Abnormal Psychology

Understanding Psychological Disorders: The Neuropsychosocial Approach 45

Twin and Adoption Studies

Twin studies compare some characteristic, or set of characteristics,

in two groups of twins, identical and fraternal. Identical twins have

basically the same genetic makeup, because they began life as a single

fertilized egg (or zygote) that then divided to become two embryos.

Such twins are monozygotic (mono-means “one”). However, even

identical twins do not have absolutely identical sets of genes: They

may differ in how often particular genes are repeated, which in turn

has effects on how the genes operate (Bruder et al., 2008). Fraternal

twins begin life as different fertilized eggs, and so are dizygotic (di-

means “two”). Fraternal twins are like any other nonidentical siblings

in terms of their genetic similarity: They have about 50% overlap in

the genes that vary among humans. When researchers compare the

characteristics of monozygotic twins and dizygotic twins, controlling

as much as possible for the environment, they can attempt to draw

conclusions about the relative contribution of genes to those char-

acteristics in that environment. For instance, such studies have sug-

gested that schizophrenia is about 50% heritable (Gottesman, 1991).

However, we must be cautious about such estimates: Not only can

identical twins have slightly different sets of genes, but they begin to

have different experiences before birth—in fact, one twin is usually heavier and

larger at birth, because of differences in the amounts of nutrients the two fetuses

receive in the womb (Cheung, Bocking, & Dasilva, 1995; Hollier, McIntire, &

Leveno, 1999).

Sometimes researchers try to discover the roles of genes and the environ-

ment in mental disorders by conducting adoption studies: They study twins who

were separated at birth and raised in different homes, then compare them to

twins who were raised in the same home. In addition, researchers also study

unrelated children who were adopted and raised together, then compare them

to unrelated children who were reared in different homes. But even in adoption

studies, it’s not easy to disentangle the effects of genes and the environment. The

reason is that genetic differences infl uence the environment—a relationship that

is characterized by the reciprocal gene–environment model. For instance, sup-

pose that a pair of twins has genes that lead them to be high-strung and very

active. Even if these twins are raised apart in different environments, their par-

ents may react similarly to them—trying to keep them calm and out of trouble,

which might mean that they wouldn’t be taken on family outings as often as

children who were less of a handful. Or perhaps their genes lead the twins to be

bright and ask a lot of questions. In this case, both sets of adoptive parents might

take them to museums and talk to them a lot. The point is that even in different

adopted households, genes can infl uence how twins are treated and what they

experience. Thus, although twin and adoption studies can be fascinating, their

fi ndings must be interpreted cautiously.

The problems with twin and adoption studies have led many researchers to

take advantage of recent technological advances in genetics: It is now possible

to assess, inexpensively and quickly (for many genes), whether a particular per-

son has a specifi c allele of a gene (Schena et al., 1995). Researchers have used

such techniques to attempt to fi nd associations between the presence or absence

of specifi c alleles and psychological disorders. For example, Rasmussen and col-

leagues (2006) found that people who develop schizophrenia relatively late in

life tend to have one particular allele of a certain gene, which may act either to

make them susceptible to late- developing schizophrenia or to delay the onset of

the disease in them until later in life. However, such research has proven diffi cult

because many genes contribute to most disorders, and they interact in complex

ways. Thus, any one gene is likely to have a relatively small role in most psycho-

logical disorders.

P S

N

Monozygotic twins

Twins who have basically the same genetic

makeup (although it may differ in how often

specifi c genes are repeated) because they

began life as a single fertilized egg (zygote),

which then divided into two embryos; also

referred to as identical twins.

Dizygotic twins

Twins who developed from two fertilized eggs

and so have the same overlap in genes (50%)

as do siblings not conceived at the same

time; also referred to as fraternal twins.

It is not easy to sort out the effects of genetics

from those of the environment by studying iden-

tical twins. Each twin has unique experiences

even before he or she is born: One twin may get

less nutrients or be exposed to more toxins while

in the mother’s womb; that is, there can be differ-

ent environments for the twins even before birth.

Paul Whitehill/Photo Edit