The secular decline in general intelligence from decreasing developmental stability: Theoretical and empirical considerations

Article history: Received 11 June 2015 Received in revised form 13 November 2015 Accepted 20 December 2015 Available online 9 January 2016 The g-loss predicted based on genetic selection is smaller than that observed across various ratio-scale measures of cognitive ability. The difference may result in part from the accumulation of deleterious mutations across generations, reducing g via their effects on developmental stability/fitness. Previously published secular trend data on a developmental stabilitymeasure, craniofacialfluctuating asymmetry (FA) size, forwhite USmales and females covering 14 and 15 decades respectively, are re-analysed.When the secular increases in FA size are rescaled as declines in latent developmental stability, and multiplied by the validity and reliability adjusted developmental stability-g correlation, g-losses of−.16 points per decade are predicted for the males, females and the combined sample. Predicted fitness losses due to mutation accumulation may account for 30% of the generational decline (−.05 points per decade), indicating only a small role for mutations in secular g-loss. The remaining 70% (−.11 points per decade)may result fromdevelopmental stability disrupting environmental change, such as increased exposure to pollutants. Adding these to the g-loss due to selection (re-estimated at −.54 points per decade) yields a combined decadal loss of−.70 points. Additional adjustments for replacementmigration and the generation length-g interaction yield a larger magnitude decadal g-loss of−1.25 points. © 2015 Elsevier Ltd. All rights reserved.