The Minnesota Study of Twins Reared Apart (MISTRA) is one of
the most important psychological studies of the last 50 years. It
began in 1979, at a time when it was widely believed that intelligence
and personality were almost infinitely malleable by the environment.
By the time the study ended 20 years later, it had played a key role
in overthrowing this dogma. It established beyond any doubt that genes
are crucial to who we are.
Long before MISTRA, it was well known that MZ twins acted very
similarly, but environmental theory held that this was because they
had grown up in the same environment and had been treated
similarly. Sometimes, however, twins have been separated at birth and
reared in different households. If separated MZ twins have similar
abilities and personalities, this cannot be because they had the same
family environment; it must be because of their genes.
Almost invariably, the reunion of identical twins was a joyful
event. Some spoke of the "ecstatic shock" of discovering someone so
similar. After just a short time together, most MZAs felt closer to
their twin than to adoptive siblings they had known all their lives.
During the evaluation week, twins completed about 15,000 paper-
and-pencil test items, and were examined for everything from gum
disease and tooth formation to heart function and blood composition.
There have been other studies of twins reared apart, but none that
gathered so much information. MISTRA data are still being analyzed
for research papers.
MISTRA yielded what amount to two different kinds of findings:
quantitative and impressionistic. The former come from personality,
intelligence, medical, and other testing, whereas the latter include
the almost eerie, unmeasurable ways in which MZA twins are alike.
The first twin pair MISTRA evaluated was particularly striking.
The two men met when they were 39, and found that both had been in
law enforcement but were now working as firemen. Both had loved
math in school and hated spelling. Both did woodworking as a hobby,
and their favorite vacation spot was Pas Grille Beach in Florida. One
had named his son James Alan and the other had named his James Al-
lan. They looked very much alike, had the same smoking habits, and
always held a beer can with a pinky under the can. Both had put on 10
pounds at the same age for no apparent reason.
Not all twins were so alike, but this book is full of astonishing
similarities. In one MZA pair, one twin was reared in Germany and the
other in Trinidad, and they had never met before they came to Minne-
sota for testing. When they arrived at the airport each was wearing a
light blue shirt with epaulettes, and wire-rimmed glasses. They both
collected rubber bands, which they wore around their wrists, and
washed their hands both before and after using the bathroom. Both
liked to startle people by sneezing loudly in elevators.
One pair of MZA women both wet the bed until age 12 or 13. When
they were teenagers they started having nightmares about the same
things: fishhooks and doorknobs. Both had problems with nightmares
for more than ten years.
A pair of female MZA twins from Australia found each other because
of a case of mistaken identity. They both worked as fashion buyers for
competing department stores, and a customer accused one of
moonlighting for the competition. They were both very elegant,
dressed with the same style and the same kind of jewelry, smoked the
same cigarettes, and had the same hairstyle, posture, tastes, and
One MZA pair of male twins were both fitness fanatics who ran their
own bodybuilding gyms. MZA twins generally have the same posture and
arrange their hands and legs in the same way while dizygotic twins do
Prof. Bouchard, who ran MISTRA, once had occasion to meet a man
who had run a smaller-scale MZA study in Denmark in the 1960s, and
asked him if he had found such astonishing similarities. The man
replied that he had, but he did not report them because was no way to
measure such similarities-and he was afraid no one would believe
Prof. Segal writes that it was "thrilling" to get to know MZAs and
discover how similar they were, but she, too, was frustrated because
it was not possible to measure or assess similarities in complex
behavior. She notes that when she interviewed MISTRA people to write
this book, many looked back with nostalgia on the excitement of their
discoveries. One researcher who administered intelligence tests to the
twins wished that he had filmed them taking the tests. As he wrote:
I sat quietly behind them. The strategies [for answering test
questions] were so different between pairs but within the MZA
pairs they were so similar. Both twins vocalized or turned
around or stared at the screen or solved the problems quickly. It
was amazing. I smiled to myself when I saw these things, think-
ing no one would believe me.
Of course, there were many findings that could be quantified, the
most obvious being intelligence. There is no better way to measure the
heritability of intelligence than to study MZA twins.1 Because their
environments are completely different-though not so different as to
include malnourishment or physical abuse-similarities in IQ can
have only genetic causes. Test results of such twins are often so
similar that it is like testing the same person twice.
The test results that certainly caused the most surprise were
measures of personality. At the time, it was common to assume that
personality was formed almost exclusively by family influence. It is
not; it is formed in about equal parts by genes and by what is called
"non-shared environment," or the micro-environment each person
makes for himself. Parents think they have a lot of influence over how
their children turn out, but they flatter themselves.
MZT twins (identical twins reared together) have very similar-but
not identical-personalities. People always assumed the similarities
came from growing up in the same environment. But MZA twins also
have very similar-but not identical-personalities, and there is no
detectable difference in the degree of similarity between twins who
grew up together and twins who grew up in different families sometimes
in different countries. The household, or the "shared environment,"
has very little effect on personality, at least by the time people are
Likewise, when biologically unrelated children are adopted and
reared in the same home, they may resemble each other slightly when
they are small, but as they grow up they become as different as com-
plete strangers. It is well known that shared environment can have an
early effect on IQ as well. "Virtual twins," or unrelated children of
the same age who grow up together, have a correlation of 0.3 for IQ at
age five, which declines to 0.11 at age 11, and to essentially zero by
And yet the personalities of MZA twins are not identical. Similari-
ties are due to genes, and the degree of similarity is a direct
measure of heritability. Dissimilarities must be the result of
environment. But remember: The similarities and dissimilarities are
about the same for MZA and MZT twins-that is to say, growing up in the
same household has little effect on personality-so what makes twins
dissimilar is the non-shared environment. This is different for all
people and not entirely understood, but is thought to consist of peer
groups, teachers, extracurricular activities etc.
Tests of the "big five" personality traits (openness to new experi-
ence, conscientiousness, extraversion, agreeableness, and neuroticism)
as well as results from the MMPI (Minnesota Multiphasic Personality
Inventory) also showed similar levels of heritability.
Needless to say, results of this kind infuriated egalitarians. The
great liberal goal is to invent "programs" that will make every child
grow up smart, law-abiding, hardworking, and happy. If intelligence
and even personality are under substantial genetic control, and
environmental influences come from micro-environments children make
for themselves, it does not leave much room for uplift. Harvard psy-
chologist Leon Kamin, who has devoted his life to trying to discredit
the concept of race and the idea that anything other than such things
as height and eye color can be heritable, practically accused MISTRA
researchers of fabricating data.
Unfortunately, a scientific legacy is not enough. The people who
make laws, run schools, and write editorials still act as though MIS-
TRA-and a hundred studies like it-had never been done.
Perhaps the most obvious example of this is the 2001 No Child Left
Behind Act. Passed by huge majorities in both houses, the law re-
quires that all normal students-those not in "special education"-
read and do math at a level of "proficient" or better by 2014. In
2002, the year after the law passed, only 36 percent of 12th graders
were "proficient" according to the National Assessment of Educational
Progress exam (NAEP), the most commonly used standard. Not sur-
prisingly, by July 2012, 24 states had already received federal
waivers from the law's requirements, and 13 more states had applied.
Anyone with the slightest understanding of the heritability of
intelligence knows that the goal of 100 percent "proficiency" was
At the official level, the United States is still mired in 1960s
illusions about universal human perfectibility. No one knows how long
it will take to shed those illusions, but at least among scientists,
MISTRA played a huge role in sweeping out the cobwebs.
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