How do new ideas spread from one culture to another?

New ideas spread from one culture to another through a process scholars call cultural diffusion: the movement of things like technologies, art, religions, and languages between people, whether inside one culture or across different ones. The concept was introduced by Leo Frobenius in 1897–98, and it covers everything from the ancient spread of the war chariot and iron smelting to the 20th-century spread of automobiles and Western business suits.

Diffusion doesn't always work the same way. Sometimes an idea stays strong where it started while also spreading outward, called expansion diffusion; sometimes it migrates somewhere new and leaves its birthplace behind entirely, called relocation diffusion. Ideas can trickle down from big cities to small towns regardless of distance, often carried by social elites, a pattern called hierarchical diffusion, or spread person to person with no regard for hierarchy at all, called contagious diffusion. One especially interesting pattern, stimulus diffusion, happens when people reject a specific idea but still adopt the concept underneath it: early Siberian peoples had no use for the domesticated cattle their southern neighbors raised, but they liked the underlying idea of keeping domesticated herds, so they applied it to an animal they already knew well, the reindeer.

How diffusion actually travels also varies. Direct diffusion happens between neighboring cultures through everyday contact like trade, intermarriage, or war, the kind of exchange that gave the US-Canada border both hockey and baseball. Indirect diffusion passes an idea through a middle culture without the original and final cultures ever meeting directly, like Mexican food reaching Canada by way of the United States. And forced diffusion happens when one culture conquers another and imposes its customs on the people it has subjugated, as in the forced Christianization of Indigenous peoples of the Americas by European powers, or the forced Islamization of some West African peoples by the Fula.

A much more extreme theory, hyperdiffusionism, claims that every culture on Earth ultimately traces back to just one source. Some of its earliest versions placed humanity's birthplace in South America, including one Spanish settler's claim that the Garden of Eden itself was located in Bolivia. In 1911, the researcher Grafton Elliot Smith revived the idea in a new form, insisting that ancient Egyptians had invented essentially everything important and spread it worldwide, and in 1939 Lord Raglan named Mesopotamia instead. Mainstream academia generally abandoned hyperdiffusionism after that, though it never entirely disappeared, but the basic phenomenon it tried to explain, how ideas cross from one culture into another, remains central to understanding history.