A very neat study design provides strong evidence for the effect of intrauterine factors on IQ: Persistent Effects of In Utero Nutrition Shocks: Evidence From Ramadan Fasting. Children of Muslim mothers (but not non-Muslim mothers) have up to 7 – 8% lower test scores as adults if their birth month lines up such that Ramadan (when Muslims fast) fell during a crucial point in their fetal development. Obvious implication is that not getting nutrition during that developmental period permanently harmed their brain. Most Muslim scholars say that God offers pregnant women the option not to fast if they make it up later, and it looks like they should probably take that offer. Also: I wonder what percent of international IQ differences this explains.Most differences in outcomes can be attributed to variation in choices, abilities and behaviors all of which are in turn influenced by general and familial culture. It is easy sometimes to overlook how both profound and widespread some of those elements can be.
7-8% is half a standard deviation. A shift of the distribution curve of that magnitude is material, particularly at the top end of the distribution curve. Of course this only affects those whose development coincides with a single month in the year but still. You add this to other cultural practices such as the prevalence of first-cousin marriages and you can begin to imagine a picture where a small handful of cultural practices end up having a determinative impact on overall outcomes. Of course this cuts both ways. It does suggest that there are positive impact cultural practices which can be emulated and expanded as well.