By their words ye shall know them: Evidence of genetic selection against general intelligence and concurrent environmental enrichment in vocabulary usage since the mid 19th century.

Michael A Woodley Of Menie,Gerhard Meisenberg,Heitor B F Fernandes,Aurelio José Figueredo

doi:10.3389/fpsyg.2015.00361

Abstract

It has been theorized that declines in general intelligence (g) due to genetic selection stemming from the inverse association between completed fertility and IQ and the Flynn effect co-occur, with the effects of the latter being concentrated on less heritable non-g sources of intelligence variance. Evidence for this comes from the observation that 19th century populations were more intellectually productive, and also exhibited faster simple reaction times than modern ones, suggesting greater information-processing ability and therefore higher g. This co-occurrence model is tested via examination of historical changes in the utilization frequencies of words from the highly g-loaded WORDSUM test across 5.9 million texts spanning the period 1850–2005. Consistent with predictions, words with higher difficulties (δ parameters from Item Response Theory) and stronger negative correlations between pass rates and completed fertility declined in use over time whereas less difficult and less strongly selected words, increased in use over time, consistent with a Flynn effect stemming in part from the vocabulary enriching effects of increases in population literacy. These findings persisted when explicitly controlled for word age, changing literacy rates and temporal autocorrelation. These trends constitute compelling evidence for the co-occurrence model.

Highlights

IntroductionEver since Galton (1869) forecast declining intelligence on the basis of the shifting demographics of the Victorian population, there has been controversy about the future of human intelligence.Early use of IQ testing seemed to confirm Galton’s (1869) predictions, as most studies found that IQ was inversely related to fertility, suggesting directional genetic selection for lower intelligence (Lynn, 2011) – a trend that persists into the present (Lynn and van Court, 2004; Meisenberg, 2010; Reeve et al, 2013; Kanazawa, 2014).Co-occurrence effects in word usageIn the West, up until the early to mid 19th century, those with high levels of socioeconomic status, wealth, and education (all of which are proxies for intelligence; Herrnstein and Murray, 1994) had higher numbers of surviving offspring relative to those with comparatively lower levels (Clark, 2007; Skirbekk, 2008), suggesting that higher intelligence may have conferred fitness advantages on individuals having to cope with extremes of cold, disease outbreaks, and conflict (Woodley and Figueredo, 2013)
The negative main effect of word debut year indicates that the more recently introduced the word, the lower the word usage, which is consistent with Curzan’s (2009) observations. These findings provide compelling evidence for the cooccurrence model, adding to the nomological net from which these and other predictions have been derived (Woodley et al, 2014a)
The findings build on this previous research by showing direct contributions to the decline in frequency among difficult words stemming from genetic selection

Summary

Introduction

Ever since Galton (1869) forecast declining intelligence on the basis of the shifting demographics of the Victorian population, there has been controversy about the future of human intelligence.Early use of IQ testing seemed to confirm Galton’s (1869) predictions, as most studies found that IQ was inversely related to fertility, suggesting directional genetic selection for lower intelligence (Lynn, 2011) – a trend that persists into the present (Lynn and van Court, 2004; Meisenberg, 2010; Reeve et al, 2013; Kanazawa, 2014).Co-occurrence effects in word usageIn the West, up until the early to mid 19th century, those with high levels of socioeconomic status, wealth, and education (all of which are proxies for intelligence; Herrnstein and Murray, 1994) had higher numbers of surviving offspring relative to those with comparatively lower levels (Clark, 2007; Skirbekk, 2008), suggesting that higher intelligence may have conferred fitness advantages on individuals having to cope with extremes of cold, disease outbreaks, and conflict (Woodley and Figueredo, 2013). Subsequent increases in global temperature, coinciding with the end of the Little Ice Age in the mid-19th century, reduced environmental harshness, boosting agricultural yields reducing ecological stress and conflict (see: Zhang et al, 2007, 2011 for a demonstration of the inverse historical relationship between temperature and conflict) This would have substantially relaxed selection against those with lower intelligence (Woodley and Figueredo, 2013). The combined effect of these was a demographic transition characterized by general reductions in fertility, which were most pronounced among those with higher intelligence (Lynn, 2011) This was mediated primarily by fertility control coupled with the increasing prevalence of opportunities to delay fertility (i.e., higher education, increasing status competition, etc., which disproportionately attenuated the fertility of high-IQ women relative to men; Low et al, 2002; Meisenberg, 2010)

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Conclusion