Why Do People Believe that Birth Order Has
Important Effects on Personality?
by Judith Rich Harris
In the first two essays on this website, I summarized the
results of more than half a century of research on birth order. My
conclusion was that birth order does not have noticeable effects on
adult personality. The question I must deal with now is: Why do most
people -- including most psychologists -- continue to believe that
birth order does have important effects on adult personality?
In this essay I will discuss several sources of the belief in
birth order, including people's subjective impressions based on
their own personal experiences, flawed or misleading research, the
tendency for research to be published and publicized only if it
supports the belief in birth order, the impressions psychotherapists
get from listening to their patients, and biological factors.
It isn't enough, however, to explain why people came to hold a
particular belief. I also have to explain why they cling to it so
tenaciously in the face of disconfirming evidence.
Impressions Based on Personal Experiences
As I explained in the first two essays on this website, birth
order effects unquestionably do exist within the context of the
family of origin. When they are with their parents and siblings,
firstborns behave differently from laterborns -- even in adulthood.
But most people don't spend their adult lives in their childhood
home; they see their parents and siblings only occasionally. In my
two previous essays (and in The Nurture Assumption), I
presented evidence showing that the patterns of behavior acquired in
the childhood home don't affect the way people behave outside that
home -- even in childhood.
Now ask yourself this: Whose birth order do you know? Certainly
you know your own, those of your sisters and brothers, and those of
your children. In all likelihood you also know the birth orders of
other relatives, your husband or wife, your close friends, and the
children of your friends and neighbors.
Of the people in your life, these are the ones you are most
likely to have seen in the presence of their parents and siblings.
To put it another way, you are unlikely to know the birth orders of
people whom you've never seen interacting with their parents and
You can see birth order effects in action when you observe
people (including your own brother or sister) interacting with their
parents or siblings (including you). So the question now becomes:
Why do you assume that people behave in a similar way when they're
not with their parents or siblings?
The answer is that humans have a strong tendency to
underestimate the effects of context on other people's behavior.
This tendency is well-known to social psychologists, who call it the
fundamental attribution error. In accounting for people's
behavior, we tend to give too much weight to what we assume are
enduring aspects of their personality and too little to the
exigencies of the situation we see them in. It's easiest to make
this error about someone we are meeting for the first time: if the
newcomer acts grouchy, we tend to peg her as a grouchy person, even
if we know that she got tied up in traffic, couldn't find a parking
space, and had her jacket splattered with mud. In a classic study
(Napolitan & Goethals, 1979), subjects were informed in advance
that, for the purposes of the experiment, a research assistant had
been instructed to act friendly (or unfriendly). Nevertheless, they
attributed her behavior to genuine friendliness (or
But the fundamental attribution error can also affect the way we
characterize people we see again and again, if we often see them in
the same setting. David Myers, a professor of social psychology at
Hope College, Michigan, is well aware that small differences in the
situation can have big effects on people's behavior, and that
students are likely to be more alert in the evening than early in
the morning. And yet, he reports, "Even knowing the effect of the
time of day on classroom conversation, I find it terribly tempting
to assume that the people in the 7:00 p.m. class are more
extraverted than the 'silent types' who come at 8:30 a.m." (Myers,
1996, p. 80).
Even knowing the effect of family get-togethers on people, you
might find it terribly tempting to conclude that Jeff -- who acts
bossy and superior whenever he's around his younger brother -- is a
domineering person with an inflated ego; and that Sarah -- who acts
clueless and irresponsible whenever she's with her parents and older
siblings -- is basically an airhead. In fact, even if you were aware
that Jeff acts bossy and superior only when he's with his
younger brother, or that Sarah acts clueless and irresponsible
only when she's with her parents and older siblings, you
might still find it terribly tempting to peg Jeff as an egotist and
Sarah as an airhead. The reason is the nurture assumption -- the
assumption that, as birth order psychologist Kevin Leman put it,
"For a young child growing up, there is no greater influence than
his or her family" (1998, p. 24). The nurture assumption causes
people to believe that, when they see Jeff interacting with his
brother, or Sarah interacting with her parents and siblings, they
are getting a glimpse of the real Jeff or the real
Sarah. But if Jeff and Sarah spend only a small fraction of their
time interacting with their parents and siblings, and if they behave
differently (in all probability, better!) when they're not
interacting with their parents or siblings, isn't that
behavior a more accurate indication of the real Jeff and the real
The characteristics often associated with the various
birth-order positions -- the belief that firstborns are bossy,
aggressive, and achievement-oriented; that middleborns are
diplomats, able to get along with anyone; and that lastborns are
irresponsible and rebellious -- are descriptions of the way these
people behave within their family of origin. These behaviors are not
enduring aspects of their personality but are reactions to the
social context in which they find themselves; in other contexts,
they behave differently. As I explained in my first essay (see "Why
Are Birth Order Effects Dependent on Context?" on this website), the
evidence shows that when children go to school or to the playground,
they leave behind the strategies they learned at home for dealing
with their siblings. The firstborn who is bossy or aggressive with
his younger siblings behaves differently with his peers: outside the
home, firstborns are not bossier or more aggressive than laterborns,
even in childhood (Abramovitch et al., 1986; Deater-Deckard & Plomin,
1999). Firstborns are often seen as high achievers by their
parents and younger siblings, but in school they perform no better,
on the average, than laterborns (Blake, 1989; Ernst & Angst,
At home, children must deal with siblings who differ from them
in age; outside the home, children in developed societies spend
their day in the company of their agemates. At home there is only
one firstborn but outside there are many; the fact that a child is
older than his siblings, and can dominate them or serve as their
teacher, doesn't matter when he's in school or on the playground.
