If you are a materialist holding the logical belief that the human brain, with all of its buzzing neural intricacies, its pulpy, electrified, arabesque chambers and labyrinthine coves, has been carved out over countless eons by the slow-and-steady hand of natural selection, then you will grant that specific brain regions evolved because they generated behaviors that were beneficial to our ancestors. When one part of the brain is compromised—through injury, disease, or some other unfortunate event—the constellation of symptoms that result are often remarkably specific. “The brain is the physical manifestation of the personality and sense of self,”writes University of Michigan neuroscientist Shelley Batts in a 2009 issue of Behavioral Sciences and the Law, “and focal damage to brain areas can result in focal changes in behavior and personality while leaving other aspects of the self unchanged.”

Not to get too technical, but if you’re unlucky enough to develop a lesion that interferes with the functioning of your dorsolateral prefrontal cortex, a specialized patch of neural tissue that’s intricately braided into your anterior cingulate cortex, then your working memory, strategy-formation, and planning skills are going to take a major nosedive. Something as simple as coming up with a list of groceries that you’ll need for the next few days becomes a major achievement.

Most of us—materialist and dualist alike—have sympathy aplenty for those patients whose brain disturbances have interfered with their everyday cognitive abilities. We’re perfectly willing to accommodate their intellectual disabilities by, say, helping them create a new mnemonic strategy or giving them a pat on the back or a word of encouragement when they’re trying to remember someone’s name. Yet when chunks of gray matter that have evolved to control and inhibit, say, our sexual appetites and other Bacchanalian drives experience a similar catastrophic blowout, are we so understanding? What if those impairments lead their victims to display … oh, I don’t know, let’s call them moral disabilities? Cases of libidinal brain systems going haywire have our kind-hearted, humanistic materialism rubbing elbows—or butting heads—with our belief in free will and moral culpability.

Although Klüver-Bucy Syndrome is relatively rare, it’s one of the most notorious neurological causes of a complete breakdown in one’s ability to control sexual urges. In 1939, neuroanatomists Heinrich Klüver and Paul Bucy removed the greater portions of both temporal lobes and the rhinencephalon from the brains of rhesus monkeys. Among a host of other peculiar effects of this rather cruel vivisection, the monkeys became incredibly randy, displaying a prominent and indiscriminate desire to copulate. The first documented case of full-blown Klüver-Bucy in humans arrived in 1955, when an epilepsy patient underwent a bilateral temporal lobectomy (a surgical excision of the lobes) and subsequently developed a ravenous sexual appetite, among other things. More often, the syndrome appears in lesser degrees, precipitated by a nasty insult to the medial temporal lobe. That might result from a case of herpes encephalitis or Pick’s disease, or from trauma and oxygen deprivation. Not all such patients experience hypersexuality, mind you, but some do. Other symptoms aren’t terribly appealing, either, however; they include hyperorality (a compulsive desire to put things in one’s mouth), apathy, emotional unresponsiveness, and various sensory disorders.

Dramatic case studies illustrating the devastating effects of Klüver-Bucy Syndrome abound in the clinical literature, and they raise intriguing philosophical questions for us to consider. That some patients so stricken are overcome with excessive carnal urges and are not simply using the disorder as a convenient excuse to become freely promiscuous, lewd, and lascivious is perhaps best demonstrated by a 1998 Clinical Neurology and Neurosurgery study by Indian neurologist Sunil Pradhan and his colleagues. In this report, a group of boys between the ages of 2.5 and 6 began to exhibit hypersexualized behaviors after partially recovering from comas induced by herpes encephalitis. One to three months after emerging from the comatose state, “all seven children,” note the authors, “demonstrated abnormal sexual behavior in the form of rhythmic hip movements (two patients), rubbing genitals over the bed (two patients) and excessive manipulation of genitals (all seven patients).” Were these children just helpless, hapless puppets of their ancient, pleasure-driven brains? The authors believe so. “As all patients [at the time of study], except one, were 4 years of age, with no possibility of environmental learning of sex, these movements most probably represented phylogenetically primitive reflex activities.”

It may be awkward enough telling other parents why your preschooler is humping everything in sight—just try rehashing the foregoing description of Klüver-Bucy Syndrome to your friends at the day care—but we do tend, as adults, to be mostly forgiving of a child’s improprieties. When this sort of hypersexuality strikes a post-pubescent individual whose sexuality is driven by orgasm-propelled desires, things become more interesting—at least, in a philosophical sense. Although it would be entirely inaccurate to portray Klüver-Bucy patients as sex-crazed lunatics, they very often display behaviors that would be considered inappropriate by conventional standards. One gentleman in his early 70s, for instance, hugged a female parishioner at his church and repeatedly kissed her. According to the clinical case report, he then asked the shocked woman, “Why don’t we do it again?” Over the ensuing years, his sexual fantasies skyrocketed and his hyperorality became unmanageable. The report notes that, according to his wife, “he would put any object in his mouth, including dog food, candles, adhesive bandages, and his wedding ring. His appetite seemed insatiable. … He died at age 77 years of asphyxiation on several adhesive bandages.”

