The human fetus develops within the mother’s body according to a relatively fixed time schedule, and fetal behavior (such as turning and kicking) also follows an orderly sequence, depending on the stage of growth. Premature infants who are kept alive in an incubator develop at much the same rate as infants who remain in the uterus full term. The regularity of development before birth provides a clear picture of what is meant by maturation.

Motor development after birth-using the hands and fingers, standing, walking-also follows a regular sequence. For example, activities, such as rolling over, crawling, and pulling up to a standing position, that prepare the child for walking occur in the same order in most children.

Unless we believe that all parents subject their offspring to the same training regime (an unlikely possibility), we must assume that growth processes determine the order of behavior. Not all children go through the sequence at the same rate; some infants are more than four or five months ahead of others. In standing alone or walking. But the order in which they go from one stage to the next is generally the same from one infant to the next.

Critical Periods in Development


1. Many skills follow a natural sequence of development. An infant is able to reach for an object before he can pick it up. We learn to walk before we run; we learn to speak words before sentences. Later skills build upon more basic ones in a fairly logical way.

2. The observation of such natural sequences of behavior has led some psychologists to suggest that development proceeds in orderly, definable stages. Certain behaviors, interests, and ways of thinking or coping with problems are characteristic of a certain stage of development, and these may change as the child progresses to the next stage.

Failure to deal adequately with the developmental problems at a particular stage may interfere with development at the next stage. The various developmental stages may interfere with development at the next stage. The various developmental stages proposed by several noted psychologists will be discussed in detail later.

3. Related to the notion of stages in development is the concept that there are critical periods during which both favorable and unfavorable circumstances have lasting and perhaps irreversible consequences. The concept of critical periods originated in the study of embryological development.


As the embryo grows, the various organ systems develop in a fixed time sequence, and each system has a critical period during which it is maximally sensitive to growth stimulation and maximally vulnerable to disruptive factors. The organ system that does not develop normally during its critical period does not get a second chance, because the focus of growth shifts to other systems.

Thus, if the mother contracts German measles during the first three months of pregnancy, the effect on the embryo depends on the exact time of infection. The infant may be born blind, deaf, or brain-damaged, depending on which organ system was in its critical phase of development at the time of the infection.

4. Critical periods have been identified in the postnatal emotional development of animals. There appears to be a critical period for taming dogs so that they will respond to human beings. Puppies were raised in a large field with tame mothers but without human contact.

They were removed at various ages to spend 10 minutes a day with a handler who was passive and waited for them to approach him. If this experience was delayed as long as 14 weeks the puppies had become wild dogs, fearful of human beings. The best time for taming was between five to seven weeks of age. When tamed at these ages, the dogs remained tame, as was seen when they were tested again 14 weeks later.


5. It is possible, but much harder to demonstrate, that the psychological development of the child has critical periods. It has been suggested that a child must form a satisfactory emotional attachment to an adult during the first three years of life in order to be capable of normal fractional relationships in later life (Bowlby, 1969). If this hypothesis is true, these years would constitute a critical period for the development of social relations.

6. A study of children born with sexual organs inappropriate to their true (genotypic) sex suggests that there is a critical period for sexual identification. These children (called pseudo hermaphrodites) can often be helped by surgery. If the sex-change operation takes place before the child is two there appear to be no personality difficulties; that is, a boy who has been initially raised as a girl can assume the male sex role and vice versa. In a child older than two, however, switching identity to the opposite sex usually causes serious problems in adapting to the new sex role (Money, et al., 1957).

7. The notion of a critical period in development implies that at a certain time during a person’s development he is optimally ready to acquire certain behaviour. If the appropriate experiences do not occur during this period, then the behavior can be learned later only with great difficulty, if at all. As we examine different areas of development we will look for evidence of critical periods.

Effects of Early Experiences


1. Because the child’s mastery of the movements necessary for sitting, standing, walking, and using hands and fingers follows such an orderly sequence, and because children in all cultures accomplish these skills at roughly the same age, motor development appears to be primarily a maturational process little influenced by the culture in which the child is reared. Although no special training is required for a child to walk at the appropriate time, a certain amount of environmental stimulation is necessary.

Children raised in institutions that are handled infrequently and given little opportunity to move about will sit, stand, and walk much later than normal. One study of an orphanage in Iran found that only 42 per cent of the children were able to sit alone at two years, and only 15 per cent could walk alone at age four (Dennis, 1960). Contrast these percentages with the norms given for home-reared children in Fig. 1.1 given below.

2. A few of the orphaned children may have been mentally retarded, but most were not. A comparative study of three different foundling homes in Iran showed a direct relationship between the amount of stimulation and experience provided and the children’s rate of motor development. Thus, although motor development is largely dependent upon maturation, experiences of moving about freely and being carried in different positions are also necessary.

3. Psychologists have done much research on the roles maturation and experience play in development. The most reasonable conclusion from the available data is that human development involves a continuous interaction between the organism and its environment.


No behavior develops solely because of maturation or solely because of learning. Certain brain and body structures must mature before a particular behavior can occur, but experience is required to develop the brain’s capacity to organize and process incoming information and to signal the appropriate response.