Genetic And Constitutional
Factors In Behavior

Few questions have generated as much emotional controversy as the issue of the relative importance of genetic and environmental factors in shaping the physical and psychologic make-up of man. No one will argue the prepotency of genetic factors in determining whether a developing human embryo will be a boy or a girl or will have typically Caucasian or Negroid features. Similarly, no one will dispute the profound influence of certain environmental factors on the eventual structure and functioning of the child. Marked differences can result from a diet grossly deficient in Vitamin D, from living in a primitive rather than a civilized society, or having an abusive, alcoholic father rather than one who is not. However, the nature-nurture controversy arises when more subtle differences are considered. These are the kinds of differences between human beings and their environments that are most numerous and the most important for an understanding of human behavior and human ills.

Controversy over the importance of genetic and environmental factors affecting human characteristics continues to rage for several reasons. Human traits, especially those of a behavioral nature, are complex and difficult to measure reliably. It is not possible to settle problems of human genetics by rigorous laboratory experimentation, i.e., by planned and controlled breeding experiments. In addition, there is a natural human tendency to oversimplify the complex, to seek a single global solution to a broad question, to ask, for example, whether a particular characteristic is mainly genetically determined or the product of environmental factors. However, the roles of genetic and environmental factors vary with the characteristic under consideration, the genetic nature of the population, and the environmental circumstances under which it now exists. For example, we might think of differences in height within a given population to be largely hereditary in nature. Children of the same age in a New York City orphanage might have very similar environments such as diet, exercise, and so forth. Differences in height might, therefore, be ascribed largely to differences in their European ancestry. Descendants of Swedes might be taller on the whole than descendants of Spaniards. On the other hand, the children of Spanish extraction in the New York orphanage might be taller on the whole than children in a poor community in Spain in which dietary deficiencies are common. Here the difference in average height of the two populations is more related to environmental factors. Even in this example, of course, many confounding factors come to mind. The separation of genetic from environmental factors in the case of complex patterns of behavior is even more difficult. However, these are just the situations which are most pertinent to an understanding of human behavior and most relevent to psychiatry.

Genetically determined characteristics may influence human behavior in many ways, some directly and some mediated by the physical or social environment. This point has been developed by Dr. Anastasi of Fordham University (1).* At one end of the continuum there are genetically determined defects which seriously limit some area of development of the individual and are little modified by environmental influences. For example, some forms of severe mental deficiency are related to genetically determined gross structural defects in the brain. This is to be contrasted, however, with the intellectual lag often seen in congenitally deaf children. Here the limitation of social interaction, so dependent on verbal communication, the slowness of language development, and the special problems of adjustment to school may lead to retarded intellectual development. In this case the intellectual limitations are not themselves inherited, but result from another inherited deficit, deafness, in interaction with other phases of development of the individuals. A still different mode of interaction is seen in the child afflicted with severe epilepsy of hereditary origin. Such a child's intellectual development may be retarded because of frequent absence from school, a poor adjustment in school, and inferior social adjustment in general. In addition, frequent uncontrolled seizures may themselves produce some intellectual deficit from brain damage. Finally, heredity may influence behavior through the mechanism of social stereotypes. For example, a child's sex, the pigmentation of his skin, and his bodily physique may greatly influence his intellectual development through the mediation of social attitudes of the family and the community toward him. Thus the determinants of intellectual development and apparent native intelligence are greatly influenced by genetic factors in complex, multiple ways.