Science in Christian Perspective



Heredity and Human Behaviour*

From: JASA 15 (June 1963): 48-51.                  

Human nature is conditioned by heredity, physical environment, and cuture. There are many different genetic causes of mental retardation, thus indicating that many genes are necessary for"normal" mental function. Recent research is directed toward the role of genetic factors in human behavior. Some implications of these observations are suggested.

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structure of hemoglobin. Many of these techniques are applicable to the investigation of human behavior (5). A standard method of approach is to select a group 

Man is different from animals in many ways but not in being free from the laws of heredity. Rife (13) lists the unique features of man as a larger brain which enables him to dominate all other animals, language ability so that he can communicate ideas with his fellow men, the use of tools, invention of the making and use of fire, and the ability to be educated and pass on the culture of one generation to the next.

What are the bases for these characteristics? Rife (13, p. 115) says, "Human beings are the products of biological inheritance, culture, and physical environment." Man's distinctive features are made possible by his genetic endowment, and certain genetic changes can drastically alter an individual's behavior pattern. One should not disregard the other two influences, however. Someone was asked, "Which is most important in making man, heredity, environment or culture?" and he replied, "Which is the most important leg of a threelegged stool?" It is the purpose of this paper, nevertheless, to deal primarily with the topic of inheritance.

Men are different from one another. They are hereditarily different. Their equality is before the law and their Creator, but not in their hereditary material. Hence, to study the nature of inheritance, we observe the variations in traits that appear among men and try to learn from such differences what are the hereditary factors.

The bearers of heredity are the genes, which act as a "code" to direct the formation of many different enzymes. These enzymes act upon the chemicals supplied through the embryonic environment, and the net result is a human body with certain characteristics or traits. We should realize that one gene may affect several characters, while several genes may have an effect on one character. Snyder (19, p. 8) states, "This point of view is especially important when it is realized that the environmental events intervening between gene actions and finished characters in man may range from such overt occurrences as trmuna and infection to the most subtle embryological, immunological, and psychological phenomena."

Much of our behavior is controlled by brain physiology. To the extent that the brain is affected in de velopment or function by genes, to that extent behavior is affected by heredity. To have a brain at all one must have "good" genes (and a suitable embryonic environment). If brain defect may be the result of "bad" genes, the converse is also true, that normal anatomy and physiology of the nervous system require "good" genes. We may learn about normal function by study ing malfunction.

The student of human genetics encounters certain problems. He cannot control mating nor standardize the environment There is a long interval between birth and reproduction, so that the "generation time" is about as long as the research life of the investigator. The large number of chromosomes (46 per cell) makes the locating of genes on specific chromosomes difficult.

Fortunately, important advances have been made possible recently by new techniques (4). Computer programs now permit complex calculations upon large masses of data. Human chromosomes can be identified readily in white blood cells from tissue culture preparations. Biochemical studies have led to the identification of genetic enzyme defects and genetic differences in the

Based partly upon a paper presented at the 1962 convention of The American Scientific Affiliation, Bethel College, St. Paul, Minn.

**Dr. Anderson is Assistant Director of the Dight Institute for Human Genetics, University of Minnesota; Dr. Mixter is Professor of Zoology at Wheaton College, Wheaton, IL.

of persons with a specific problem, such as schizophrenia. The frequency of the same condition among parents, brothers and sisters, and children can then be compared with what would be expected with different patterns of inheritance (1). The effects of family environment would often produce a different pattern, and this possibility can be tested as well. If the disorder being studied is a rare one, and if the affected children often have parents who are blood relatives, this information by itself is good evidence for a simple genetic cause.

The study of twins can also be revealing. Twins are called identical if they have the same sex and are similar in many traits, such as blood groups and eye color. They are labeled fraternal if they show differences. Identical twins are assumed to have the same inheritance from one egg and one sperm, but fraternal twins are the products of two sets of germ cells.

By comparing the occurrence of characteristics in identical twins with the occurrence in fraternal twins, we get a clue to traits which are inherited. If a trait occurs more often in both identical twins than in both fraternal twins, the trait is considered to have an hereditary basis. The text by Stern (20) has a very clear description of the logic behind such reasoning.

If a husband and wife have a child born without a brain, the likelihood of another brainless child is about three percent. However, it turns out that among twins "discordant identical twin pairs are almost the rule, that is, one affected and the other not. Even so, this does not dispose of a familial tendency; it merely shows that environmental peculiarity of some sort is necessary to evoke the reaction to which both twins must be susceptible" (10, p. 75).

A number of different twin studies have been carried out for schizophrenia, a mental disorder which occurs in about one percent of the population. Certain problems in the analysis of twin data have led to criticism of this approach, but Rosenthal (14) concluded that the total weight of the evidence from all studies still strongly favors the hypothesis of a genetic contribution to the etiology of schizophrenia. Among identical twins both members are affected in two-thirds of the pairs, while only one-sixth of fraternal twin pairs show the abnormality in both members.

