velop and most outgrow at an early age. Mostly anec-
dotal evidence of twin ESP typically focuses on one
twin feeling phantom pain when her co-twin is in-
jured. Beyond this intrinsic interest, twins are very
useful for science. Identical twins share the same ge-
netic code and thus are natural clones. How similar
or different are they to each other? How important
are genes and environment for development? Be-
cause identical twins share all of their genes, it is the
environment—rather than genetics—that accounts
for any differences between them. In a study design
that includes both identical and fraternal twins, a re-
searcher can estimate the relative influence of genes
and environments on behavioral differences among
people.
Identical, or monozygotic, twinning occurs when
one egg is fertilized by one sperm then splits after
conception into two genetically identical halves.
These twins share all of their genes and are the same
sex. Fraternal, or dizygotic, twinning occurs when two
eggs are released by the mother and fertilized by two
separate sperm. Fraternal twins share on average half
of their genes, just like typical siblings. They can be
the same sex or opposite sex. About two-thirds of all
twins are fraternal.
People differ from one another because of the
complex interaction between their genetic endow-
ment and their environment. Effects of genes and en-
vironments cannot be separated for individuals, but
they can be separated at the population level through
twin studies. Twins are the same age as each other,
and in the twins-reared-together design, they are
raised in the same home. Heritability is implied if
identical twins (who share all of their genes) are more
similar to each other than fraternal twins (who share
half of their genes) for the trait under study. Other
less common types of twin studies include the twins-
reared-apart design, which focuses on twins who were
adopted into separate homes when they were infants.
These twins do not share their environments so ge-
netic influences are thought to account for their simi-
larities. A more experimental approach is the co-twin
control design; one twin receives treatment while the
other twin is studied as the control. For example, one
twin may attend an after-school program to enhance
gymnastic ability and the other will not. Learning ef-
fects are examined by comparing the twins’ gymnastic
abilities before and after the program.
Twin studies yield heritability estimates. Herita-
bility is the proportion of the differences among indi-
viduals on a particular trait that are due to genetic
differences. For example, the heritability of child-
hood attention deficit hyperactivity disorder (ADHD)
is around 80 percent. Thus, most of the differences
among individuals on symptoms of ADHD are due to
genetic differences. On the other hand, the heritabili-
ty of childhood delinquency is approximately 20 per-
cent to 40 percent, suggesting that both genes and
environment account for individual differences in de-
linquency. There are several important limitations to
the heritability statistic that are often misunderstood.
Heritability describes the variance, or the differences
among people, in a particular population at that time.
It does not apply to the development of single indi-
viduals nor to differences between populations.
Heritability also can change at different points in
the lifespan as genes turn on and off and environ-
mental conditions change. Behavior that is heritable
is modifiable by environmental conditions to a great
degree. It is important to emphasize that genetic in-
fluences are not deterministic but rather increase the
probability of the behavior in some environments.
Early brain development, for example, follows a ge-
netic program with many opportunities for environ-
mental modification. Appropriate prenatal nutrition
is needed for proper dentition, and visual experience
is needed for proper sight.
Arnold Gesell was one of the first to use the twin
method to study early development. He noticed that
identical twins were very similar both physically and
behaviorally and used the identical co-twin control
method to examine the effects of training on physical
development. He studied a pair of twins, neither of
whom could climb stairs when they were forty-six
weeks of age. One twin was given daily practice and
encouragement to climb stairs, while the co-twin had
no stairs in his environment. After six weeks of prac-
tice, the trained twin could climb the stairs and the co-
twin could not. One week later, however, the co-twin
could also climb the stairs. Replicating this result in
several similar studies, Gesell demonstrated that
physical training can cause physical skills to appear
sooner but that identical co-twins who were trained
later performed the same after a relatively shorter pe-
riod of training. Gesell later became interested in in-
dividual differences and the individual’s role in
creating his or her own environment. At the begin-
ning of the twenty-first century, this dynamic view of
person-environment relationships was being studied
with newer sophisticated statistical techniques.
Contemporary twin studies have underscored the
ways in which the environment influences human be-
havior. Studies show that the majority of environmen-
tal influences on behavior act to make siblings
different from each other. This does not mean, how-
ever, that global family variables, such as parenting
style, are unimportant for the development of chil-
dren’s behavior. Common factors, such as parenting
style, can affect individuals differently and thus may
contribute to individual differences. The impact of420 TWIN STUDIES