Flynn effect
The Flynn effect refers to the observed phenomenon of substantial increases in average IQ test scores over generations, typically amounting to about 3 points per decade, primarily documented through standardized intelligence assessments in numerous countries during the 20th century.[1] This trend, which applies more pronouncedly to measures of fluid intelligence such as Raven's Progressive Matrices than to crystallized knowledge, implies that if modern norms were applied to earlier cohorts, previous generations would appear to have lower average intelligence.[1] Named after New Zealand political scientist James R. Flynn, who systematically analyzed and popularized the pattern in the 1980s using data from Dutch military testing and other longitudinal records, the effect challenges static conceptions of intelligence by highlighting generational shifts uncorrelated with genetic changes within populations.[2][3] The gains have been uneven across domains, with larger rises in visuospatial and abstract reasoning tasks compared to verbal or arithmetic abilities, suggesting influences tied to modernization rather than uniform enhancement of general cognitive ability (g).[1] Proposed explanations emphasize environmental factors, including improved nutrition, widespread education, reduced family sizes allowing greater parental investment, and increased exposure to complex stimuli through technology and media, though no single cause fully accounts for the variance observed globally.[4] Empirical evidence indicates the effect has been strongest in developing nations during recent decades, while in high-income countries like Scandinavia and the United States, IQ gains have slowed, stagnated, or reversed since the late 20th century, with some studies reporting declines of 2-7 points per generation in fluid intelligence metrics.[5][6] Controversies persist regarding the effect's implications for interpreting IQ as a measure of innate potential versus malleable skills, with critics noting that score inflation may reflect test-specific familiarity or motivational changes rather than true cognitive gains, and that the observed g-loading of increases is often weak or negative.[6] Recent reversals have fueled debates over causal realism, including potential dysgenic pressures from differential fertility rates favoring lower-IQ individuals or immigration patterns altering population averages, though mainstream explanations prioritize ongoing environmental deteriorations like educational quality or health factors.[7] These developments underscore the need for updated norming of tests to avoid misdiagnosis in clinical and educational contexts, while highlighting academia's occasional reluctance to explore politically sensitive genetic or selective explanations despite empirical patterns.[8]Historical Background
Discovery and Naming
The Flynn effect, denoting the observed rise in IQ test scores across generations, received its name from New Zealand psychologist James R. Flynn, who systematically compiled and analyzed evidence of these gains in a landmark 1984 paper published in Psychological Bulletin.[2] Flynn's analysis revealed average IQ increases of about 0.31 points per year—or roughly 3 points per decade—between 1932 and 1978, based on 18 comparisons using Stanford-Binet and Wechsler scales across the United States.[9] This work highlighted the pervasiveness of the trend, countering earlier dismissals of sporadic reports as anomalies or artifacts of specific tests or populations.[2] The term "Flynn effect" was coined by Charles Murray in reference to Flynn's contributions, which extended prior isolated findings into a coherent, cross-national pattern documented in his subsequent 1987 paper on gains in 14 nations.[2] Flynn, then at the University of Otago, emphasized that these gains applied broadly to both fluid and crystallized intelligence measures, challenging assumptions about the stability of IQ norms and prompting reevaluation of what intelligence tests truly capture.[10] While pre-1984 studies had noted secular IQ rises—as early as 1936—Flynn's synthesis demonstrated their magnitude and consistency, establishing the phenomenon's empirical foundation.[11]Precursors to Systematic Recognition