Some may suspect that a similar environment has caused the appearance of mental illness in both of identical twins. But Stem (20, p. 580) reviews Kallmann's account of identical twins reared apart who both became schizophrenic. One worked in a factory and had an illegitimate child at the age of fifteen; the other was a housekeeper for a good family. The fact that both developed the disease in spite of different backgrounds suggests a constitutional weakness. Shields (17) studied a number of identical twins and found that in some personality traits those pairs reared apart were more alike than those reared together. It is possible that twins reared together develop somewhat different habit patterns in an attempt to establish a personal identity.

There are many different factors which can lead to mental retardation. Some of these may be classed as environmental-irradiation of the developing fetus, birth injury, or infection, for example.

In addition there are at least fifty different types of mental retardation which result from genetic causes (2, 16). A wide range of biochemical defects are included, involving the body's use of amino acids, carbohydrates, lipids, or hormones. Some types cannot be described biochemically as yet, but the family history shows them to follow simple laws of inheritance.

We are ignorant of the existence of a gene until we detect a mutation of it. Then we know that all along there was a "normal" gene in place of the new "abnormal" one. Thus, if hereditary changes are identified which lead to mental retardation, we are justified in saying that many genes are required for normal mental function.

An exciting break-through in recent years was the discovery of an explanation for mongolism, also called Down's syndrome. The size of the problem can be seen from the fact that among four million babies born in the United States each year about 8,000 will show the signs of this disorder (12). In 1959, French scientists Lejeune, Turpin, and Gautier identified the cause as an extra chromosome (number 21) in the cells of mongoloids. Normal babies have 46 chromosomes and mongoloids have 47.

What causes the presence of the extra chromosome? Apparently some process which accumulates with age causes non-disjunction, that is, puts an extra chromosome into a germ cell. The chance of having a child with Down's syndrome increases sharply with the age of the mother, reaching about two percent for mothers over forty.

Other mental abnormalities result from unusual numbers of chromosomes (8). Severe retardation accompanied by multiple physical defects results from an extra chromosome in the group numbered 13-15 or an extra chromosome 18. Men who have extra X chromosomes are retarded and have undeveloped sex organs.

Stem (20, p. 590) summarized the studies which have been made on the response of normal individuals to intelligence tests. Identical twins (reared together) differed by only 3.1 points, as compared with an 8.5 point difference between non-identicals. Even identical twins reared apart have only a 6 point difference. A large part of the latter difference occurred in four pairs of twins who also had the most marked difference in amount of schooling. Stern concluded that the twin studies show: (a) that the I. Q. score is modified by differences in environment, and (b) that identical twins, whether reared together or apart, show greater similarity in I. Q. than non-identical twins reared in the same home.

Differences in Behavior

A number of studies have been made on the gross differences in behavior seen in the psychotic disorders such as schizophrenia and manic-depressive psychosis (12, 18). When all are taken into account, there is good evidence for genetic differences in ability to respond to environmental circumstances. Paul Meehl, in a presidential address before the American Psychological Association (9, p. 837), stated his personal conviction that psychologists will be able "to make a unique contribution in the near future, using psychological techniques to establish that schizophrenia, while its content is learned, is fundamentally a neurological disease of genetic origin."

Attempts to find a single biochemical change in all cases of schizophrenia have been unsuccessful. But there is growing evidence of unusual biochemical findings in a few psychotic persons (11). A different enzyme defect has been observed in persons who appear normal except for mild speech problems (6). We can expect to find a series of other biochemical causes for some cases of behavior disorder.

Another type of behavior problem is criminality. Stern (20, p. 605) concluded a discussion on this point: "The facts on criminality in twins actually show only that identity in genes plus the close similarity in social experiences, at least in early childhood, are more identical twins into prison than two On the basis of present data, one cannot exclude the possibility that the higher concordance for criminality of identical twins is mainly, or even exclusively, the result of their more similar social experiences; nor can one exclude the opposite possibility that their higher concordance is mainly the result of their identical genotypes . . . If the present evidence is insufficient to indicate reliably that criminals are 'born,' it does point to a genetic component for the type of crime committed, if a crime is committed at all."

Some data are available on behavior differences among the general population. In his study of identical twins brought up apart and together, Shields (17, p. 156) found a significant resemblance between twins in both groups with respect to "intelligence, extraversion, neuroticism, and a variety of personal characteristics, ranging from voice and mannerisms to smoking habits." Gottesman (7) tested twins with two personality inventory tests and found an appreciable hereditary effect on certain of the rating scales. When the data were compared with the profiles for various psychoneurotic syndromes, the importance of genetic factors was highest for schizoid/schizophrenic reaction and anxiety reaction. Conversion reaction or hysteria rated the lowest. Vandenberg (21, p. 235) used a different battery of psychological tests and found that "hereditary factors play a role in many areas of human skilled performances, often in spite of the fact that these skills are highly practiced."

