By now, everyone’s got an opinion about 50 Shades of Grey: It's trash; it’s fun -fodder; it’s misogynist; it’s empowering for women; it’s silly. While the 50 Shades media saturation has grown tiresome, one must admit that it’s compelled a societal discussion of sexual practices involving bondage, discipline, sadism, and masochism ( ) that are otherwise not broadly considered. Leaders of the BDSM community are quick to point out that 50 Shades is not an accurate representation of BDSM sexual practice where “safe, sane, and consensual” are the watchwords and that the term “BDSM” is broad, like the term “sports.” It includes people with highly divergent sexual desires and personae. Just because you like to be flogged doesn’t mean that you necessarily like to be humiliated as well.

For those outside of this group, a failure to understand the appeal of BDSM practice usually comes down to this: How can one experience pain, either the physical pain of a smack on the tush or the emotional pain of , as pleasurable? Aren’t pain and pleasure diametrically opposed?

Source: Photo by Dennis Jarvis via Flickr, CC ShareAlike 2.0

You don’t have to be a masochistic enthusiast to know that pleasure and pain can be felt simultaneously: think of the pleasures of a delicious meal laden with spicy chili peppers or the blissful ache following a long-distance run. In the lexicon of cognitive , both pleasure and pain indicate salience, that is, experience that is potentially important and thereby deserving of . Emotion is the currency of salience and both positive emotions like euphoria and love and negative emotions like and disgust signal events that we must not ignore.

How is salience built into neural pathways? We have an evolutionarily ancient and highly interconnected pleasure circuit in our brains. When neurons in a brain region called the ventral tegmental area become electrically active, thereby triggering the release of in a structure called the nucleus accumbens, this evokes the feeling of pleasure from both our vices (eating food when hungry, having an , drinking ) and our virtues ( , learning, giving to charity).

Here are the key findings that help to explain the pleasure-pain connection. When subjects in a brain scanner received in injection into the jaw muscles that produced a protracted aching type of pain, this triggered dopamine release in the nucleus accumbens and the greatest release was seen in those subjects who rated the pain as most unpleasant. In rats, one can examine this phenomenon in greater detail. Electrical recordings from single dopamine neurons of the ventral tegmental area revealed that all of these neurons responded to the presentation of a tasty sugar-droplet, yet some of these neurons responded to a brief painful footshock with a decrease in their ongoing rate of activity while others responded with an increase. In other words, these latter dopamine-using neurons were salience detectors, releasing dopamine in response to either pleasure or pain. We also know, from different experiments, that protracted physical pain and protracted emotional pain (resulting from social rejection) can cause the release of endorphins, the brain’s own -like molecules and that these endorphins can activate dopamine neurons in the ventral tegmental area. The end result is that there is an innate rewarding component to both pleasurable and painful experiences.

How then, can we account for individual differences? Why do surveys reveal that only 5 to 10 percent of people enjoy receiving pain in a sexual context? The short answer is that we don’t entirely know. Understanding how sexual kinks develop has not been a funding priority for government agencies and biomedical research charities. There are variant forms of dopamine receptor that attenuate the experience of pleasure and increase and novelty-seeking behavior. However, it’s not clear that these gene variants or any others (such as those related to endorphin signaling or pain perception) are linked to the practice of .

Perhaps the best hypothesis for sexual masochism comes by analogy from studies of another painful practice: chili pepper consumption. If you grow up in a community where chili peppers are readily eaten, you will reject them as an infant, but by about age 5, you will almost certainly develop a taste for these painful foods. Rats and mice, by comparison, cannot be trained to choose chili peppers in their food no matter how their upbringing is manipulated by scientists. It is likely that there is a human predisposition to learn to find certain forms of pain to be rewarding. This seems to be the case when pain is survivable and doesn’t lead to permanent damage as in both masochistic sexual practice and chili pepper eating. However, it is only when that human predisposition is combined with aspects of one’s particular life experience (as influenced by cultural and religious ideas) that the brain’s neural salience circuits are modified to forge the pleasure-pain connection in a sexual context.