Subfertility among the very intelligent: An examination of the American Mensa

Recent data on post-war birth cohorts in the U.S.A. show a negative relationship between IQ and fertility, both actual and expected. Previous investigators (e.g. Bajema, the Reeds and Waller) had found a slightly positive relationship for certain pre-war birth cohorts. This secular change in the relationship between IQ and fertility is replicated here for data on the membership of the American Mensa. Members born before the war exhibit a fertility below but close to that of the equivalent cohorts in the nation as a whole. Members born during and after the war exhibit a fertility that is substantially lower than that of the equivalent national cohorts. Apparently, rising fertility, which characterized the period in which the pre-war birth cohorts bore their children, causes a convergence in birth rates across IQ classes. Declining and/or low fertility, which characterizes the period in which the post-war birth cohorts are bearing their children, has the opposite effect, i.e. a divergence in birth rates across IQ classes, with the lower birth rates in the higher-IQ classes. The membership of Mensa may not be a representative sample of the high-IQ population as a whole, as it seems to attract a disproportionate number of low procreators even for this population. However, there is no reason to believe that changes in fertility rates across cohorts within Mensa, relative to changes in the national fertility rates, do not reliably reflect changes in fertility rates across cohorts within the high-IQ population of the U.S.A. as a whole, again relative to those of the nation, though these rates may be fluctuating around different means.