Child-rearing practices and cultural norms also seem to play a part. A cross-sectional study of 288 normal full-term babies from the Yucatan peninsula in Mexico found that at 3 months of age these babies were ahead of American babies in motor skills. By 11 months, however, the Mexican babies were so far behind American babies that an American baby at the typical level of a Mexican baby might be considered neurologically impaired (Solomons, 1978). The differences again might reflect child-rearing practices. The Mexican babies were slow by American norms, but were not considered retarded under the standards of their own culture. The Mexican babies' delayed skills in moving about may be related to several conditions in their lives: as infants, they are swaddled, and so their freedom of movement is restricted; as older children, they continue to be restrained by being carried more, by sleeping in hammocks (which become net "cages," compared with the open space of a firm-mattressed crib), and by not being put on the ground to play (partly because of tropical insects and partly because of local beliefs about the dangers of cold floors). By the same token,

Mexican babies may be more advanced in manipulative skills, because, not having toys to play with, they discover and play with their fingers earlier than American babies.

The difficulty of coming up with environmental explanations for differences like these, however, is highlighted by the finding that Navajo babies—who are also swaddled for most of the day—begin to walk at about the same age as other American babies (Chisholm, 1983).
We also see cultural differences in childrens' pern -mance on tasks like those in the Denver Developmetr Screening Test, such as playing pat-a-cake, picking up i\; sins, and dressing themselves at a certain age. In one stuc (V. Miller, Onotera,  & Deinard,  1984), southeast Asi.: children did not do these things, but that did not indiar-abnormally slow development. In their culture, children c not play pat-a-cake, raisins look like a medicine they :•.¦ told to avoid, and they are not expected to dress themsek • until a later age than western children. We must be ca. tious, then, in applying the norms derived from one culUr-to children who are brought up differently.

Other  research   has  also  pointed  to  environmen:. causes for differences in gross motor skills. Children of tr • Ache in eastern Paraguay, for example, do not learn to v\ a • until 18 to 20 months of age—about 9 months later thi; American babies (Kaplan & Dove, 1987). This delay in,: stem from early child-rearing practices. Ache mothers r hibit their children from exploring in the forest by

pullir-their babies back to their laps when the babies begin • crawl away. These mothers supervise their babies ve-closely, not only because of the hazards of nomadic lit-but also because their primary responsibility is child rearr -rather than subsistence labor. It is possible that childrt whose mothers spend less time with them become ind> pendent sooner because their caretakers are less vigilar This may apply to American babies now, when day care prevalent and when children seem to be developing mo--quickly.

Even when motor development is slowed, childrt-often catch up, given a favorable environment. We see th with the Ache children, who in their early years show th-slowest motor development reported for any human grou; but who typically catch up later on. Ache 8- to 10-year-olc climb tall trees, chop branches, and play in ways that v-hance their motor skills. Development, then, may I-viewed "as a series of immediate adjustments to currc conditions as well as a cumulative process in which sir ceeding stages build upon earlier ones" (Kaplan & Dou 1987, p. 197).

Short-term experiments suggest that it is hard to spec up motor development (whether this would be desirable -yet another question), but these illustrations show how cer-tain child-rearing practices may advance or retard it. W see that there is no universal standard of what is best f<¦• children and that cultures encourage their children to develop along somewhat different lines. Before we adopt < ¦ condemn another culture's child-rearing practices, we ha\ • to ask, "What is best for our children at this time and in th -place?"

INFANCY AND TODDLERHOOD
ENVIRONMENTAL INFLUENCES ON MOTOR DEVELOPMENT
Human beings appear to be genetically programmed to perform many different activities such as sitting, standing, and walking. All these skills unfold in a regular, largely predetermined pattern, and children have to reach a certain level of physiological maturity before they can exercise them.
The environment also plays a role, influencing children's development through specific child-rearing practices. (See Box 5-3.) However, motor development does not appear to be affected by sex, place of residence, or parents' education (Bayley, 1965). When children are well nourished, receive good health care, have physical freedom, and are given the opportunity to practice motor skills, their motor development is likely to be normal (Clarke-Stewart, 1977). But an environment that is grossly deficient in any of these areas may retard development significantly, as can be seen in the

following classic study of orphans in institutions in Iran.

How Environment Can Retard Development
In two Iranian orphanages, the children were hardly ever handled by the overworked attendants. The younger babies spent practically all their time lying on the backs in the crib. They drank from propped bottles. They were never put in a sitting position or placed stomach down. They had no toys and were not taken out of bed until they could sit, without help (often not till 2 years of age, much later than the average American child). And once a sitting child was put on the floor, there was no child-sized furniture or play equipment. These children were retarded in their motor development because of the deficient environment, which initially kept them from moving around and provided little stimulation.

The children in a third orphanage were fed in the arms of trained attendants, were placed on the stomach and were propped up so that they could sit and had many toys. These children showed normal levels of motor development.

When the children in the first two orphanages did start to get about, they moved around in a sitting position, pushing their bodies forward with their arms and feet, rather than creeping on their hands and knees. Since they had never been placed on their stomachs, they had had no opportunity to practice raising the head or pulling the arms and legs beneath the body—the movements

needed for creeping. Also, since they had never been propped in a sitting position, they had not practiced raising the head and shoulders to learn how to sit at the usual age. However, their retardation appeared to be temporary. Older children in one of the two "poor" institutions, who presumably had also been retarded as toddlers, worked and played normally (Dennis, 1960).

Such severe levels of environmental deprivation are fortunately rare. But the environment does play a part in motor development, and the more deficient it is, the greater its effect can be.Can Motor Development Be Speeded Up?

Although (as we saw in Box 5-3) pervasive cultural practices can affect the rate of motor development, in limited experiments it has not been possible to train children to walk, climb stairs, or control the bladder and bowels earlier than usual.
In a classic experiment, Gesell (1929) examined one set of identical twins to study the effects of training babies to perform a number of motor activities. He trained twin T, but not twin C, in climbing stairs, building with blocks, and hand coordination. As the twins got older, twin C became just as expert as twin T Gesell concluded that "the powerful influence of maturation on infant behavior

patterns is made clear." Even though this study was conducted more than 5( years ago on only two infants, its conclusion still seems to stand. Children perform certain activities only when they are ready, despite the efforts of parent and psychologists.

Toilet training, for example, is often begun long before babies can control the necessary muscles. When a child is successful at a very early age, it is usualh because the parent is trained to recognize the child's readiness and can get the child to the potty in time. Before children can control elimination, they have to learn a great deal. Initially, elimination is involuntary: when an infant's bladder or bowels are full, the appropriate muscles open automatically. To control these muscles, children have to know that there is a proper time and place to allow them to open. They have to become familiar with the feelings that indicate the need to eliminate and they have to learn to tighten the muscles until they are on the potty, and only then to loosen them.