Lust on the Brain

The Talk

It’s time to have the talk. Not exactly the one that I was so afraid of having with my parents that I disposed of the American Girl Guide to the Care and Keeping of You before my mom could give it to me. It’s time to talk about the neurobiology of sex. Because as Stefanie told you last week, Christian of Denmark exhibited a lot of concerning behavior, but none concerned his contemporaries (or me) more than his addiction to masturbation. While his physician believed the masturbation was driving his other psychiatric symptoms and abnormal behavior, modern medicine suggests that it was the opposite way around. Hypersexuality, as we will see, is actually a symptom of a disorder we have talked about before: manic depression, also known as bipolar.

IYKYK. Amazon

Can(‘t) Keep My Hands to Myself

As we’ve previously discussed on the blog, bipolar disorder is a serious mental illness characterized by periods of mania followed by periods of depression. During manic episodes, patients talk more, sleep less, have an inflated sense of self, and engage in risky behavior. Christian has been described as having periods of clarity, consistent with the cyclical nature of this disease. A hallmark symptom of bipolar is the presence of delusions during mania, which is consistent with Christian’s disbelief that he was king and tendency to live in a “fantasy world”. He also experienced paranoia, having trouble trusting the people around him and fearing he would be assassinated. And he was an exceptionally poor decision maker, especially when it came to his rock and roll lifestyle.

Since the 1970s, manic episodes have been associated with “hypersexuality”. Studying this connection has proved to be difficult because “even though hypersexuality is firmly tied to bipolar illness both in the professional and lay literature, the exact definition of this term with regard to this illness has not been established. This lack of definition is problematic as sexual norms are strongly dependent on culture and historical period, making it difficult to compare studies across different time periods or cultures” (Kopeykina et al, 2016). This is similar to modern interpretations of Caligula’s behavior that we talked about previously.

Even so, increased sex drive during mania has consistently been associated with bipolar disorder. The majority of the research on the connection between libido and bipolar has been focused on “risky” sexual behaviors. Studies have found that bipolar patients having a manic episode are more likely to engage in prostitution, be the victim of sexual violence, have extramarital affairs,or be exposed to sexually transmitted diseases. It has been proposed that manic patients, “ [progress] from heightened sexual thoughts and activity in the early stages of mania, into sexual preoccupation, “provocativeness”, and sexual delusions” (Kopeykina et al, 2016).

Despite these concerning findings, the biology underlying hypersexuality, especially during mania, remains a mystery. What we do know for sure is how the brain gives rise to desire and arousal, and understanding how the neurobiology of sex works under normal conditions can gives us some clues as to what’s going wrong during disease.

Animal Instincts

I will assume that you are familiar with the basics of the biology of sex (although if not, you wouldn’t be the first person to be failed by high school sex ed). But what you probably didn’t learn from your PE teacher is that the brain is involved at all stages of sexual function. This is a massive topic, and I am nowhere near an expert. Luckily, I still have my notes from a fantastic behavioral endocrinology class that I took at Northeastern University by Dr. Jade Zee, and I am going to condense the information that she condensed from the extremely complex field. And please keep in mind that this is the last thing I want to talk about on the blog, but we are being purely scientific so please be mature, Mom. 

Let’s start with the ladies. As we have talked about before, hormones fluctuate across the menstrual cycle, giving rise to changes in physiology and mood. These hormone changes can also cause changes in a woman’s interest in and receptivity to sex. Basically, the brain and body scheme together so that a woman’s sex drive is highest at ovulation, when she is most fertile, because of the effects of estrogen. In addition, you’ve probably heard about oxytocin, the so-called love hormone. Most people are familiar with research that it is released in the brain during sex and facilitates pair bonding between partners. This hormone is also thought to trigger the reward pathways (more on that later!) in the brain as a result of sex, and has important roles in uterine contraction to facilitate fertilization.

This graph demonstrates that the proportion of successful mating attempts (open circles) in mice peaks at ovulation (dark circles), when estrogen is highest.