All that matters is how he compares -- in size, strength,
intelligence, knowledge, and so on -- with other children his own
age. Even if he goes out into the world assuming that it will just
be a larger version of his home (and the evidence suggests that he
makes no such assumption), he will soon find out that he is wrong.
Behaviors that worked at home will not work in the wider world
Various personality traits have been ascribed to firstborns and
to laterborns, but there is one theme that crops up again and again:
the description of firstborns can be summed up as "more mature."
They're more responsible, more adult-like in their behavior, take
life more seriously. Well, firstborns are more mature than
their younger siblings, for the simple reason that they're older!
Although the age difference between siblings becomes less
important as they get older, I suspect that popular ideas about
birth order are based largely on comparisons between siblings (such
as the offspring of friends and neighbors) who are still living at
home, which means that an older child is being compared to a younger
one. Popular ideas about birth order are also influenced by people's
thoughts and feelings about their own siblings. But people's
thoughts and feelings about their own siblings date back to the
years when they lived together under the same roof and saw each
other every day -- a time when an age difference of two or three
years meant a big difference in maturity.
Resistance to Disconfirming Evidence
When people believe something, their belief influences their
perception of new information. Confirmation bias is the
tendency to seek, notice, and remember evidence that confirms one's
belief, and to ignore, forget, or explain away contrary evidence.
It's a universal human failing, demonstrated over and over again in
laboratory studies and in real life (Myers, 1996). In one
experiment, for example, subjects were asked to evaluate two
(fictitious) research studies on the deterrence effect of the death
penalty: one supporting, the second disconfirming. The subjects who
started out believing in the death penalty readily accepted the
first study but were highly critical of the second. Those who were
opposed to the death penalty accepted the second study but found
flaws in the first. Regardless of their prior belief, all the
subjects ended up even more convinced that they were right --
despite the fact that they had all been exposed to exactly the same
evidence (Lord, Ross, & Lepper, 1979).
When faced with mixed evidence, people focus on the parts that
confirm their beliefs and ignore the parts that don't. A believer in
birth order who didn't happen to know the birth order of Bill Gates,
the founder of Microsoft, might think he was a firstborn, because of
his leadership ability and drive to succeed. A believer in birth
order who happens to know that Bill Gates is the second of three
siblings will focus on some other aspect of his personality, such as
his openness to new ideas or his loyalty to old friends. Because a
wide variety of personality characteristics have been attributed to
the different birth orders (laterborns, for example, have been
described as rebellious, easy-going, cooperative, competitive,
irresponsible, shy, sympathetic, open-minded, adventurous,
affectionate, and lacking in self-confidence), most people can find
something in the list that fits them -- just as most people can find
something pertinent in their astrological forecast in the newspaper.
Of the three or four vague and cryptic statements in an astrological
forecast, we pay attention only to the one or two that seem to apply
If the disconfirmatory evidence can't be ignored, it can be
explained away. Kevin Leman, who accurately describes himself as
"North America's 'pop' birth order psychologist" (1998, p. 13),
believes that firstborns are meticulous, well-organized
perfectionists, but was undismayed when he encountered a firstborn
with a messy desk. "Your desk is sloppy," he observed, "but can you
find what you need on it?" "Of course," was the firstborn's reply.
Satisfied, Leman went on to speculate: "My guess is you are
something of a perfectionist and perfectionists are known for having
sloppy desks as a means of covering their discouragement for not
always having life go just the way they want it" (1998, p. 93). It's
a win-win setup for Leman: If a firstborn has a neat desk, it's
because he's a perfectionist, and that confirms Leman's theory. If a
firstborn doesn't have a neat desk, it's because he's covering up
his discouragement, and Leman's theory is again confirmed.
