Across cultures as distant as Amsterdam, Athens, and Beijing, nearly every human being has been tickled — and responds to it in the same way. A new cross-cultural study confirms that ticklishness is not a social custom but a biological inheritance, one we share with our primate cousins and perhaps even with rats. The question science is now pressing is not whether tickling is universal, but why evolution would wire laughter and vulnerability together — and what that ancient reflex tells us about the bonds between people.