Income per capita level at which dogs get leashed

As countries move from low to higher income per capita levels, people who live there start leashing their dogs. This happens at GDP per capita levels that are surprisingly similar across societies. The cutoff seems to be about $10,000. Below that, almost all dogs run around free. If it's much lower than that, they're starving, too; if it's just below, they look well-fed--but still do not seem to belong to anyone. When income exceeds ten grand per (human) head, all those stray dogs suddenly get leashed.

One of the great mysteries for social science to tackle.