Is there any disconfirming evidence that Leman can't explain
away? On one occasion he went out on a limb and guessed that Bryant
Gumbel, co-host of the Today Show at the time, was a firstborn. When
he was told that Gumbel actually has a brother three years older, he
explained it this way: "Clearly, there had been a 'role reversal' in
the Gumbel family. Bryant had taken over as the 'firstborn' while
his brother, who today is a broadcaster and certainly no slouch in
his own right, became 'number two' " (Leman, 1998, p. 30).
The idea of sibling role reversals is a useful escape clause for
psychotherapists who are followers of birth order theorist Alfred
Adler (1870-1937). According to Adlerian theory, it's
psychological birth order, not actual birth order, that
matters: "A child's birth order position may be seized by another
child if circumstances permit." This explains the firstborns who act
like secondborns and vice versa. But what about the people who don't
fit any of the stereotypes? The Adlerians have an answer for that,
too: "Birth order is sometimes not a major influence on personality
development" (Stein, 2001).
There is no question that siblings differ from one another in
personality. But, as I explained in my previous essay (see "Why Can't
Birth Order Account for the Differences Between Siblings?" on this
website), two children reared in the same home don't differ more
than two reared in separate homes. The evidence does not show that
growing up together causes children to develop distinctive
characteristics that make them less alike. Birth order theories,
including those that allow "role reversals," are based on the idea
that sharing a home creates or widens differences between siblings.
This idea is erroneous.
Most people not only believe in birth order: they also believe
that there is plenty of research to support their belief in birth
order. Indeed, many studies have found statistically
significant differences in personality between firstborns and
laterborns. On the other hand, many other studies -- especially the
largest and most rigorous ones -- have failed to find birth order
differences. Why the discrepancy? Sometimes it's just a matter of
chance: so many birth order studies are carried out that some of
them are bound to pay off. But sometimes it's the result of the
research method. Certain research methods can produce misleading
Making use of context effects.
Because birth order effects do exist within the family of
origin, the easiest way to demonstrate them is to put people back,
psychologically, into that context. Researchers can do this quite
easily: for example, they can ask their subjects to compare
themselves to their siblings, or they can assess their subjects'
personalities by asking their parents to describe them. Many of the
studies that reported significant effects of birth order on
personality have used such techniques. The results of these studies
are not inaccurate; the differences they report are real. But they
are misleading, because the researchers would like us to believe
that the results tell us something about how the subjects behave in
contexts outside the family -- in their careers, for example -- and
they do not.
Context effects are discussed in detail in my first birth order
essay (see "Why Are Birth Order Effects Dependent on Context?" on
this website), so I won't belabor this point here.
Failure to control for family size and socioeconomic status.
People from large families and people from small families
differ, on average, in at least three measurable ways: socioeconomic
status (SES), education, and IQ. Large families are more common at
lower SES levels. On the average, mothers who have many children
have less education and lower IQs than mothers of the same age with
fewer children. On the average, children who have many siblings have
lower IQs than children with few or no siblings, and they are less
likely to graduate from high school (Blake, 1989; Ernst & Angst,
1983, Rodgers et al., 2000).
Of a sibship of two (sibship size is the number of offspring in
a family), half are firstborns. Of a sibship of four, only a quarter
are firstborns. Small families produce a relatively larger
proportion of firstborns; large families produce more laterborns. A
firstborn plucked at random from the population is more likely to be
from a small family, a laterborn more likely to be from a large
Taken together, the facts in the previous two paragraphs imply
the following: a firstborn is more likely than a laterborn to come
from a higher SES family, to be born to and reared by intelligent
parents, to have a higher IQ, and to be better educated.
Any study that fails to take family size and/or SES into
account, and simply lumps together subjects from large and small
sibships, is likely to find differences between firstborns and
laterborns. But these differences are misleading: they are due to
the fact that, on average, the firstborns come from higher SES homes
and have higher IQs than the laterborns.
The best way to control for the effects of family size is to do
the birth order comparisons separately for each sibship size:
compare firstborns from sibships of two with laterborns from
sibships of two, firstborns from sibships of three with laterborns
from sibships of three, and so on. To do this properly requires
collecting data from a large number of subjects -- hundreds or,
better still, thousands. Small samples don't contain an adequate
number of subjects from each sibship size.
Nonetheless, small studies often make it into the news. Here,
for example, is a description of a study that got a lot of publicity
in June, 2001:
BIRTH ORDER AFFECTS CAREER INTERESTS, STUDY SHOWS
Columbus, Ohio - A child's place in the family birth order may
play a role in the type of occupations that will interest him or her
as an adult, new research suggests. . . . These results fit
into theories that say our place in family birth order will influence
our personality, said Frederick T. L. Leong, co-author of the study and
professor of psychology at Ohio State University. . . .