Early anecdotal and empirical observations of rising performance on intelligence tests emerged in the 1930s, primarily in the United States, as test administrators noted discrepancies between older norms and contemporary scores.[10] The first documented report came from Runquist in 1936, who analyzed high school seniors in Minneapolis from 1929 to 1933 and found median scores increasing from 33.7 to 42.3, though exact IQ equivalents were not calculated.[10] Subsequent U.S. studies corroborated these trends: Roesell (1937) reported a 4.0 IQ point gain over 14 years among Minnesota youth aged 13-18 (equivalent to 2.9 points per decade); Johnson (1937) observed a similar 4.0-point rise over 14 years in St. Louis children aged 5-11; and Merrill (1938) documented a 3.0-point gain over 21 years nationwide for ages 5-11 (1.4 points per decade).[10] These U.S. findings highlighted patterns such as greater gains in younger children and on non-verbal subtests. Wheeler (1942) measured a 10.8-point increase over one decade in Tennessee schoolchildren aged 6-16, with gains diminishing by age (e.g., 7.9 points at age 6 versus 6.5 at age 16); Smith (1942) in Hawaii noted a 6.0-point gain over 14 years for ages 10-14, with non-verbal scores rising faster than verbal (6.0 versus 2.6 points per decade).[10] By the mid-20th century, adult samples showed comparable shifts: Tuddenham (1948) reported an 11.5-point gain over 26 years in U.S. adults tested from 1917 to 1943 (4.4 points per decade).[10] Norm updates for major tests reflected this obsolescence; Terman and Merrill (1973) observed a 9.9-point gain over 40 years (1932-1972) on the Stanford-Binet across ages 2-18, with larger effects in preschoolers (2.42-2.70 points per decade) than school-age children (0.47-0.97 points per decade).[10] Parallel observations appeared in the British Isles and elsewhere, though often smaller in magnitude. In Scotland, Thomson (1949) found a 1.47-point gain per decade at age 11 between 1932 and 1947 surveys; Boyne and Clark (1959) reported 3.3 points per decade from 1947 to 1957.[10] England's Cattell (1951) documented a modest 0.60-point gain per decade for ages 9-11 from 1936 to 1949.[10] Internationally, early reports included Binning's (1958) 4.33-point gain per decade in Canadian 14-year-olds from 1946 to 1955, and Elley's (1969) 2.19 points per decade in New Zealand youth aged 10-14 from 1936 to 1968.[10] These scattered studies, spanning multiple nations and tests, indicated consistent but uneven secular rises, typically 1-5 IQ points per decade, yet lacked comprehensive synthesis or causal analysis, remaining peripheral to mainstream psychometric discourse until later aggregation.[10]Empirical Observations
Magnitude and Patterns of 20th-Century Gains
![Ourworldindata_wisc-iq-gains-over-time-flynn-2007.png][float-right] In the United States, IQ scores on Wechsler and Binet tests increased by approximately 3.1 points per decade from 1932 to 2001.[12] James Flynn's analysis of U.S. data from the 1930s to the 1970s showed a rise of 13.8 IQ points over 46 years, equating to about 3 points per decade.[9] Globally, a meta-analysis of 271 samples spanning 1909 to 2013 estimated an average gain of 2.31 IQ points per decade across the 20th century, with stronger effects in earlier decades and in developing regions catching up to industrialized nations.[13] Gains exhibited domain-specific patterns, with larger increases on fluid intelligence measures like Raven's Progressive Matrices—often 5 to 6 points per decade—compared to crystallized intelligence tests such as vocabulary, where rises were closer to 1-2 points per decade.[2] In Europe, similar trends emerged; for instance, Norwegian conscript data from 1957 to 1999 indicated consistent gains of around 3 points per decade on verbal and performance scales.[14] Cross-national variations showed higher rates in post-World War II Western Europe (e.g., 20 points over 30 years in the Netherlands on non-verbal tests) versus more modest increases in the U.S. on full-scale IQ batteries.[2][9] These patterns were not uniform across age groups or socioeconomic strata; gains were often more pronounced among children and lower-SES populations early in the century, reflecting broader societal improvements.[12] By mid-century, the effect permeated most demographics in developed countries, though test-specific norms occasionally masked full magnitude due to uneven restandardization.[9] Overall, 20th-century gains totaled roughly 20-30 IQ points in many Western populations, shifting normative distributions such that the average person in 2000 scored comparably to the top 5-10% in 1900.[15]Variations Across Populations and Test Types
