Able But Unwilling: Intelligence is Associated with Earlier Puberty and Yet Slower Reproduction

Abstract

Purpose

Research using system integrity theory (SIT) has shown that more intelligent men have higher-quality semen, which is puzzling because although reproductive capability should predict fertility, more intelligent men have fewer children. The current research addresses this puzzle by highlighting the distinct obligate and facultative outcomes that emerge when SIT is integrated with life history theory (LHT) and evolutionary novelty theory (ENT). Specifically, we propose that SIT accounts for more rigidly obligate physiological traits whereas LHT encompasses both obligate traits and flexibly facultative behaviors and, thus, permits the ENT-driven expectation that brighter individuals would act in evolutionarily novel ways—e.g., slower reproduction despite possessing capacities for faster reproduction.

Methods

We examined this logic using another obligate reproductive trait: the timing of puberty. Based on our proposed synthesis of SIT, LHT, and ENT, we tested the prediction that more intelligent people would experience puberty earlier and yet have sex later, engage in less sexual activity, and have fewer children using two nationally representative and generationally distinct samples from the NCDS and Add Health.

Results

Data across both samples confirmed that higher intelligence predicted earlier puberty and indicators of slower reproduction over and above several potential confounds, thus constituting a robust validation of our propositions.

Conclusions

Findings are discussed with regards to the importance of considering the interplay between obligate and facultative traits, particularly when opposing directions might occur due to evolutionarily novel preferences associated with intelligence, as well as in the context of evolutionary mismatch in modern settings. Future directions inspired by this novel synthesis are offered.