One of the strongest findings was the fact that only
children and first-born children tended to have more cognitive and
analytical interests, while later-borns were more artistic and
oriented to the outdoors. "Parents of only children may discourage
pursuit of physical or outdoor activities because they are more
fearful of physical harm to the child," Leong said. Parents may also
encourage only or first-born children to pursue interests that could
lead to a prestigious career like lawyer or doctor, he explained.
(OSU News, 2001).
Dr. Leong's conclusions were based on two small studies: one
with 159 subjects and one with 119 (Leong et al., 2001). That's not
nearly enough to control for family size. The results he reported on
career choice were exactly what you would expect if the firstborns
and laterborns in his samples differed, on average, in socioeconomic
class and intelligence.
There is likely to be a surplus of firstborns in any group of
eminent people -- famous scientists, political leaders, and so on.
This surplus is explained by the correlations between birth order,
family size, SES, IQ, and education. Even strong believers in birth
order effects admit that "Once eminence is controlled for family
size and social class, differences by birth order generally
disappear (Sulloway, 1996, p. 109). The same is true of IQ: once the
research method controls for family size and parental intelligence,
firstborns are no more intelligent than their younger siblings
(Rodgers et al., 2000).
In previous generations, another factor may have contributed to
the surplus of eminent firstborns. Before it became common for
parents to see that all their children received an education, the
offspring who was most likely to be educated was the firstborn son.
This educational edge helps to explain, for example, the slight
surplus of firstborns and only children among U.S. presidents.
(Fourteen of the 42 U.S. presidents -- 33% -- have been firstborns
or onlies. The expected percentage, based on historical family
sizes, is 23%, according to Somit et al., 1996.)
Another complication is that the proportion of firstborns in the
population -- and therefore the decision about whether the
proportion in a given sample is higher or lower than expected --
varies over time, due to changes in average family size. In U.S. and
Canadian populations, the proportion of firstborns was 27% in 1960,
39% in 1970, and 44% in 1984 (Somit et al., 1996).
Average SES and educational levels have also risen steadily over
time. These social changes may affect the outcome of a birth order
study even if the subjects are all the same age, because the parents
of a laterborn are likely to be older than the parents of a
firstborn (to put it another way, there's likely to be a bigger age
gap between a laterborn and his parents than between a firstborn and
his parents). Older parents grew up at a time when average SES and
educational levels were lower. Thus, the laterborns in a sample may
have come from lower SES homes than the firstborns, even if the
researchers controlled for family size.
That age gap between parents and their offspring -- the gap that
is larger for laterborns than for firstborns -- can also contribute
to the erroneous impression that laterborns are more rebellious.
During times of rapid cultural change, firstborns' ideas are likely
to be more similar to those of their parents, because they are
closer to their parents in age and thus were reared in more similar
"Birth order research seems very simple," the Swiss researchers
Ernst and Angst wryly observed (1983, p. xi). Its apparent
simplicity makes it a perennial favorite among students in search of
a science fair or thesis topic. In reality, this kind of research
requires sophisticated knowledge of statistics and sociology, as
well as a sizable pool of subjects.
Massaging the data.
Very few researchers go to the trouble of carrying out a study
unless they have a idea -- a belief or theory -- that they hope will
be supported by the results of the study. They are disappointed if
the results don't support their idea. They may do another study to
see if a change in the procedure will demonstrate the effect they
are looking for, or they may look more closely at the data to see if
they missed something. Some give up. A very small minority change
When lots of studies get done, some of them are bound to produce
statistically significant effects, just by chance. If
significant is defined in the usual way, as "only a 5%
probability that these results were due to chance," there is a 5%
probability that the results were due to chance. An honest pair of
dice can, on occasion, come up snake eyes four times in a row, due
to chance. Chance is probably the reason why a larger-than-expected
proportion of a sample of stripteasers, and a larger-than-expected
proportion of the scientists who founded SETI (the Search for
Extraterrestrial Intelligence), were firstborns (Skipper & McCaghy,
1970; Shermer, 2001). Taking off one's clothes in public
and searching for extraterrestials are not the sort of dull but
prestigious occupations we usually think of in connection with the
Chance can't happen in the absence of opportunity. The more
times you throw the dice, the more opportunities you give yourself
to produce a run of four snake eyes. Many studies of birth order
measure several different variables at once, thus increasing the
chances that significant birth order effects will be found in at
least one of them. Even if the researchers are interested in only
one outcome variable -- say, achievement motivation -- they may have
two or three different ways of measuring it. This doubles or triples
their chances of finding a significant effect.
Another way of increasing the chances of finding a significant
effect is to look for statistical "interactions": effects that turn
up in one subgroup of subjects but not in another. Many of the
significant findings reported in Ernst and Angst's (1983) survey of
the birth order literature were interactions, rather than "main
effects." Significant birth order effects might be found in male
subjects but not in females (or vice versa), or in high-SES subjects
but not in low-SES ones (or vice versa), or in subjects from large
families but not from small ones (or vice versa). Researchers who
are unsophisticated about statistics might see nothing wrong with
dividing up their data in as many ways as they can think of and
reporting only the one that pays off.
