Child Development

tertwined and influence most aspects of human emo-
tion, behavior, and cognition in some ways. Given the
prevailing views in current psychology, most re-
searchers would agree that the violent acts committed
by these boys probably stemmed from an unfortunate
interaction among various hereditary and environ-
mental factors. Researchers, however, may disagree
on (1) the extent to which heredity and environment
each influences particular developmental outcomes
and (2) the way in which a mixture of hereditary and
environmental factors relate to each other. In other
words, the controversy involves the extent of contri-
bution as well as the nature of interaction among a va-
riety of genetic and environmental forces. How do
researchers address these issues?

Exploring Heredity and Environment:

Research Methods

Since as early as the 1930s, researchers have at-
tempted to estimate the contribution of hereditary
and environmental factors to various aspects of
human cognition, by comparing pairs of individuals
varying in genetic relatedness. These studies are
often called kinship studies, and twin studies and
adoption studies represent two of the most common
types of such studies. They have been extensively con-
ducted to estimate the heritability of a wide variety of
human characteristics.

Twin Studies
In traditional twin studies, monozygotic (identi-
cal) twins and dizygotic (fraternal) twins are com-
pared in terms of their emotional, behavioral, and
cognitive similarities. In the process of cell divisions
upon formation of a zygote, sometimes the resulting
cells fully multiply and produce two identical babies;
they are called monozygotic twins, since they come
from a single zygote and are genetic ‘‘carbon copies.’’
In other words, any genetic information concerning
physical and psychological predispositions should be
exactly the same for these twins.

By contrast, dizygotic twins develop from two sep-
arate zygotes, as a result of two eggs being fertilized
by two sperms independently. Consequently, the ge-
netic profiles of the resultant babies are similar only
to the extent that they share the same set of biological
parents. By comparing the correlations of a particular
dimension, such as intelligence test scores, between
identical twins and those between fraternal twins, re-
searchers can theoretically compute the relative influ-
ences of nature and nurture on the dimension. For
example, Sandra Scarr reported an interesting find-
ing in the book Intelligence, Heredity, and Environment.
She found a correlation for IQ test scores of .86 for
identical twins and .55 for fraternal twins, indicating

These children—slumdwellers in 18th-century London—faced few

positive prospects as they grew to adulthood. Scientists and

philosophers have long struggled with the debate as to how much

environmental conditions, both positive and negative, influence

individual development. (Archive/Hulton Getty Picture Library)

that identical twins’ scores are more like one another

than those of fraternal twins. Some influence of he-

redity, therefore, is evident. If IQ scores were 100

percent genetically determined, however, the correla-

tion for identical twins would have been 1.00. In this

example, therefore, heredity appears to play an im-

portant, but not definitive, role in explaining the de-

terminants of what is measured through IQ tests.

In addition to these heritability estimates, re-

searchers also study concordance rates: the rates at

which both twins develop the same, specific character-

istics. The absence or presence of a particular mental

illness would be a good example. If both twins had

clinical depression in all pairs examined in a study,

then the concordance rate would be 100 percent for

this sample. On the other hand, if all twins in a study

had one individual with clinical depression and an-

other with no depression, then the concordance rate

is 0 percent. Reportedly, concordance rate for clinical

depression is reportedly about 70 percent for identi-

cal twins and about 25 percent for fraternal twins.

This appears to demonstrate a sizable genetic contri-

bution involved in the development of depression.

Despite scholars’ consensus that genetic contribu-

tions are not to be ignored, these correlational data

HEREDITY VERSUS ENVIRONMENT 183