[jimar]

Actually, "heritability" in the scientific context can be counterintuitive sometimes. I'd bet that the heritability of slavery in the 19th century was actually pretty low - I wouldn't imagine that monozygotic twin pairs are any more/less likely to be be slaves than same-sex dizygotic counterparts. Indeed, the majority of variation of the "is a slave" phenotype would be due to environment as slaves get set free.

Think of a trait like hair colour in East Asians. Having black hair is completely determined by genes, but heritability is pretty much zero since (with maybe a few extreme exceptions) variation in hair colour in that population is entirely environmental (use of hair dyes, etc.). In Europeans however, heritability is probably much higher due to greater variation in hair colour.