Earlier I said that confirmation bias -- the tendency to notice
and accept evidence that supports our beliefs and to ignore or
explain away contradictory evidence -- is a universal human failing.
Scientists are not exempt from this failing. Some sciences are more
advanced than others in finding ways to guard against researcher
bias. Medicine is way ahead of psychology in this respect.
Medical researchers use a variety of procedures to reduce the
chances that researchers' biases will affect the outcome of their
studies. In testing the effects of a new drug, for example, the
condition of the patients at the end of the trial must be assessed
by physicians who don't know which patients received the drug and
which received the placebo. Otherwise it would be too easy for the
physicians who make the assessment to err in the direction of their
prior bias. Trusting physicians to do their best to be objective
turned out not to be good enough; nowadays medical researchers use
double-blinding whenever possible.
Researcher bias is a psychological phenomenon; when it does
affect the results of a study, it usually happens without the
researcher's conscious awareness. One would think that
psychologists, of all people, would be alert to its dangers. Yet
procedures such as double-blinding are virtually never used in
psychology. Unlike medical researchers, psychologists simply trust
each other to do their best to be objective.
Some methods used to study birth order are particularly
susceptible to the effects of researcher bias. For example, there's
the question of whether biological birth order or "functional" birth
order affects personality. Someone who was biologically his parents'
secondborn child, for instance, would be a functional firstborn if
his older sibling died in infancy and he grew up as the oldest child
in the family. In this case it makes sense that functional birth
order is what matters. The problem is that researchers may be more
motivated to ask questions about an individual's functional birth
order if his biological birth order doesn't support their views. The
problem becomes more serious if the individual in question has been
dead for years: it takes a good deal of effort to ferret out the
kind of personal information needed to assess the functional birth
order of a historical figure. Jeremy Freese and his colleagues
(1999) have explained how researcher bias could have influenced the
results reported in Born to Rebel (Sulloway, 1996), a book
that is mainly about historical figures:
Sulloway's evolutionary explanation of birth-order effects
depends crucially on this distinction between "functional" and
"biological" birth order. However, the finding that biological birth
order is unimportant once functional birth order is controlled is
based on a test of only 29 biologically laterborn scientists who
were raised as "functional" firstborns (Sulloway 1996:465). Given
the high rates of infant mortality and other sources of childhood
instability in the eras predominantly represented in Sulloway's
sample, it is almost certain that more than 29 cases of laterborns
raised as "functional" firstborns exist among these scientists. This
raises the possibility that the "functional" birth status of some
sample members was investigated more thoroughly than others,
precisely because they otherwise would have been exceptions to
the study's general findings. (Freese et al., 1999, p. 213n;
italics in the original)
When a researcher re-examines results that go against his
theory, and accepts without question results that support it, the
researcher's biases can determine the outcome of a study. The same
thing can happen in an entire field, if a bias is widely shared. As
medical epidemiologist Alvan Feinstein observed,
If the results confirm what we believe, the customary human
tendency is to assume that they must be right. The research methods
need not be examined closely because there is no need to do so.
Having produced the right answer, the methods must also be correct.
Conversely, if the results are contrary to what we believe, the
research methods must be wrong, no matter how good they seem.
(Feinstein, 1985, p. 408)
An example is the debunking of the work of the late British
psychologist Cyril Burt by an American psychologist, Leon Kamin.
According to Kamin (1974), Burt faked some of the data he used to
support his view that intelligence is largely inherited. Whether
Burt was or was not a phony is not the issue here: the point is that
Kamin looked for anomalies in Burt's data only because he strongly
disagreed with Burt's conclusions: Kamin doesn't believe that genes
have any effect on intelligence. Researchers who fake their data (or
who use other means to nudge their results in the desired direction)
are much more likely to go undetected if the results happen to agree
with widely held beliefs.
More studies get done than get published. If the results don't
come out the way the researchers expected or hoped, they may never
see the light of day. Publication bias is the tendency for
statistically significant findings to get published and
nonsignificant findings -- or findings contrary to expectations --
to be tossed, with a shrug of the shoulders, into a file drawer or
the recycling bin.
The failure to publish no-difference results is a problem in all
fields of science, including medical research, where such results
can be vitally important. If a new drug, or a new surgical
procedure, is no better than the standard treatment, physicians and
their patients need to know that. (Would you want to undergo surgery
if it didn't increase your chances of getting better?) And yet, even
in medicine, studies that fail to find a significant difference are
less likely to be published (Ioannidis, 1998).
