Questions

-Does sadomasochism have a biological purpose, i.e. does it increase reproductive success under at least some circumstances, and if so, how?

Sadomasochism is so common, that it is very likely to have some purpose, otherwise it would have long since evolved away.

-Why is the occurrence of sadomasochism strongly correlated with clothing fetishes involving leather, rubber, PVC and any other shiny fabric?

This is almost the hardest question for any theory that claims to explain these phenomena. Most authors put it in the too hard basket, or even just ignore it.

- What is the full range of sexual preferences and activities associated with sadomasochism?

An objective physiological criterion for S&M is needed to answer this question. There may turn out to be a few surprises.

-Is there one basic phenomenon that underlies all those preferences and activities?

Probably yes.

-Is there some part of the brain specifically devoted to sadomasochism? Is there some specific neurotransmitter involved?

First part: very probably, if sadomasochism has a specific function. Second part: maybe.

-What effects do psychoactive drugs have on sadomasochism, and can they be distinguished from effects on overall sex drive?

Do not attempt to answer this question by experimenting outside a proper laboratory that is equipped and licenced to perform such experiments. This is most revelant if the answer to the second part of question 5 is yes, as psychoactive drugs generally act on neurotransmitter systems (rather than on specific locations).

- Is there a group of people who are sadomasochists distinct from those people who are not sadomasochists, or is it variation along a continuum?

Further question: is there anything unusual about those at the extreme vanilla end of the spectrum?

- Are there any genes that determine sadomasochism?

Possibly, given the extreme variation that is observed.

-What is the concordance rate for identical twins?

This is the standard way to answer the previous question. It ignores all difficulties as to whether the genetic effect is a complex one involving many genes or a simple one involving one or just a few genes.

-Are there environmental causes or triggers for sadomasochism? If so, do they relate to the biological purpose (if any) that sadomasochism has?

Probably yes. One would expect sadomasochism to be triggered by those circumstances in which the purpose of it was more relevant, for example when opportunities for sexual satisfaction are difficult or infrequent.

-Is anything like sadomasochism observed in non-human animals?

Probably not.

The basic mechanism of sexual sadomasochism is the following: response suppression , linked in some way to a sexual opportunity.

The response being suppressed can be an actual behavioural response, or it can be one of a set of emotional responses that includes anxiety, embarrassment and guilt.

The response can be suppressed internally, i.e. by deciding not to respond to something that one normally responds to, or externally, for example by means of physical restraint. The implication is that there exists some part of the brain that measures the non-occurrence of expected responses, and which is connected to those parts of the brain involved in sexual arousal.

The biological purpose of sadomasochism is to encourage the sadomasochist to do something that they would not normally do or put up with something that they would not normally put up with in order to make the most of a difficult or infrequent opportunity for sexual satisfaction.

"Shiny clothes" fetishism is a side-effect of sadomasochism. The responses that get suppressed are the movements that the eyes would normally make when looking at reflected images seen in the shiny fabric. These movements are suppressed in favour of those made in order to look at the fabric itself (which always has a residual matt component in its reflectivity), the shape of which reveals the sexually attractive shape of the body underneath the clothes.

A Typical S&M Scenario

A ticklish masochist, when tickled, normally reacts to the tickling by squirming, laughing and trying to avoid or prevent it. But suppose that his sexual partner starts tickling him. He may become sexually aroused as she tickles him. As he becomes aroused, his normally ticklishness seems to disappear, in that his normal responses to tickling cease to occur. But as she continues to tickle him, he becomes more aroused. If the situation terminates in orgasm, and she continues to tickle him, she can observe that his normal response to tickling, i.e. squirming and laughing, has very suddenly returned.