The Psychobiology of Sexual and Gender
Cindy M. Meston and Penny F. Frohlich
Interest in human sexual function has increased in the past decade, in large part as a result of increased recognition of the sexual side effects of various medications, the high incidence of sexual dysfunction among men and women, and the highly publicized success of some treatments for sexual dysfunction (e.g., Viagra for erectile dysfunction). This paper will describe the present knowledge of the endocrine, neurotransmitter, and central and peripheral nervous system mechanisms governing sexual function and dysfunction. The primary focus will be the underlying physiological processes although it should be noted at the outset that it would be misleading to assume that sexual dysfunction is best conceptualized in this manner.
Psychological problems, such as depression or anxiety, and relationship issues, such as marital discord or stress, can have a profound effect on sexual functioning. Although such cognitive and emotional factors are often integral to a sexual problem, these aspects will be reviewed only briefly here.
Sexual desire is commonly defined as the broad interest in sexual objects or experiences. One of the difficulties in diagnosing inhibited desire is determining exactly what constitutes low desire. Sexual desire cannot be measured exclusively by frequency of sexual activity — a person may desire sexual activity a great deal more or less often than their actual level of activity. It is problematic to measure sexual desire based on a discrepancy between partners; a man who desires sexual activity once a day may be frustrated by a partner who desires sexual activity twice a week, yet both partners have a level of sexual desire that falls within the normal range. Because there is no objective physiological criterion for desire, it is generally inferred by self-reported frequency of sexual thoughts, fantasies, dreams, wishes, and interest in initiating and/or engaging in sexual experiences. However, it is also problematic to diagnose hypoactive sexual desire based on a simple comparison with typical levels of desire. A couple who both prefer sexual activity only once a month would be exhibiting levels of desire below normal, yet it is unlikely that they would be unsatisfied with their degree of activity (LoPiccolo and Friedman, 1988). In order to meet the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV), criteria for hypoactive sexual desire disorder, the person must not only experience a persistent or recurrent deficiency or absence of sexual fantasies and desire for sexual activity, but the situation must create marked distress or interpersonal difficulty—indeed, it should be noted at the outset that in order to be diagnosed with any of the sexual disorders a person must be experiencing significant distress or interpersonal difficulty (American Psychiatric Association, 1994).
Hypoactive sexual desire disorder is much more common in women than in men. Thirty-two percent of women between the ages of 18 and 29 years old reported a lack of sexual interest compared to 14% of men in the same age group. Women did not demonstrate a change in rates of inhibited desire according to age while men were significantly more likely to report lack of sexual interest as they aged, particularly after age 50 years old. Women did not differ in rates of inhibited desire based on marital status whereas married men were significantly less likely to report inhibited desire compared to divorced or never married men. Women who had less than a high school level of education reported significantly higher rates of inhibited desire compared to women with more education. Perhaps more educated women are more open to improving sexual communication and sexual knowledge. Exploring what is sexually pleasurable, and communicating sexual needs are techniques used for enhancing sexual desire. Unlike women, men showed no significant differences in desire according to education. AfricanAmerican women reported significantly higher rates of inhibited desire compared to Caucasian or Hispanic women whereas men demonstrated no ethnic differences in sexual desire (Laumann et al., 1999).
Cases of low desire in men are often related to medical conditions or treatments that affect hormone levels. Hypogonadal men (i.e., men with deficient secretion of gonadal hormones) receiving testosterone replacement therapy demonstrated a significant drop in sexual interest following removal of the hormone treatment, and a return in sexual interest when the hormone treatment was resumed. This indicates that very low testosterone levels may impair sexual desire in men. Once testosterone levels reach a certain threshold, additional testosterone does not affect sexual desire—thus, testosterone administration to a male with normal testosterone levels will not increase sexual desire. In adolescent males, higher testosterone levels are associated with increased frequency of sexual fantasies and sexual activity but this relationship does not hold true in adult men. Perhaps during and around puberty internal factors (e.g., hormones) trigger sexual appetite while in adulthood external cues (e.g., relationship factors) play more of a central role. Some evidence suggests that oestrogen and progesterone administration reduces sexual desire in men with excessive or inappropriate desire,