When positive findings are published and no-difference or
negative findings are not, it can lead physicians to the wrong
conclusions. To guard against this, medical researchers have devised
a statistical procedure called a funnel plot to test for
publication bias. They gather together all the published studies
they can find on a given topic and check to see if the smaller
studies -- those involving fewer patients -- tended to produce
larger effects (that is, more substantial differences between the
tested drug or procedure and the placebo). If larger effects are
found for smaller studies, it's an indication of publication bias
(Egger et al., 1997). The reasoning is that a big study is likely to
be published regardless of its outcome, whereas a small study that
doesn't produce a statistically significant effect may just be
tossed out. Thus, publication bias results in a dearth of small
studies producing no-difference or nonsignificant results.
Though I didn't realize it at the time, I demonstrated a
publication bias for birth order studies in Appendix 1 of The
Nurture Assumption (p. 372), when I showed a clear trend
among the birth order studies reviewed by Ernst and Angst
(1983): no-difference or nonsignificant results
were reported far less frequently for the smaller studies than for
the larger ones. There is no question that publication bias exists
in birth order research. It probably exists in all areas of
Psychologists sometimes use a simple calculation called the
file-drawer test to check for the effects of publication
bias. This test isn't used by medical researchers. I will have more
to say about the file-drawer test in my fourth essay ( "The
Mystery of Born to Rebel: Why Did Sulloway's Results Differ
from Those of Ernst & Angst?", forthcoming on this website).
As I said at the beginning, most psychologists do believe in
birth order. Their beliefs influence their choice of a research
topic, the methods they use in collecting and analyzing data, and
whether they decide to write up their results and submit them to a
journal. Journal editors and people who review manuscripts for the
journals also have their biases. Albert Somit and his colleagues
(1996), who looked for birth effects on political behavior and
concluded that there weren't any, discovered that it is very hard to
change people's minds:
The reasons are twofold. First, there is the inherent
non-rational nature of deeply held beliefs -- whether in the social
sciences or elsewhere. Second, and apparently almost equally
intractable, given our repeated experiences the past several years,
are the curious and often perverse stances taken by many social
science journal editors and reviewers with regard to birth order
research. The net effect is to make it extremely difficult, though
fortunately not completely impossible, to publish [our] research.
(Somit et al., 1996, p. vi)
Fortunately, not completely impossible. But the extensive work
by Somit and his colleagues has received little or no publicity,
whereas the small study by Frederick Leong got a fair amount of
attention. Getting research mentioned in a newspaper or magazine
article is far more difficult than getting it accepted by a journal.
I'll bet you've never seen a newspaper article with the headline
STUDY FINDS NO DIFFERENCE BETWEEN FIRSTBORNS AND LATERBORNS. And yet
there have no doubt been thousands of such studies.
Observations by Psychotherapists
Some of the most dedicated believers in the importance of birth
order are psychotherapists -- psychiatrists and clinical
psychologists who help people with their personal problems.
Therapists believe in birth order not only because it fits in well
with an idea that Freud made popular -- the idea that the
psychological problems of adults have their origins in things that
happened to them in early childhood -- but also because it fits in
well with what their patients tell them.
Why do people who seek psychotherapy talk so much about their
parents and siblings? Why does what they say tend to reinforce their
therapist's belief in the importance of these family
The first question is easy to answer. Thanks to Freud and his
followers, almost everyone who decides to go to a psychotherapist
takes it for granted that he or she will be expected to talk about
Mom and Dad. Since one of the ways Mom and Dad cause problems is by
apportioning love, attention, responsibility, and blame unequally
among their children, memories involving brothers and sisters are
also assumed to be relevant.
The answer to the second question is that talking about their
parents and siblings puts the patients back into the context of the
family they grew up in -- the one context in which birth order is
truly important. By encouraging their patients to relive their
childhood experiences with their parents and siblings, therapists
are evoking the feelings associated with being the firstborn, or the
lastborn, or a middle child, or an only child. What patients say
under these conditions is likely to reinforce the therapist's belief
in the power of family relationships to shape, and perhaps to
damage, a child's personality.
Whatever type of therapy they administer, most psychotherapists
feel that they're helping their patients. If their patients talk
about their childhood experiences with their parents and siblings
and end up feeling better, therapists interpret the improvement as
confirmation of their belief that the childhood experiences were at
the root of their patients' troubles. But most troubled people feel
better after a while whether or not they go to a psychotherapist.
