LINKAGES Psychological Research and Behavioral Genetics

37
Research Methods in Psychology
LINKAGES
Is behavior influenced by our
genes or by our environment?
(a link to Biology and Behavior)
behavioral genetics The study of how
genes and environments combine to
affect behavior and mental processes.
O
LINKAGES
ne of the most fascinating and difficult challenges in psychology is to
Psychological Research and
find research methods that can help
us understand the ways in which genes and
Behavioral Genetics
the environment—sometimes called nature
and nurture—combine to influence behavior
and mental processes (Moffitt, Caspi, & Rutter, 2005). Consider Mark and John,
identical twins who were both adopted at birth because their parents were too poor
to care for them. John grew up with a couple who made him feel secure and loved.
Mark went from orphanage to foster home to hospital and, finally, back to his
biological father’s second wife. In other words, these genetically identical people had
encountered quite different environments. Yet, when they met for the first time at
the age of twenty-four, they discovered similarities that went beyond physical
appearance. They used the same after-shave lotion, smoked the same brand of
cigarettes, brushed with the same imported brand of toothpaste, and liked the same
sports. They had joined the military within eight days of each other, and their IQ
scores were nearly identical.
How had genetic influences operated in two different environments to result in such
similarities? Exploring questions such as this has taken psychologists into the field of
behavioral genetics, the study of how genes and environments work together to
shape behavior and mental processes. They have already discovered that most behavioral tendencies can be influenced by many different genes but also by many environmental events and conditions, before and after birth. Accordingly, research in behavioral genetics aims to explore the relative roles of genetic and environmental factors in
creating differences among people in personality, mental ability, mental disorders, and
other phenomena. It also seeks to identify specific genes that contribute to hereditary
influences.
Some behavioral genetics research takes the form of experiments, mainly on the
selective breeding of animals (Suomi, 2004). For example, Stephen Suomi (1999)
identified monkeys whose genes predisposed them to react strongly or weakly to
stress. He then mated strong reactors with other strong reactors and mated weak reactors with other weak reactors. Within a few generations, descendants of the strongreactor pairs reacted much more strongly to stressors than did the descendants of the
weak-reactor pairs. Selective-breeding studies must be interpreted with caution,
though, because it is not specific behaviors that are inherited. What is inherited are
differing sets of physical structures and capacities that make certain behaviors more
or less likely. These behavioral tendencies are often very specific, and they can be
altered by the environment (Grigorenko, 2002). For example, when Suomi (1999)
placed young, highly stress-reactive monkeys with unrelated “foster mothers,” he discovered that the foster mothers’ own stress reactivity amplified or dampened the
youngsters’ genetically influenced behavioral tendencies. If stress-reactive monkeys
were placed with stress-reactive foster mothers, they tended to be fearful of exploring their environments and had strong reactions to stressors. But if equally stressreactive young monkeys had calm, supportive foster mothers, they appeared eager to
explore their environments and were much less upset by stressors than their peers
with stress-reactive foster mothers.
Research on behavioral genetics in humans must be interpreted with even greater
care. Legal, moral, and ethical considerations obviously prohibit the selective breeding of people, so most research in human behavioral genetics depends on correlational studies, not controlled experiments. These usually take the form of family
studies, twin studies, and adoption studies (Plomin, DeFries, et al., 2001; Rutter et
al., 2001).
In family studies, researchers look at whether close relatives are more likely than
distant ones to show similarities in behavior and mental processes. If increasing
similarity is associated with closer family ties, the similarities might be inherited.
38
1.8
Family and Twin Studies of
Schizophrenia
The risk of developing schizophrenia, a
severe mental disorder, is highest for the
siblings and children of patients with
schizophrenia and lowest for those who
are not genetically related to anyone with
schizophrenia. Does this mean that schizophrenia is inherited? These results are consistent with that interpretation, but the
question cannot be answered through
family studies alone. Environmental factors, such as stressors that close relatives
share, could also play an important role.
Studies comparing identical and nonidentical twins also suggest genetic influence,
but even twin studies do not eliminate the
role of environmental influences.
50
Percentage schizophrenic
FIGURE
Chapter 1 Introduction to the Science of Psychology
40
30
20
10
Relationship to schizophrenic person
Identical twin
Child
Nonidentical twin
Niece/nephew
Nontwin sibling
Grandchild
Spouse or
general public
For example, data from family studies suggest a genetic basis for schizophrenia, as
Figure 1.8 shows. Remember, though, that a correlation between variables does not
guarantee that one is causing the other. The appearance of similar disorders in close
relatives might be due to environmental factors instead of, or in addition to, genetic
ones. After all, close relatives tend to share environments, as well as genes. So family
studies alone cannot establish the role of genetic factors in mental disorders or other
characteristics.
Twin studies explore the nature-nurture mix by comparing the similarities seen in
identical twins with those of nonidentical twin pairs. Twins usually share the same environment and may also be treated very much the same by parents and others. So, if
identical twins—whose genes are the same—are more alike on some characteristics
than nonidentical twins (whose genes are no more similar than those of other siblings),
those characteristics may have a significant genetic component.
TWINS AND BEHAVIORAL GENETICS
Like other identical twins, each member
of this pair has identical genes. Twin
studies and adoption studies help to
reveal the interaction of genetic and environmental influences in human behavior
and mental processes. Cases in which
identical twins who have been separated
at birth are found to have similar interests, personality traits, and mental abilities suggest that these characteristics
are significantly influenced by genetic
factors.