Oh yeah, that's another good proxy. But the one you brought up, polo, is also a good one, because I remember about a statistics example, hopefully not made up -

Oh yeah, that's another good proxy. But the one you brought up, polo, is also a good one, because I remember about a statistics example, hopefully not made up - where a correlation was found between keeping horses and good health, and horses just turned out to be a proxy for wealth.