Carefully controlled studies have failed to support the view that
talking about childhood experiences has therapeutic value. In fact,
the forms of psychotherapy that these studies have shown to be the
most effective are those that focus on people's current problems,
rather than their ancient history (DeRubeis & Crits-Christoph,
1998). Alan Stone, a professor of psychiatry at Harvard and a former
psychoanalyst, has admitted in print that he now questions the
premises he started out with: that every psychological disorder has
its roots in the experiences of infancy and childhood, and that
reconstructing these experiences is an essential part of
Our problem is that, in light of the scientific evidence now
available to us, these basic premises may all be incorrect. Our
critics may be right. Developmental experience may have very little
to do with most forms of psychopathology, and we have no reason to
assume that a careful historical reconstruction of these
developmental events will have a therapeutic
effect. . . . If there is no important connection
between childhood events and adult psychopathology, then Freudian
theories lose much of their explanatory power. (Stone, 1999, p.
Stone no longer puts his patients on a couch and asks them to
free associate; he has changed the way he does psychotherapy. "My
focus," he explains, "is almost entirely on the here and now, on
problem-solving, and on helping patients find new strategies and new
ways of interacting with the important people in their lives" (p.
39). He means the important people in their current
According to Ray Blanchard, a researcher at the Centre for
Addiction and Mental Health in Toronto, Canada, birth order is
related to sexual orientation: laterborn males who have older
brothers are more likely to be gay. "The best available data,"
Blanchard reports, "indicate that each additional older brother
increases the odds of homosexuality by 33%" (2001, p. 106). Older
sisters don't have this effect; nor do older siblings of either sex
increase the odds of homosexuality in a female.
The explanation of the connection between sexual orientation and
"fraternal birth order" is almost certainly biological. Blanchard
has proposed that it results from a reaction by the mother's immune
system to a male fetus, and that this reaction becomes stronger with
each pregnancy that results in a baby boy. Male fetuses produce
substances called male-specific Y-linked minor histocompatibility
antigens. These antigens, which are not normally present in a
female's body, get into the mother's bloodstream and are regarded as
foreign invaders by her immune system, which responds by producing
antibodies to them. "When that happens," Blanchard conjectures,
"these antibodies partly prevent the fetal brain from developing in
the male-typical pattern" (2001, p. 109). The result might be a
child who is less masculine in his interests than most boys and who,
as an adult, will be attracted to men rather than women.
I was intrigued when I encountered Blanchard's theory, because
it can explain a finding reported years earlier by the Swiss
researchers Ernst and Angst (1983). In addition to reviewing the
work of earlier birth order researchers, Ernst and Angst did a study
of their own -- to my knowledge, the largest study of personality
and birth order ever carried out: 7,582 subjects took part. Among
subjects from sibships of two, there were no differences between
firstborns and secondborns in any of the twelve dimensions of
personality that were measured. But for subjects from sibships of
three or more, Ernst and Angst found one statistically significant
difference: the youngest sibling was a little lower in masculinity
than the older ones.
Other investigators have described laterborns as less dominant,
less aggressive, and less conventional than firstborns (Sulloway,
1996). If laterborn males are less masculine than other men, that
could explain why they are less dominant and less aggressive. If
laterborn males are more likely to adopt a gay lifestyle, that could
explain why they are seen as unconventional. Thus, as researchers
Jeremy Beer and Joseph Horn (2001) have pointed out, some of the
characteristics attributed to laterborns may be due, not to their
experiences with parents and siblings, but to prenatal influences on
It's a good explanation but there may be nothing for it to
explain. The evidence for birth order differences in dominance,
aggressiveness, and conventionality is very weak -- perhaps
nonexistent, if you eliminate judgments by parents and siblings (see
my discussion of the study by Deater-Deckard and Plomin in "Why Are
Birth Order Effects Dependent on Context?" on this website). Not all
cases of male homosexuality can be attributed to the prenatal
effects of the mother's prior pregnancies: some gay men are
firstborns. In a typical birth order study of 1000 subjects or less,
the number of laterborn gay men is probably too small to affect the
When Deeply Held Beliefs Come Head to Head with Science
The knowledge that there is a lot of conflicting or
disconfirming evidence on birth order doesn't stop people from
believing in it. Here is a statement from psychologist Ginger Moore,
of Duke University:
"There's a lot of debate about that, a lot of disagreement,"
said Ginger Moore, assistant professor of social and health sciences
psychology. "There are some who think [birth order] is incredibly
important for personality development. There are others who believe
it's one factor of many other factors."
(Notice that Moore offers only two choices: either birth order
is incredibly important or it's one factor among many. She doesn't
even consider the possibility that it might have no effect at all.)
"There's contradictory evidence. Some reports say it influences
intelligence, other reports say it doesn't," she said, noting that
the same is true of personality. "I'm sure it influences all these
things, but I'm not sure in what direction." (Brumm, 2001)
Moore is sure that birth order influences personality; she's
just not sure in what direction!
