You can see how the still-nascent disciplines of child psychology and pediatrics pounce on the problem of masturbation. As the sociologist Steven Ward has noted, by establishing masturbation as a deeply worrying, deeply important problem, psychologists established themselves as an indispensible authority. (The medical historian Jonathan Gillis has made a similar argument about thumb-sucking and pediatricians.) Ultimately, the masturbation madness might not have been very helpful for the parents or the children involved, but it was very useful for the psychologists.

The 1920s were the height of behaviorism in child psychology: the belief that children can be taught to do, or not to do, pretty much anything. Many psychologists, including the infamous behaviorist John Watson, argued that masturbation and thumb-sucking were habits that could be untaught, and moreover, that they were so malignant they were worth preventing at all costs -- even if the cost was the child not being able to move. Infants had their nightgowns pinned to their cribs. Their elbows were strapped to splints (to prevent wayward arm-bending), or their knees were strapped together (to prevent wayward leg-opening). Their hands were stuffed in mittens or tied down.

This treatment was not some weird, deviant aberration from the standard advice. It was the standard advice. "It was doubtless an exaggeration to claim that tied hands were commonplace at the beginning of the twentieth century," the authors of a cultural history of masturbation write, "but the recommendations to this effect and the examples that were cited are numerous. A Scottish woman recalled that, as a child during this period, her hands had sometimes been so tightly tied that her piano teacher questioned her about her striped wrists."

There were scarcely any remedies too severe. Some parents were so scared that they threatened to remove the cause of the problem altogether. This was not a new tactic: for many decades, physicians, when presented with chronic cases, had been said to brandish a butcher knife.

Then the fever breaks. By the 1940s, American childrearing has taken on a new and more empathetic, Freudian-lite tenor -- the tone that Benjamin Spock would master in his books. After decades of hysteria over masturbation, of punishments and strictures and end-is-nigh warnings, the subject is dropped. The end was not in fact nigh.

The revisions in Infant Care alone are enough to induce whiplash. In 1914, the manual told mothers that masturbation would wreck their children for life. In 1942, it told mothers that they should ignore the habit altogether. Pity the grandparents. They must have been deeply confused.

What happened? Masturbation had lost its power to shock. Partly this was because the activity had been drained of its deviant content -- it was no longer seen as actually being masturbation. The interest of a infant in his genitals was explained away as a sort of meandering, meaningless curiosity. "His main active aim was to explore his world; autoeroticism was an incidental by-product of such exploration," the sociologist Martha Wolfenstein wrote at the time, summing up the conventional wisdom.