<< Factors Effecting Variation >>

Morphologic differences between individuals are the product of both hereditary and environmental factors. Differences in behavior, on the other hand, are dependent not only upon hereditary morphology and upon the direct effects of environment on that anatomy, but upon psychologic conditioning and social pressures as well. Because of the larger number of factors involved, variation in behavior is much greater than variation in anatomic structures.

The most important biologic factors affecting the nature and frequency of sexual response in the human animal are the hereditary forces which account for the differences between male and female. Within either of these sexes, heredity must also account for some of the variation in sensory structures and in the mechanisms which are concerned with emotional response; but there is little precise information on this point. Variation within the lifetime of a single individual is effected by such biologic factors as age, general metabolic level, nutrition (Miles 1919, Jackson 1925), vitamins (Biskind and Falk 1943, Moore 1942), general health, changes in neurologic conditions, and still other situations. Age is the one biologic factor that most strongly affects variation in the sex life of an individual and which, therefore, accounts for the differences between populations of different age. Sex hormones are the biologic factors with which there has been the most experimentation. In general, an increased availability of male hormone (up to the point of its optimum effect) increases the frequency of sexual activity (Hamilton 1937, Moore 1942, Pratt 1942, Lisser and Curtis 1943, Heller, Nelson and Roth 1943). Less often noted in the literature and less widely utilized for experimentation, thyroid hormones produce, if anything, more marked results, and our histories include some persons who have had the intensity of their sex drive and the frequency of their activity considerably increased by the administration of thyroid extracts. Since thyroid so directly affects the general metabolic level, it is probable that its influence on sexual frequencies is by way of its relation to metabolism in general rather than through any immediate action of the hormone. The master gland of the hormonal system, the pituitary, regulates both thyroid and sex glands and thus (probably in this indirect fashion only) affects the sexual activity of the individual.

Psychologic conditioning accounts for a larger part of the variation in behavior in a population. All living organisms, from the lowest to the highest, are modified by the experiences through which they pass. This modifiability is one of the intrinsic qualities of living protoplasm. In any creature with a central nervous system which is as highly developed as that found in the vertebrates, particularly in the primates, this conditioning becomes a paramount factor in determining the animal’s behavior. Whether an individual is located at some lower point or at a higher point on the total curve of outlets depends in part upon the experience which he has previously had and the incentive which that experience provides for a repetition or avoidance of further activity. Whether an individual depends upon masturbation or heterosexual intercourse for his pre-marital outlet depends in part upon the early experience he happens to have had. Whether exclusively heterosexual or exclusively homosexual patterns are followed, or whether both heterosexual and homosexual outlets are utilized in his history, depends in part upon the circumstance of early experience.

A third group of factors effecting variation in human sexual behavior is the sociologic group. As later chapters in this volume will show, the mores are the prime forces which produce variation in the sources of sexual outlet in different groups. Patterns of sexual behavior are, in an astonishingly high percentage of the cases, merely reflections of the patterns of the particular social level to which an individual belongs. In most cases, the individual rationalizes his particular pattern and thinks that he himself has logically chosen the regimen which is most satisfactory, socially profitable, or morally right; but mores which are many hundreds of years old are, in reality, the sources of most of these decisions.

There are, then, a variety of factors which may modify the frequency and sources of sexual activity within the lifetime of a single individual; but within any limited period of time — within a five-year period, for instance — changes are effected chiefly by physical health and by modifications of situations which affect the opportunity for sexual contact. Most individuals maintain a surprisingly constant position on the outlet curve for periods of several years, changing mostly because of advancing age.

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