I'll try to read the book because maybe it answers this, but I struggle to see how they determine causal relationship in the direction of "prohibiting cousin ma

I'll try to read the book because maybe it answers this, but I struggle to see how they determine causal relationship in the direction of "prohibiting cousin marriage" to "more individualism and less kinship and less nepotism". Seems more likely that it could have been the other way around (or, more directly, cultures with strong kinship were just less likely to prohibit things that strengthen those bonds, like allowing cousins to marry).