Understanding Psychology (10th Ed)

FIGURE 1
Characteristics infl uenced 
signifi cantly by genetic factors. Although 
these characteristics have strong genetic 
components, they are also aff ected by 
environmental factors.
Height Memory Shyness
Weight Intelligence 
Extraversion
Obesity 
Age of language acquisition 
Emotionality
Tone of voice 
Reading disability 
Neuroticism
Blood pressure 
Mental retardation 
Schizophrenia
Tooth decay 
Anxiety
Athletic ability 
Alcoholism
Firmness of handshake
Age of death
Activity level
Physical
Characteristics
Intellectual
Characteristics
Emotional
Characteristics
and Disorders
Study Alert
The nature–nurture issue is a 
key question that is pervasive 
throughout the fi eld of 
psychology. It explores 
how and to what degree 
environment and heredity 
produce their joint eff ects.
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 Module 
35 
Nature and Nurture: The Enduring Developmental Issue 
383
Determining the Relative 
Infl uence of Nature and Nurture
Developmental psychologists use several approaches to determine the relative infl u-
ence of genetic and environmental factors on behavior. In one approach, researchers 
can experimentally control the genetic makeup of laboratory animals by carefully 
breeding them for specifi c traits. For instance, by observing animals with identical 
genetic backgrounds placed in varied environments, researchers can learn the effects 
of specifi c kinds of environmental stimulation. Although researchers must be careful 
when generalizing the fi ndings of nonhuman research to a human population, fi nd-
ings from animal research provide important information that cannot be obtained for 
ethical reasons by using human participants. 
Human twins serve as another important source of information about the rela-
tive effects of genetic and environmental factors. If  identical twins  (those who are 
genetically identical) display different patterns of development, those differences 
have to be attributed to variations in the environment in which the twins were 
raised. The most useful data come from identical twins (such as Gerald Levey and 
Mark Newman) who are adopted at birth by different sets of adoptive parents and 
raised apart in differing environments. Studies of nontwin siblings who are raised 
in totally different environments also shed some light on the issue. Because they 
have relatively similar genetic backgrounds, siblings who show similarities as adults 
provide strong evidence for the importance of heredity (Sternberg, 2002a; Vitaro, 
Brendgen, & Arseneault, 2009).
Researchers can also take the opposite tack. Instead of concentrating on people 
with similar genetic backgrounds who are raised in different environments, they 
may consider people raised in similar environments who have totally dissimilar 
genetic backgrounds. For example, if they fi nd similar courses of development in 
two adopted children who have different genetic backgrounds and have been 
raised in the same family, they have evidence for the importance of environmental 
infl uences on development. Moreover, psychologists can carry out research involv-
ing animals with dissimilar genetic backgrounds; by experimentally varying the 
environment in which they are raised, they can determine the infl uence of envi-
ronmental factors (independent of heredity) on development (Petrill & Deater-
Deckard, 2004).
Developmental Research 
Techniques
Because of the demands of measuring behavioral change across different ages, devel-
opmental researchers use several unique methods. The most frequently used,