You probably wouldn't detect it since there are so many effects at play here. For example, the parents of those born after WW2 in western countries are on avera

You probably wouldn't detect it since there are so many effects at play here. For example, the parents of those born after WW2 in western countries are on average much younger than before (especially men) or after. Parental age has a significant effect on mutation rate and hence mental illness. As a consequence, bare intelligence is slightly increased in the same generation.