Humans are ticklish due to two main types of responses: knismesis, a light irritation like a bug on the skin that prompts brushing it away, and gargalesis, intense repeated touching of sensitive areas that triggers laughter and withdrawal.

Types of Tickling

Knismesis acts as a protective reflex against parasites or insects and can be self-induced. Gargalesis, however, cannot be self-induced because the brain does not perceive self-touch as a threat, involving the hypothalamus for involuntary emotional and fight-or-flight reactions.

Evolutionary Purpose

Ticklishness likely evolved to safeguard vulnerable spots like the abdomen, throat, feet, and armpits, where nerve endings are dense, by prompting defensive movements. It may also build touch sensitivity, encourage social bonding, or train reflexes, especially in young children.

Why Laughter Occurs

Tickling activates brain areas linked to pain, touch receptors, and autonomic responses, causing laughter even if unpleasant, similar to crying from onions. Individual differences arise from nerve density, genetics, mood, and sensitivity levels.