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

• Published data on secular trends in craniofacial fluctuating asymmetry are reanalyzed.
• Significant linear increases in FA are found for white males and females are found spanning 160 years.
• This suggests a secular decline in developmental stability.
• FA and g are robustly negatively correlated.
• Equivalent g-loss for the combined sex sample is −.16 points per decade.

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 stability measure, craniofacial fluctuating asymmetry (FA) size, for white US males 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 from developmental 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 replacement migration and the generation length-g interaction yield a larger magnitude decadal g-loss of − 1.25 points.