Somit and his colleagues (1996, p. vi) referred to "the inherent
non-rational nature of deeply held beliefs" in their attempt to
explain the persistence of the faith in birth order. Deeply held
beliefs are often part of a culture and are extremely resistant to
The Yanomamö, natives of the Amazon rain forest, believe
that when someone in their village falls ill, it's due to witchcraft
-- an evil spell perpetrated by one of their enemies. From our
standpoint, this is clearly a non-rational belief. But the
Yanomamö support their beliefs with evidence, just as we do. A
stray gust of wind or the sudden cry of a bird is accepted as proof
of the presence of the enemy's malevolent spirit (Valero, 1970).
Our ancestors, too, believed in witchcraft. It was medical
science that persuaded them to give up the idea that illnesses are
caused by evil spells. Scientific evidence and reasoning can, in the
long run, win out over cultural myths. Call me an optimist, but I
believe that psychology will someday catch up with medical
I thank Ray Blanchard, Allen Esterson, Jeremy Freese, Charles S.
Harris, Marilyn Heins, David G. Myers, Albert Somit, and Frederic
Townsend for their helpful comments on earlier versions of this
Abramovitch, R., Corter, C., Pepler, D. J.,
& Stanhope, L. (1986). Sibling and peer interaction: A final
follow-up and a comparison. Child Development, 57,
Beer, J. M., & Horn, J. M. (2000). The influence of rearing
order on personality development within two adoption cohorts.
Journal of Personality, 68, 789-819.
Blake, J. (1989, July 7). Number of siblings and educational
attainment. Science, 245, 32-36.
Blanchard, R. (2001). Fraternal birth order and the maternal
immune hypothesis of male homosexuality. Hormones and
Behavior, 40, 105-114.
Brumm, M. (2001, October 19). Scientists doubt birth order
studies. The Chronicle -- The Independent Daily at Duke
University, Vol. 97, No. 39,
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,
DeRubeis, R. J., & Crits-Christoph, P. (1998). Empirically
supported individual and group psychological treatments for adult
mental disorders. Journal of Consulting and Clinical
Psychology, 66, 37-52.
Egger, M., Smith, G. D., Schneider, M., & Minder, C. (1997,
September 13). Bias in meta-analysis detected by a simple, graphical
test. British Medical Journal, 315, 629-634.
Ernst, C., & Angst, J. (1983). Birth order: Its influence
on personality. Berlin, Germany: Springer-Verlag.
Feinstein, A. R. (1985). Clinical epidemiology: The
architecture of clinical research. Philadelphia: W. B. Saunders.
Freese, J., Powell, B., & Steelman, L. C. (1999). Rebel
without a cause or effect: Birth order and social attitudes.
American Sociological Review, 64, 207-231.
Ioannidis, J. P. A. (1998, January 28). Effect of the
statistical significance of results on the time to completion and
publication of randomized efficacy trials. Journal of the
American Medical Association, 279, 281-286.
Kamin, L. J. (1974). The science and politics of IQ.
Hillsdale, NJ: Erlbaum.
Leman, K. (1998). The new birth order book: Why you are the
way you are. Grand Rapids, MI: Fleming H. Revell.
Leong, F. T. L., Hartung, P. J., Goh, D., & Gaylor, M.
(2001). Appraising birth order in career assessment: Linkages to
Holland's and Super's Models. Journal of Career Assessment,
Lord, C. G., Ross, L., & Lepper, M. (1979). Biased
assimilation and attitude polarization: The effects of prior
theories on subsequently considered evidence. Journal of
Personality and Social Psychology, 37, 2098-2109.
Myers, D. G. (1996). Social psychology (5th ed.). New
Napolitan, D. A., & Goethals, G. R. (1979). The attribution
of friendliness. Journal of Experimental Social Psychology,
OSU News (2001, May 24). Birth order affects career interests,
study shows. Ohio State University News Research Archive,
Rodgers, J. L., Cleveland, H. H., van den Oord, E., & Rowe,
D. C. (2000). Resolving the debate over birth order, family size,
and intelligence. American Psychologist, 55, 599-612.
Shermer, M. (2001). The borderlands of science: Where sense
meets nonsense. New York: Oxford University Press.
Skipper, J. K., & McCaghy, C. H. (1970). Stripteasers: The
anatomy and career contingencies of a deviant occupation. Social
Problems, 17, 391-405.
Somit, A., Arwine, A., & Peterson, S. A. (1996). Birth
order and political behavior. Lanham, MD: University Press of
Stein, H. T. (2001). Adlerian overview of birth order
characteristics. Alfred Adler Institute of San Francisco,
Stone, A. A. (1997, Jan/Feb). Where will psychoanalysis survive?
Harvard Magazine, 99(3), 35-39.
Sulloway, F. J. (1996). Born to rebel: Birth order, family
dynamics, and creative lives. New York: Pantheon.
Valero, H., as told to E. Biocca (1970). Yanoáma: The
narrative of a white girl kidnapped by Amazonian Indians (D.
Rhodes, Trans.). New York: Dutton.