Significance
Using administrative register data with information on family relationships and cognitive ability for three decades of Norwegian male birth cohorts, we show that the increase, turning point, and decline of the Flynn effect can be recovered from within-family variation in intelligence scores. This establishes that the large changes in average cohort intelligence reflect environmental factors and not changing composition of parents, which in turn rules out several prominent hypotheses for retrograde Flynn effects.
Abstract
Population intelligence quotients increased throughout the 20th century—a phenomenon known as the Flynn effect—although recent years have seen a slowdown or reversal of this trend in several countries. To distinguish between the large set of proposed explanations, we categorize hypothesized causal factors by whether they accommodate the existence of within-family Flynn effects. Using administrative register data and cognitive ability scores from military conscription data covering three decades of Norwegian birth cohorts (1962–1991), we show that the observed Flynn effect, its turning point, and subsequent decline can all be fully recovered from within-family variation. The analysis controls for all factors shared by siblings and finds no evidence for prominent causal hypotheses of the decline implicating genes and environmental factors that vary between, but not within, families.
Average IQ score by birth year (A) and distribution of IQ scores (B). IQ scores are computed from stanine scores (s) using the conversion IQ = 100 + 7.5 × (s − 5). In A, the shaded region depicts 95% confidence intervals around the cohort mean score. n = 736,808.
Within-family estimates of Flynn effects. The sample underlying estimates in A consists of all families with at least two scored brothers (n = 355,438), B of families with scored brothers in the first two parities (n = 215,514), and C of siblings born 1962–1991 in all families with sons in the first two parities (n = 236,934). Two-brother samples exclude twins and brothers born the same year. The dashed line depicts the trend for firstborn sons (n = 320,739 in A and B and 353,476 in C). Confidence intervals are computed from SEs clustered within families. The shaded region in C covers percentile values from the posterior distribution of the Bayesian model.
IQ score coverage in all families and missing IQ data in two-brother sample. A shows data coverage for all boys present in Norway on their 18th birthday (n = 817,611). B shows noncoverage rates for younger brothers in the two-brother sample; for legibility the figure depicts rates for three 5-y intervals only (n = 65,363; see SI Appendix, Table S4 for the complete series).
