Sinistrality is associated with (slightly) lower general intelligence: A data synthesis and consideration of the secular trend in handedness

The relationship between the general factor of intelligence (g) and handedness is investigated using a combined sample of 23511 respondents from three large databases: the NLSY'79 (US), NLSY'97 (US) and NCDS (UK). Dextrals - those who use their right hands were found to be 1.22 IQ points higher than sinistrals (left handers) after controlling for sex and age and correcting for sources of measurement error. To see if the association between IQ and handedness was strongest on the abilities that were the best measures of g, the method of correlated vectors was used to test for moderation. Across the three studies, g was found to very weakly negatively moderate the association between ability measure and handedness (ρ = −.023, K = 3, N = 23511), however in the NLSY'79, the coding speed subtest was an outlier in terms of the strength of its association with handedness. Its removal yielded indications of positive moderation in this dataset, which when aggregated boosted the overall vector correlation value to .539 (K = 3, N = 23511), suggesting that g might be an important moderator of this relationship. Secondary analysis of secular trend data on the changing percentage of sinistrals in Western populations indicates that overall, sinistrality has increased, entailing a g decline of .106 points over 150 years (.006 points per decade). The secular increase in sinistrality is consistent with other data indicating long-term declines in developmental stability and may stem from some combination of increasing mutation load and environmental stress in Western populations.