In all species, sexual behavior is directed by a complex interplay between steroid hormone actions in the brain that give rise to sexual arousability and experience with sexual reward that gives rise to expectations of competent sexual activity, including sexual arousal, desire, and performance. Sexual experience allows animals to form instrumental and Pavlovian associations that predict sexual outcome and thereby directs the strength of sexual responding. Although the study of animal sexual behavior by neuroendocrinologists has traditionally been concerned with mechanisms of copulatory responding, more recent use of conditioning and preference paradigms, and a focus on environmental circumstances and experience, has revealed behaviors and processes that resemble human sexual responses.
Understanding human sexual behavior requires continued research and a hefty dose of patience and creativity. We must balance the simplicity of definitions of normal and abnormal arousal, desire, performance, and orgasm with the bewildering array of exceptions to those definitions that characterize sexual function and dysfunction in real people.
All too often our questions are obscured by scientific blinders and constrained by research review committees, with certain moral limitations imposed by ethics review boards and government agencies pressured to enforce "community standards." There is much that we simply cannot do, either because of ethical concern, impracticality, or the lack of sufficient technology. This is most obvious when we ask questions about the neurobiology of sexual behavior.
Although we can view human brain activation in sexual circumstances or monitor the sexual behavior of individuals with specified brain damage or following drug treatments, it is difficult to study these phenomena experimentally. Most people will not knowingly allow themselves to be rendered sexually dysfunctional by some experimental manipulation, and ethics review boards generally have a hard time allowing researchers to monitor human copulatory behavior firsthand. Despite this, real progress has been made in the past decade in understanding the neuroanatomical and neurochemical mechanisms of erection, ejaculation, and other sexual responses, and in the design of rational pharmacological treatments for certain sexual dysfunctions. We have begun to examine the mechanisms that underlie desire, and how sexual stimulation and reward impact on attractiveness and mate choice. Progress in these areas could not have been made without the help of animal models.
The comparative approach to the study of sexual behavior was championed by Beach, who issued a "call to arms" in his 1950 presidential address to the American Psychological Association, in a delightful treatise called "The Snark was a Boojum" (Beach, 1950). His call was heeded by those in the fledgling science of behavioral neuroendocrinology, but at the expense of a unified approach to the study of sexuality. By the early 1990s it was as if two camps had emerged: clinicians who studied people and neuroendocrinologists who studied animals.
Those camps rarely shared their insights at scientific meetings. And why bother? After all, human copulatory behavior doesn't really resemble copulatory behavior in animals. There is no human counterpart to lordosis (at least not as an unambiguous, estrogen-dependent postural display of sexual receptivity in females), and human sexual behavior is so shaped by experience and learning that it seems to defy hormone actions that are critical to the display of animal sexual behavior.
But insights into the human experience could indeed be derived from animals. The links started forming around the study of sexual pharmacology. For example, the dopamine receptor agonist apomorphine induces erection in rats and men whereas the dopamine antagonist haloperidol reduces sexual arousal and desire in rats and men.. Such results allowed researchers to make a predictive link between the sexual responses of male rats and men and gave rise to an important theoretical implication that certain brain systems had been conserved in evolution to subserve similar or identical functions among species.
One interpretation of Beach's argument was that a true understanding of the neurobiological or behavioral rules that underlie sexual responding required us to study how well they fit across species. If they did, then animals could be used as models to study aspects of human sexual behavior that otherwise could not be studied experimentally in humans. If they did not, then they represented either interesting examples of biological diversity or evolutionary dead-ends. Of course, either possibility required a full account of what animals do, or can do if asked the proper questions. Some hard reflections here I reckon!