While this does not translate to humans, a very nice model of the effects of sex hormones on female sexual behavior is lordosis in mice. Lordosis is a particular position female mice assume when they are receptive to the advances of a suitor. Lordosis is particularly interesting because it is dependent on both sex hormones and neurotransmitters. Sexual stimuli lead to the release of hormones like estrogen in the central nervous system. Estrogen increases excitatory neurotransmitters that promote neuronal activity in the hypothalamus. If hypothalamic activity is blocked, then lordosis does not occur, presumably because the neurons in the spinal cord controlling the relevant muscles do not get the message.

On the male side, sex hormones play a similarly critical role. Castration of rodents leads to reduced sexual behavior and desire (those of you who have neutered animals know what we are talking about here), which can be reversed by administration of testosterone. Testosterone is rapidly metabolized by the body into byproducts that drive these effects. At the penis, the metabolite dihyroxytestosterone controls sensitivity to stimuli and reflexes. In the brain, testosterone is actually converted to estrogen. While estrogen is normally thought of as a female hormone, it has profound effects on the male brain and is critical for sexual behavior, acting on different brain regions to modulate libido, sensory integration, and motor behavior. There is some super interesting research that becoming a father and spending time with your children actually reduces testosterone levels, with the theory being that men need biological reinforcement to slow the roll on baby making (the pain of childbirth is believed to fulfill that role in women).

Castrated rodents showed decreased sexual behavior after castration, which was ameliorated by testosterone treatment. Grunt and Young, 1952

Neurotransmitters also participate in male sexual behavior. A gaseous neurotransmitter called nitrous oxide (NO) can be produced in the brain or locally at the penis by nerves. NO initiates a signaling cascade to promote blood flow necessary for erection. The discovery that Viagra works by preventing the termination of NO’s signaling cascade was actually an impetus for increased research into neurobiological control of sex. 

I hope you can appreciate that the brain and reproductive system are intimately connected. Hormones and neurotransmitters influence desire, arousal, and consummation through a complex network of interactions. Bipolar disorder is characterized by alterations in neural communication, but it remains unclear which of these changes affects sexual behavior at what level. It could be that sexual stimuli have a different potency for patients with bipolar. It could be that their libido is increased. But modern studies of hypersexuality hint that in cases where people engage in compulsive sexual behaviors, the brain’s reward system is often to blame. Let’s dive into that research and ponder whether the same mechanism could be at play in bipolar.

Bad Habits

While I am advocating that Dr. Struensee’s theory of “masturbatic insanity” was incorrect, I don’t want to neglect the fact that recent studies suggest self-pleasure habits can affect the brain. Specifically, there is an emerging interest in the neurobiological effects of pornography. Porn has never been easier to access thanks to the internet, and people have taken full advantage. I know these are wide ranges, but the American Psychological Association (APA) estimates that between 50 and 99% of men and 30 to 86% of women consume porn, based on studies from across the globe. And despite the fact that there is good evidence that viewing porn compulsively can lead to relationship problems and workplace indiscretions, the APA declined to include a diagnosis of “hypersexuality” in the DSM-5, because of the implications of judging someone’s sexual behavior. But that means that people who have sex and porn addictions (which the APA fails to recognize) may have trouble getting the care that they need.

Luckily, neuroscientists have jumped in where the psychologists won’t to try to prove that pornography can be addictive just like alcohol or drugs. At the center of all addiction is something called the mesolimbic pathway. The mesolimbic pathway connects the ventral tegmental area (VTA) with a region of the midbrain called the striatum. When the VTA releases a neurotransmitter called dopamine onto the striatum, a person experiences a biological reward: pleasurable feelings associated with something that is good for you evolutionarily, like sex. But objects of addiction “hijack” the pathway and become the only stimulus capable of triggering a dopamine hit. Therefore, I think the strongest evidence that porn can be addictive is that compulsive porn consumption hijacks the mesolimbic pathway. 