In a 2005 letter to the editor of European Psychiatry, two physicians describe the case of a 14-year-old schoolgirl (“Ms. A”), who, prior to developing Klüver-Bucy Syndrome after being in an encephalitis-caused coma, “was an intelligent and social girl with a good academic record.” This quiet, well-behaved teenager became somewhat challenging, to say the least, after recovering from her illness. You think you’re raising a difficult teen? Consider what these parents were dealing with:

[T]he patient started … disrobing in front of others, manipulating her genitals, and making sexual advances toward her father. She would lick any object lying on the ground and whenever she got an opportunity, she would rush to the toilet and try to put urine and feces into her mouth (urophagia and coprophagia, respectively).

In another case, an epileptic woman underwent an unsuccessful left temporal lobectomy to help stop debilitating seizures. Klüver-Bucy symptoms, including hypersexuality, emerged following the surgery. She began masturbating in public and aggressively soliciting her family members and neighbors for sex. After having another seizure, she was brought to the emergency room, where, after a half hour in the waiting area, she began performing fellatio on an elderly cardiac patient. (This may or may not be one of the few examples where one person’s syndrome is another’s lucky day; it’s also unclear if this was a display of hypersexuality or hyperorality, but it’s inevitable, perhaps, that the twain should occasionally meet.)

Other temporal-lobe epileptics have also exhibited hypersexuality in the “postictal” state, which is the period of recovery time following a seizure. In a recent issue of Epilepsy and Behavior, New York University neurologist Vanessa Arnedo and her colleagues present the case of a 39-year-old man who began having semi-frequent seizures during the middle of the night. After nocturnal convulsions, he’d sleep for another 10 minutes, wake up, and then rape his wife. (In the authors’ more delicate wording, he was described as “becoming sexually aggressive toward his wife by forcing intercourse.”) Importantly, however, “the tremendous remorse and abhorrence for what he had done when he learned of his actions led him to pursue possible surgery mainly to eliminate this postictal behavior.” Other people with similar epileptic profiles also become hypersexualized in the postictal state. To his later horror, one man motioned for his 12-year-old daughter to join him and his wife in the bedroom following a nighttime seizure.

It is these last few examples, where Klüver-Bucy Syndrome manifests in criminal behavior, such as rape or child molestation, that our materialistic convictions are really put to the test. In 2003, University of Virginia neurologists Jeffrey Burns and Russell Swerdlow described how an otherwise well-behaved, 40-year-old man developed a case of “new-onset pedophilia” after suffering the appearance of a right orbitofrontal tumor. The man denied any pre-existing interest in children; he did have a predilection for pornography before the tumor, say Burns and Swerdlow, but now he was downloading child porn and making subtle sexual advances to his prepubescent stepdaughter. His hypersexuality applied to full-grown women, too—so much so, in fact, that he couldn’t keep himself from fondling female nurses and staff during a neurologic examination. Long story short, when the man’s tumor was removed, his prurient interests and behaviors all but disappeared, and since he was no longer deemed a threat to his stepdaughter, he returned home. But his headaches returned, his tumor regrew, and so did the criminal impulse. A “re-resection” of the tumor was accomplished, the man became a good citizen again, and, as far as we know, that remains true today. (In a more recent case co-published by famed neuroscientist Oliver Sacks, and neatly summarized by the Neuroskeptic blog, a 51-year-old man without any criminal history had a portion of his right temporal lobe removed to prevent seizures. Following this, he developed telltale signs of Klüver-Bucy, including hypersexuality. His was another case of “new-onset pedophilia” but as Sacks laments, in spite of this he was nevertheless sentenced to several years in prison for downloading child porn.)

What’s the take-away message? I’ll let you do the hard work of thinking through the implications for our belief in free will and how it might or might not apply. But another intriguing question emerges, too: If a “good” person’s brain can be rendered morally disabled by an invasive tumor or an epileptic fuse-shortage, subsequently causing them to do very bad deeds, then isn’t it rather hypocritical to assume that a “bad” person without brain injury—whose brain is anatomically organized by epigenetics (the complex interplay between genes and experiences)—has any more free will than the neuroclinical case? After all, perhaps it’s just a matter of timing: The “good” are born with brains that can “go bad,” whereas the “bad” are hogtied by a morally disabled neural architecture from the very start. And although it may be less common, if a “bad” person behaves in an upstanding manner, could that be the result of fortuitous brain damage or epilepsy, too?

It’s all brain-based in the end, including the parameters by which one can contemplate and, especially, execute their free will. Perhaps we’re only as free as our genes are pliable in the slosh of our developmental milieus.

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