Understanding Genetic Influence

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Chapter 9
Human Development
FIGURE 9 .1
Motor Development
When did you start walking? The left end
of each bar indicates the age at which 25
percent of infants were able to perform a
particular behavior; 50 percent of the babies were performing the behavior at the
age indicated by the vertical line in the
bars; the right end indicates the age at
which 90 percent could do so (Frankenberg & Dodds, 1967). Although different
infants, especially in different cultures,
achieve milestones of motor development
at slightly different ages, all infants—
regardless of their ethnicity, social class,
or temperament—achieve them in the
same order.
Walks
alone
Stands
alone well
Walks holding
onto furniture
Stands holding
onto furniture
Sits without
support
Bears some
weight on legs
Rolls
over
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Age in months
Understanding Genetic Influence
behavioral genetics The study of the
effect of genes on behavior.
Most developmental psychologists now accept Piaget’s idea that both nature and nurture contribute to development. Guided by research in behavioral genetics, the study
of how genes affect behavior, they explore how genes and the environment influence
specific aspects of development. Their studies have demonstrated that nature and nurture jointly contribute to development in two ways. First, nature and nurture operate
together to make all people similar in some respects. For example, nature influences all
of us to achieve milestones of motor development in the same order and at roughly
the same rate. But supportive nurture, in the form of proper nutrition and exercise, is
also necessary to allow normal maturation to unfold. Second, nature and nurture operate together to make each person unique. The nature of inherited genes and the nurture of widely different family and cultural environments produce differences among
individuals in athletic abilities, intelligence, speech patterns, personality, and many
other dimensions (Plomin et al., 2002; Spinath et al., 2004).
Behavioral geneticists are concerned with the differences between individuals or
groups of individuals, not with the characteristics of a single individual. Consider
height. Whether raised together or apart, identical twins (who have identical genes) are
much more similar in height than fraternal twins (who share no more genes than other
siblings) or unrelated individuals. This finding suggests that height is more strongly