Sexologists agree that the incidence of gender identity/role disorders is greater in males than in females, though there are as yet no fixed statistics. The embryology of prenatal hormonal regulation of sex differentiation clearly shows that nature’s first choice is to differentiate the morphology of a female. The differentiation of male morphology requires that something be added (the “Adam principle”). This something is, for the most part, androgen released by the fetal testes. A second substance, known only as mullerian inhibiting substance, suppresses development of the mullerian ducts which otherwise would form a uterus in the male. By inference from animal experiments to human beings, androgen also influences brain pathways or thresholds that mediate erotic and mating behavior. Apart from some rodent studies, the neuroanatomy and neurochemistry of this influence still must be demonstrated.
It is, by hypothesis, likely that prenatal androgen has a masculinizing effect on brain thresholds sub-serving the relationship of visual signals and images to erotic attraction and arousal. In lower mammals, including primates, an odor or pheromone from the ovulating female’s vagina serves as a sex attractant. In man, the sense of sight overrides smell as a sex attractant. Both sexes respond to visual erotic signals, but woman is more dependent on touch for complete arousal, according to present evidence. In man, the visual stimulus prompts the initiation of an erotic approach. Nature demonstrates the primacy of the visual image in male eroticism in the phenomenon of the pubertal orgasm dream (wet dream) for which there is no exact counterpart in the female.
The actual image that is erotically stimulating is not phyletically programmed so as to be identical in all human males. If it were, any two males and females could pair-bond, that is, fall in love with each other. But nature, in its own wisdom, has designed us as a species rich in the diversity of individual differences, erotic individual differences included. Thus, the image of erotic arousal is no more innate than is native language. Like native language, the image of erotic arousal is established in response to early life experience, and it becomes engramed or imprinted. The so-called errors of imagery, manifested as transpositions of gender identity/role, or as the intrusion or displacement paraphilias, also become engramed or imprinted. The infant and juvenile male appears more vulnerable to such an imprinting error than is his female counterpart, probably because of the greater importance of the visual image to erotic arousal in nature’s design of the human male.