Identical Twins Are Not Genetically
Identical
Potential consequences for the Minnesota Twin Study
Published on November 25, 2012 by Berit Brogaard, D.M.Sci., Ph.D in The Superhuman Mind
When I was a child one of my favorite books was about a pair of identical twins who decided to
switch clothes. They looked so much alike that their parents had had to dress one in blue and the
other in green. The twin boys fooled their parents for a long long time. An obedient 3-year old, I
was thrilled by their ingenuity and boldness.
Though parents can usually tell the difference between their identical twins, grandparents,
teachers, neighbors and peers sometimes cannot. And for good reasons. Identical twins very
often look almost exactly alike. No surprise there, if identical twin share all of their DNA.
Recent research presented at the 2012 American Society of Human Genetics meeting, however,
suggests that identical twins may not be as genetically similar as hitherto suggested. Identical, or
monozygotic, twins come from the same fertilized egg. So, at some point during cell division
(before 14 days post-conception), identical twin embryos share virtually all of their DNA. During
early fetal development, however, identical twins undergo more than 300 genetic mutations, or
copy errors, on average. As human cells divide trillions of times during their lifespan, a few
hundred genetic mutations could lead to millions or trillions of genetic differences in the DNA of
identical twins over the years. Chemical factors can furthermore activate or suppress gene
expression, which means that the same subset of genetic material can lead to the formation of
different proteins.
The results, which were presented by McGill University epidemiologist Rui Li, could have drastic
consequences for what we know about the heritability of diseases, addictions, personality and
intelligence—or what is more popularly known as the nature versus nurture debate.
A good chunk of the information we have about whether traits are passed down from parent to
child through genes or whether they are a result of the environment comes from the Minnesota
Study of Twins Reared Apart. The Minnesota Twin Study is a project originally led by Minnesota
Professor of Psychology Thomas Joseph Bouchard, Jr. The initial project took place from 1979 to
1999 and consisted in periodical educational, psychological, medical and dental testing of
individuals in an extensive population of identical (monozygotic) and fraternal (dizygotic) twins
and their families.
Starting around 1990 Professor Bouchard and his team published numerous results from the twin
study project. The majority of the conclusions of the twin studies are based on answers to the
question of whether identical twins (who were thought to share all their genes) are more similar
than those of fraternal twins (who share an average of 50 percent of their genes). It was
concluded, among many other things, that identical twins are about 85 percent similar for IQ,
whereas fraternal twins are about 60 percent similar. This would seem to indicate that half of the
variation in intelligence is due to genes.
What, then, are the consequences of the recent discovery that identical twins are not completely
genetically identical for the results of the Minnesota Twin Study? The answer to that question
depends on how different identical twins are. Assuming that early fetal genetic mutations multiply
significantly as time passes, there may be reason to question some of the previous twin study
results. The main conclusions at risk are those that concern traits, diseases and conditions that
we thought were a result of environmental influences.
Suppose a large number of pairs of identical twins separated at birth turn out to have very similar
IQs. Setting aside skepticism about IQ tests as a measure of intelligence, we should be able to
conclude that the environment does not significantly affect intelligence. The studies done by Li
and her colleagues do not affect this conclusion in any interesting way. However, suppose that
we find that a large population of pairs of twins separated at birth have very different IQs. Can we
conclude that intelligence isn't inherited? The answer to this question is "no." The reason for this
is that variation in intelligence may be grounded in genetic material that identical twins do not
share. So the environment could affect intelligence a lot less than we once thought.
Li and her colleagues used blood cells to calculate mutation rates. As blood cells divide slower
than most other cells in the body, the consequences for diseases (e.g., cancer) could be more
austere than we think. That is, some diseases that we believe are not genetic could be genetic
after all. Blood cells, however, divide faster than brain cells. So, genetic differences in brainbased traits, such as personality and intelligence, may not be as austere as differences
elsewhere.