Relocation and Expansion Diffusion

Diffusion is just culture spreading from where it started. The question is always how it travels: does it need bodies to move, or can it hop from person to person without anyone relocating? Relocation diffusion is the body-dependent kind. A person or group picks up and moves, and the trait goes wherever they go. If nobody had migrated, the trait simply would not exist at the new site yet.

Expansion diffusion works differently: the trait spreads outward through contact, imitation, or authority, but the original source keeps it. Draw both on the same map and the difference shows up immediately. Expansion diffusion looks like ripples spreading from a stone dropped in water, still touching the stone. Relocation diffusion looks like someone picking up a single drop of that water and carrying it across the map to start a brand new, separate ripple far away, with nothing connecting the two except the person who carried it.

A trait's hearth does not lose the trait just because someone leaves with it. That is the detail that trips people up: migration explains the new location, but it says nothing about whether the old location keeps practicing it. Often both diffusion types run on the same trait at once, which is why real distribution maps look messy. A religion might expand gradually through nearby conversion while also appearing abruptly, as an island, wherever its migrants resettled. Reading a map well means asking, for every new cluster, whether it connects smoothly to a neighboring region or whether it looks like an isolated transplant, since that tells you which mechanism produced it.