Tickling is two different things. In 1897 psychologists G. Stanley Hall and Arthur Allen separated knismesis, a light moving itch that annoys without laughter and is shared with animals like scratching dogs and twitching horses, from gargalesis, the harder pressure that brings helpless laughter and is found mostly in humans and other primates, though rats emit ultrasonic chirps when tickled. In 1939 Yngve Zotterman at the Karolinska Institute showed in cats that tickle depends on the same nerves that carry pain. Cutting pain nerves to treat chronic pain dulls the tickle response, and German brain studies link gargalesis to defensive circuits in the hypothalamus. The soles of the feet top the list of ticklish spots, then armpits, neck, under the chin and ribs, while the nerve rich palms do not.

Darwin argued tickling needs the anticipation of pleasure, since a stranger’s tickle brings fear. In a survey of 84 college students, only 32 percent liked being tickled and 36 percent disliked it, yet some who disliked it smiled more than those who enjoyed it. Galileo, in The Assayer, used tickling to argue sensations exist in the perceiver, and Francis Bacon noted that grieving men still laugh when tickled. The combat theory, from J.C. Gregory in 1924 and later psychiatrist Donald W. Black, notes that ticklish spots match the body’s most vulnerable areas in a fight.

• Christenfeld and Harris’s blindfolded tickle machine, which drew as much laughter as a person
• Aristotle’s puzzle about self tickling and Blakemore’s 1998 brain imaging showing the cerebellum cancels the sensation
• Why some people with schizophrenia or related traits can tickle themselves
• Heinz Heger’s account of guards at Flossenbürg torturing a prisoner with goose feathers, and the Japanese punishment kusuguri-zeme
• Vernon Wiehe’s research with 150 adults abused by siblings, including forced tickling that caused vomiting and loss of consciousness