The experimental analysis of behavior differences is receiving a good deal of attention in the emerging field of behavior genetics (3, 5). It is clear that gentic factors are involved in behavior differences between strains of fruit flies, of mice, or of dogs. It is equally clear that behavior patterns may be conditioned and modified by early experience. In young animals, for example, there appear to be critical periods during which the basic social relationships can be established most readily (15). Such findings will give us new ideas and methods which can be adapted for the study of human behavior.

From this discussion, one can see the importance of biological factors in our understanding of the nature of man. The abnormal development of the nervous system or a change in blood chemicals can drastically alter behavior. Furthermore, there may be considerable inborn variability from person to person.

In a number of different aspects of behavior one's genetic endowment probably establishes a "repertoire" of mechanisms for reacting. Early environment is then crucial in permitting or restricting the actual expression of these ways of responding.

a) Such thoughts should sharpen the problem of human responsibility and sin. This does not mean at all that the power of sin is reduced. But it does suggest that oar diagnosis of what constitutes sin in another person may not be as accurate as we assume. Is it not possible that for certain behavior problems a medical treatment is more helpful than prayer?

b) Further research into die complex interactions between heredity and environment should be encouraged. "The need for more and more knowledge concerning die genetic bases for human behavior provides the greatest challenge of all to the human geneticist" (13, p. 241).

c) Variability in human behavior is desirable. Certain severe disorders are clearly a problem, but aside from these there is no set of traits that is the best. Diversity should be accepted and encouraged.

d) Much work is needed to find ways of correcting the effects of gross behavior disorders. It is possible that different methods of education are required for different groups of children in order to help each child make the best use of his potential abilities.

1. Anderson, V. Elving, "Statistical Studies of Probands and Their Relatives," Annals of the New York Academy of Sciences, 91:781-796, 1961.

2. Anderson, V. Elving, "Genetics in Mental Retardation," in Review of Research in Mental Retardation, Chicago: University of Chicago, In press.

3. Bliss, E. L. Roots of Behavior: Genetics, Instinct, and Socialization in Animal Behavior, New York: Harper and Brothers, 1962.

4. Burdette, Walter J. (Ed.), Methodology in Human Genetics, San Francisco: Holden-Day, 1962.

5. Fuller, John L., and Thompson, W. Robert, Behavior Genetics, New York: John Wiley, 1960.

6. Ghadimi, H., Partington, M. W. and Hunter, A., "A Familial Disturbance of Histidine Metabolism," New England journal of Medicine, 265:221-224, 1961.

7. Gottesman, Irving L, "Differential Inheritance of the Psychoneuroses," Eugenics Quarterly, 9:223-227, 1962.

8. Hirschhorn, Kurt, and Cooper, L., "Chromosomal Aberrations in Human Disease: A Review of the Status of Cytogenetics in Medicine," American journal of Medicine, 31:442-470, 1961.

9. Meehl, Paul E., "Schizotaxia, Schizotypy, Schizophrenia, American Psychologist, 17:827-838, 1962.

10. Penrose, Lionel S., "Genetics of Growth and Development of the Foetus," in L. S. Penrose, ed., Recent Advances in Human Genetics, 1961, pp. 56-75.

11. Perry, T. L., "N-methylmetanephrine: Excretion by Juvenile Psychotics," Science, 139:587-589, 1963.

12. Reed, Sheldon C., Counseling in Medical Genetics, Philadelphia: W. B. Saunders, 1963.

13. Rife, David C., Heredity and Human Nature, New York: Vantage Press, 1959.

14. Rosenthal, David, "Problems of Sampling and Diagnosis in the Major Twin Studies of Schizophrenia," Journal of Psychiatric Research, 1:116-134, 1962.

15. Scott, J. P., "Critical Periods in Behavioral Development,"
Science 138:949-958, 1962.

16. Scriver, Charles R., "Hereditary Aminoaciduria," Progress in Medical Genetics, 2:83-186, 1962.

17. Shields, James, Monozygotic Twins Brought up Apart and Brought up Together: An Investigation into the Genetic and Environmental Causes of Variation in Personality, London: Oxford University Press, 1962.

18. Shields, James, and Slater, Eliot, "Heredity and Psychological Abnormality," in H. J. Eysenck, ed., Handbook of Abnormal Psychology, New York: Basic Books, 1961, pp. 298-343.

19. Snyder, Laurence H., "Old and New Pathways in Human Genetics," American Journal of Human Genetics, 3:1-16, 1951.

20. Stern, Curt, Principles of Human Genetics, San Francisco:
W. H. Freeman and Company, 2nd ed., 1960.

21. Vandenberg, Steven G., "The Hereditary Abilities Study: Hereditary Components in a Psychological Test Battery," American journal of Human Genetics, 14:220-237, 1962.