A simple but effective infographic illustrating why addictive substances take over the mesolimbic circuit. Wikidot

In a 2008 case study, doctors from the Mayo Clinic reported on a man who had been struggling with pornography, masturbation, and infidelity. His marriage and career were suffering and he did not respond to treatments for obsessive compulsive disorder. His psychiatrist suggested he try naltrexone, a drug used to treat alcoholism. Naltrexone works by blocking opioid receptors on neurons of the VTA, thus preventing dopamine release. The patient reported that he felt, “‘a measurable difference in sexual urges. I wasn’t being triggered all the time. It was like paradise.’” After three years of consistent naltrexone in combination with an OCD medication, he said, “‘I occasionally slip, but I don’t carry it as far, and I have no desire to meet anyone.’” Of course, this was only one person and the data is preliminary, but it is provocative.

The hypothesis that pornography can be addicting was further backed by a 2014 study out of Germany that conducted MRI imaging on over 60 healthy men. The researchers found that the more hours of pornography participants cosumed per week, the smaller a portion of the striatum, called the caudate nucleus, was on the right side of the brain. In addition, the caudate on the right side showed less communication with part of the prefrontal cortex important for impulse control and decision making. Men who watched more porn showed less brain activation in response to sexually explicit images, consistent with desensitization seen in other kinds of addictions. Moreover, they found correlations between risk for internet sex addiction and alcohol misuse, suggesting the two forms of addiction could be related. However, the authors point out that it’s possible that the abnormal size and activity of the caudate could make men predisposed to pornography addiction, rather than being a result of the compulsion. So while I don’t think Christian’s compulsive masturbation led to his psychiatric demise, there might be something to that “masturbatic insanity” theory after all.

As hours of porn viewed per week increased, right caudate nucleus volume decreased. Kuhn and Gallinat, 2014

You Will Get Pregnant and Die

Too much of a good thing is a bad thing, and for Christian, sex definitely became that thing. While his sexual habits were believed to be the cause of his insanity, we now know that hypersexuality is a common feature of bipolar disease. And, as we saw, sexual addiction is not uncommon in the broader population. Sadly, the deeply personal nature of sex and the delicacy of the topic has prevented the APA from recognizing sex addictions as a diagnosable mental illness, limiting the amount of research that can be done, particularly when it comes to clinical trials. Acknowledgment that not all sexual behavior is healthy could help a lot of people, just as understanding that Christian’s hypersexuality was a result of mental illness could have had ripple waves on history. More on that next week when our series concludes.

You know who would have advocated for a clinical definition of hyper sexuality? This man!! Seventeen


Bostwick, J. M., & Bucci, J. A. (2008). Internet Sex Addiction Treated With Naltrexone. Mayo Clinic Proceedings,83(2), 226-230. doi:10.1016/s0025-6196(11)60846-x

Geller, B., Zimerman, B., Williams, M., Delbello, M. P., Frazier, J., & Beringer, L. (2002). Phenomenology of Prepubertal and Early Adolescent Bipolar Disorder: Examples of Elated Mood, Grandiose Behaviors, Decreased Need for Sleep, Racing Thoughts and Hypersexuality. Journal of Child and Adolescent Psychopharmacology,12(1), 3-9. doi:10.1089/10445460252943524

Kopeykina, I., Kim, H., Khatun, T., Boland, J., Haeri, S., Cohen, L. J., & Galynker, I. I. (2016). Hypersexuality and couple relationships in bipolar disorder: A review. Journal of Affective Disorders,195, 1-14. doi:10.1016/j.jad.2016.01.035

Kühn, S., & Gallinat, J. (2014). Brain Structure and Functional Connectivity Associated With Pornography Consumption. JAMA Psychiatry,71(7), 827. doi:10.1001/jamapsychiatry.2014.93

Zee, J., Dr. (2018). Male and Female Reproductive Behavior. Lecture presented at Behavioral Endocrinology in MA, Boston.

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