Understanding Psychology (10th Ed)

 PHYSICAL DEVELOPMENT 
Children’s physical growth provides the most obvious sign of development. During 
the fi rst year of life, children typically triple their birthweight, and their height 
increases by about half. This rapid growth slows down as the child gets older—think 
how gigantic adults would be if that rate of growth was constant. From age 3 to the 
beginning of adolescence at around age 13, growth averages a gain of about 5 pounds 
and 3 inches a year (see Figure 4). 
The physical changes that occur as children develop are not just a matter of 
increasing growth; the relationship of the size of the various body parts to one another 
changes dramatically as children age. As you can see in Figure 5, the head of a fetus 
(and a newborn) is disproportionately large. However, the head soon becomes more 
proportional in size to the rest of the body as growth occurs mainly in the trunk 
and legs. 
DEVELOPMENT OF SOCIAL BEHAVIOR: 
TAKING ON THE WORLD 
As anyone who has seen infants smiling at the sight of their mothers can guess, at 
the same time that infants grow physically and hone their perceptual abilities, they 
also develop socially. The nature of a child’s early social development provides the 
foundation for social relationships that will last a lifetime. 
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398 Chapter 
12 
Development
 Attachment, 
the positive emotional bond that develops between a child and a 
particular individual, is the most important form of social development that occurs 
during infancy. The earliest studies of attachment were carried out by animal ethologist 
Konrad Lorenz (1966). Lorenz focused on newborn goslings, which under normal 
circumstances instinctively follow their mother, the fi rst moving object they perceive 
FIGURE 4 
The average heights and 
weights of males and females in the 
United States from birth through age 20. 
At what ages are girls typically heavier 
and taller than boys?
 
(Source: National 
Center for Health Statistics, 2000.)
72
30
32
34
36
38
40
42
44
46
48
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58
60
62
64
66
68
70
Height (in.)
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
30
25
20
35
40
45
50
55
60
65
70
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80
85
90
95
100
105
110
115
125
135
145
155
120
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160
Weight (lb)
Age (years)