult
personality, when personality is measured by standard,
well-substantiated methods in studies that include all the necessary
controls. Effects of negligible size are occasionally reported in
such studies, but they have generally not been replicated. Thus,
birth order probably has no effect at all on adult personality.
The reason birth order does not affect adult personality is that
children discover at an early age that many of the behaviors they
acquired at home are irrelevant or counterproductive in other social
contexts. Fortunately, they are not compelled to retain these
behaviors -- they are free to acquire new patterns of social
behavior, better suited to their outside-the-family contexts.
Childhood is preparation for the future. As Erik Erikson observed
many years ago, the child has "no workable future within the womb of
his family" (1963, p. 259). If children are destined to leave
that family behind, why should they retain behaviors that were
useful within the family but are not useful outside of it?
I thank Stephen L. Black, William T. Dickens, Charles S. Harris,
Steven Pinker, and Robert Plomin for their helpful comments on
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