Estimating the strength of genetic selection against heritable g in a sample of 3520 Americans, sourced from MIDUS II

• 3520 Americans born in the 1960's were investigated for genetic selection against g.h.
• g.h should decline by − .262 points per decade due to selection.
• Adjusting for mutation accumulation increases the loss to − 1.072 points per decade.
• These g.h losses co-occur with s.e gains (Flynn effect) of 3.6 points per decade.

The relationship between IQ and completed fertility among a sample of 3520 Americans from MIDUS II (1960's birth cohorts) is examined using a common factor comprised of eight cognitive ability measures, in order to determine the rate of phenotypic IQ loss due to genetic selection. Negative correlations are present in both the male and female subsamples, and are associated with a predicted loss in heritable g (g.h) of − .262 points per decade, increasing to − 1.072 points when the additive effect of mutation accumulation is considered. The ability–fertility associations showed Jensen effects at the level of the whole sample (.167), and also separately for each sex (.185 and .147 for the females and males respectively). The magnitude of the expected g.h loss in this cohort due to selection is comparable to that derived from a meta-analysis of disattenuated decadal g.h declines from eight US studies (− .44 points per decade; N = 127,389). There is a Flynn effect in the US amounting to gains of 3.6 points per decade, which are concentrated on more environmentally plastic and specialized sources of ability variance (s.e) suggesting co-occurrent socio-ecological specialization with respect to narrower cognitive abilities in the present cohort.