I whole-heartedly back minx's opinion, except for the assertion that she implies at the end that problem solving capability should not be genetic. On a molecular level, there might be slight mutations that don't change protein shape according the hydropathy, rendering them totally worthless, hence retarded, but some of the specialized proteins in the brain might have slightly different affinity values (k-max, that type stuff, for those who understand) that slightly reduce nerve conduction velocity due to mutations in the opening of voltage-gated Ca2+ channels and elsewhere. Its possible but do you really want to blame genetics for everything?