The Flynn Effect is …” the substantial and long-sustained increase in both fluid and crystallized intelligence test scores measured in many parts of the world from roughly 1930 to the present day.” (Thank you Wikipedia)
And there have been many reasons mentioned why this has happened, but this study has a quite novel explanation for the effect from the seventies on: lead, or rather the decrease in U.S. blood lead levels. Still, someone in the back is coughing ‘correlation’, and indeed it is.
Abstract of the study:
The dramatic decrease in U.S. blood lead levels (BLLs) since the 1970s has been documented-however, the anticipated societal impact on intelligence quotient (IQ) has not. The objectives of this study were to determine whether mean IQs of American adults, adjusted for demographics, have increased in concert with society׳s decreasing BLL.
Mean IQs of eight normal adult cohorts (N=800), tested individually in 2007 by trained clinicians, were compared using ANCOVA and correlation analysis. Cohorts ranged in mean societal BLLs from 4 1/2μg/dL (born 1985-1987) to 19μg/dL (born 1963-1972). IQs were adjusted for confounders-education, gender, ethnicity, region, urban status. To control for age, we analyzed IQ data for a second adult sample (N=800), tested in 1995-all born when BLLs were high (1951-1975, BLL ≥15μg/dL).
When controlling for education, gender, ethnicity, and region, the regression of IQs on BLLs was significant (r=-0.84, p<0.01); the modeled change in BLLs from 20 to 4μg/dL suggests predicted increases of 3.8 IQ points (95% CI, 1.4-6.2). Also controlling for urban status produced significance (r=-0.88, p<0.01) with predicted increases of 5.2 points (95% CI, 2.4-8.0). Control analyses ruled out aging as a confounder.
The dramatic societal decreases in BLLs in the U.S. since the 1970s were associated with a 4-5-point increase in the mean IQs of Americans. This effect is consistent with researchers’ predictions; however, other variables (e.g., medical advances) may have contributed to the IQ gains.