The history of medicine is replete with theories that reflect religious beliefs rather than scientific facts. One of the most interesting, and most instructive examples of this phenomenon is the claim that masturbation causes insanity.

Masturbatory insanity pre-occupied medical professionals for almost 250 years. It arose apparently de novo in Europe in the early eighteenth century, and was not finally put to rest until the middle of the twentieth century. Its history of the idea is recounted exhaustively in the scientific paper Masturbatory Insanity: The History of an Idea by E.H. Hare published in the Journal of Mental Science in January, 1962.

Classical medical professionals made no reference to any harmful effects of masturbation. It was not until the early eighteenth century that the idea gained currency upon publication of the book Onania, or the Heinous Sin of Self-Pollution. The author is believed to have been a "clergyman turned quack" and not a reputable medical professional. Like most quacks he advertised an extremely expensive secret remedy.

The assertions gained wide popularity, though. Voltaire, writing in his Dictionnaire Philosophiqe in 1764 reports that the book was then in its 80th edition.

The idea passed into the realm of official medical thought with Tissot's Onania, or a Treatise upon the Disorders produced by Masturbation in 1758. According to Tissot:

...[L]oss of semen ... occasions general debility and so opens the way to consumption, deterioration of eyesight, disorders of digestion, impotence, and so on…[T]he more serious effects are on the nervous system and this is due to the sexual act causing an increased flow of blood to the brain. "This increase of blood explains how these excesses produce insanity. The quantity of blood distending the nerves weakens them; and they are less able to resist impressions, whereby they are enfeebled."

... it (masturbation) may be a forerunner of mania, of dementia and even of senile dementia; it leads to melancholy and suicide; … it is a grave obstacle to cure in those of the insane who frequently resort to it during their illness

The third natural family I would assign to the masturbators… I think it cannot be denied that that vice produces a group of symptoms which are quite characteristic and easily recognized, and give to the cases a special natural history: the peculiar imbecility and shy habits of the very youthful victim; the suspicion and fear and dread and suicidal impulses and scared look and feeble body of the older offenders, passing gradually into Dementia or Fatuity.

... the typical age of onset of masturbatic insanity is between 13 and 20 years; it is at least five times as common in males as in females because of the greater rarity of masturbation in females; in the majority of cases there is a rapid decline into agitated dementia, but where deterioration is less rapid "the obtrusive selfishness, cunning, deception, maliciousness and cruelty of such patients" is such that "the [doctor] may find it impossible to reconcile himself to regarding them as anything else than repulsive eye-sores and a source of contamination to other patients, physically and morally."

Nor is it difficult, from our present standpoint, to see how this came about. Masturbation is most commonly and most frequently practised during adolescence and therefore most likely to be observed when mental disease insidiously deprives an adolescent of his normal social inhibitions.