ready" is aware of the validity of this formula. To the frustrated parent, it seems as though the infant is "just not trying." He can utter all the sounds that make up spoken words and can say things that are wordlike. Wanting to talk is certainly a factor, of course, but the point is that talking requires the maturation of a great many complexly interrelated muscles and controls, and hence cannot appear until the infant is neurologically and physiologically "ready."
Although growth is basic to all development, the term growth is usually applied to changes in size and complexity, whereas development refers to changes in character or function. Growth ceases with maturity, but development continues throughout the life span. It is difficult to observe the process of development, and our conclusions must be based on data gathered at different points in time.
At the point of conception, the basic patterns for the neurophysical development of the individual are set by genes: large and complex molecules found in chromosomes, which in turn are located in the nuclei of living cells. Half of the 46 chromosomes contained in each cell are contributed by one parent, and half by the other. Deoxyribonucleic acid (DNA) provides the chemical basis for genetic transmission, and ribonucleic acid (RNA) molecules are the "messengers" that determine the structure and function of body tissue. The complex process of rearranging and trading genes between chromosomes which takes place shortly after conception, results in a virtually infinite number of possible genetic combinations. Genetic effects are most obviously detected in physical traits common to families; shared behavioral traits are harder to pin down and may be caused by social inheritance. Research on genetic transmission of physical traits has focused on defects or abnormalities, like hemophilia or allergy to ragweed pollens. The chromosomal abnormality of the type known as, XYY for example, has been thought to predispose males to violent aggressive behavior, but review of research indicates that the risk has been exaggerated. Environmental stresses, such as are experienced by children raised by adults who mistreat them, are more likely causes of deviant behavior in later years.
Human beings are plastic organisms, in the sense that they can acquire an endless variety of behavior patterns. Hence it is difficult to determine whether individual and group differences are brought about by genetic causes or by environmental ones. The relative effects of these two sources have been studied through the observation of identical (monozygotic or MZ) twins and of unlike (dizygotic or DZ) twins. Such studies permit some control of genetic and environmental factors, inasmuch as MZ twins have identical genetic elements, whereas the