Fecund
What theory are you referring to? The race and crime one?
I do think ethnicity does have an affect on many things. Different ethnic groups have different values, that can't be explained by differences in income alone. For example, Mexicans tend to throw big birthday parties for their kids, regardless of their income level. People from certain Asian countries tend to emphasize education, regardless of their income level. Germans tend to be more stoic than Italians, regardless of their income level.