Showing their true colours: Possible secular declines and a Jensen effect on colour acuity — More evidence for the weaker variant of Spearman's Other Hypothesis

• Secular trends with respect to color acuity are investigated
• Color acuity is strongly correlated with g
• A secular decline is found
• Color acuity is associated with the Jensen effect

ABSTRACTSpearman's Other Hypothesis predicts that the common factor amongst sensory discrimination measures corresponds to general intelligence (g). The co-occurrence model predicts that low-complexity physiological information-processing indicators reliably measure g across cohorts, and should therefore decline with time due to genetic changes in the broader population. As strong relations exist between general sensory discrimination and g, such measures should show evidence of secular declines. This is tested using N-weighted temporal regression of square-root Total Error Scores (√ TES), obtained from four Western normative samples published in the 1980s, 90s and 2000s (combined N = 752) evaluated using the Farnsworth–Munsell 100-Hue colour acuity test (disattenuated g loading = .78). A significant temporal β value of .37 was found (controlling for national IQ), suggesting a decline in colour acuity equating to a reduction in g of − 3.15 points per decade. Analysis of the subset of the cohorts aged 20–29 years, in which colour acuity is maximized, reveals a larger secular decline (β = .67, N = 199, − 5.85 points per decade). The small number of studies employed in these analyses makes these findings tentative however. Also consistent with a weaker variant of the Other Hypothesis is the finding that 100-Hue acuity-IQ correlations are associated with the Jensen effect. The aggregate vector correlation across two studies is .63 (N = 932.5, p < .05).