I've made clear in other posts my beliefs on this issue: a lot of factors have a demonstrable impact on IQ, and as such the causal density of this issue is super-high. One hypothesis that explains the inter-group differences in IQ is the one that says genetics account for the majority of the difference, however this is established almost solely through inference as the actual genes that would cause such differences have not been identified. As such, the validity of explanations pointing to differences in socio-economic status, culture or a combination of these is at least equal or even better as there are well-understood mechanisms at work.