Spain and Shakeshaft aren't relevant.
Sure they are. The mutations involved in mutation load are, almost by definition, rare; if they were singly or in aggregate with large effects, that should show up in surveys on the high end. Instead, we just get that with the low end, which is consistent with there being occasional very rare or de novo mutations which can drastically reduce below average, but not that they will increase multiple or many SDs for the above average who already escaped the retardation-bullets.
If there were aggregate effects, how would they show up in Spain and Shakeshaft? Just going by the abstract, Spain is looking for genes where the rare variant has a positive effect. That is the opposite of the mutational load theory, and they don't find any. I think Shakeshaft reaches the same conclusion by pedigree analysis.
Say that there are 10k genes, MAF=0.01, each worth 1.5 IQ points. What would Spain detect? If the TIP population is 10/3σ,* then these 10k genes each appear as the mutant 2/3 as often, 20 hits, rather than the expected 30. That's a 2 ...
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.