The Flynn effect exhibits substantial variation across national populations, with documented IQ gains ranging from 5 to 25 points over a single generation in 14 primarily developed countries, as reported in Flynn's 1987 analysis of standardized test data.[16] These disparities reflect differences in the pace of societal modernization, test familiarity, and environmental factors, with some of the largest increases observed in nations like the Netherlands on Raven's Progressive Matrices (approximately 20 points from 1952 to 1982).[16] In contrast, gains in verbal and arithmetic tests were smaller, often under 10 points, highlighting uneven application across cognitive domains even within countries.[16] More recent meta-analyses confirm that the effect persists but at reduced rates in many developed nations, averaging 2.3 IQ points per decade overall, while remaining robust in developing countries where socioeconomic transitions continue to drive larger secular increases.[9] For instance, positive Flynn effects are prevalent in regions undergoing rapid urbanization and education expansion, such as parts of sub-Saharan Africa and South Asia, contrasting with minimal or negative trends in Scandinavian countries like Norway since the 1990s.[17] Within the United States, subgroup analyses from the National Longitudinal Survey of Youth-Children (1986–2004) reveal no significant differences by gender, race/ethnicity, or urbanization, but accelerated gains for children of higher maternal education (slope 0.24 IQ points higher per year for ages 7–13) and household income (0.13 points higher per year).[18] This pattern suggests that in advanced economies, the effect may disproportionately benefit advantaged groups benefiting from enriched cognitive environments.[18] Regarding test types, gains are consistently larger on measures of fluid intelligence—tasks requiring novel problem-solving, such as Raven's Progressive Matrices (up to 7 points per decade)—than on crystallized intelligence subtests assessing accumulated knowledge, like vocabulary or similarities (around 3.6 points per decade on Wechsler scales).[9] Nonverbal and performance-based tests, which minimize cultural loading, show the most pronounced increases, often exceeding 5 points per decade, whereas verbal tests exhibit smaller or negligible effects in some cohorts.[9] Subtest-specific variations further underscore this: for example, memory and visuospatial abilities display parallel gains comparable to fluid measures, while semantic and arithmetic subtests lag, potentially due to ceiling effects in test exposure among later generations.[9] These differences imply that the Flynn effect primarily captures shifts in abstract reasoning skills rather than uniform enhancements across all cognitive facets.[17]Evidence of Slowing, Plateau, and Reversals
In Norway, longitudinal data from military conscripts born between 1962 and 2009 reveal a reversal of the Flynn effect, with IQ scores rising by approximately 3 points per decade until the 1975 birth cohort, followed by a decline of 7 IQ points (about 0.26 points per year) in subsequent cohorts. This pattern aligns with earlier indications of slowing; Sundet et al. (2004) reported gains of 1.3-2.4 points per decade in verbal and performance IQ from 1962 to 1991, but with evidence of deceleration in later years. Comparable reversals have emerged across other Nordic countries since the mid-1990s. In Denmark, conscript IQ scores increased steadily until around 1990 before declining by roughly 1.5-3 points per decade.[19] Sweden and Finland show similar post-1995 losses averaging 6-7 IQ points when extrapolated over 30 years, particularly in measures of fluid intelligence and visuospatial reasoning.[19] These trends persist even after adjusting for test familiarity and socioeconomic factors, suggesting a genuine diminution in cognitive performance rather than artifacts of measurement.[20] A systematic review by Dutton and Lynn (2016) documented negative Flynn effects in multiple European nations beyond Scandinavia, including a 2-4 point generational decline in the UK since the 1980s, losses in the Netherlands and Belgium from the 1990s onward, and reductions in France and Germany. In the United States, Dworak et al. (2023) analyzed over 394,000 Woodcock-Johnson test administrations from 2006 to 2018, finding declines of 0.3-0.5 points per year in fluid reasoning, short-term memory, and crystallized knowledge, offset only by gains in visual-spatial processing. Australian data from Raven's matrices similarly indicate a plateau or slight reversal since the 1980s.| Country/Region | Onset of Slowing/Decline | Estimated IQ Loss | Key Measures Affected | Source |
|---|---|---|---|---|
| Norway | Mid-1970s | 7 points (1975-1991 cohorts) | Full-scale IQ | Bratsberg & Rogeberg (2018) |
| Denmark | ~1990 | 1.5-3 points/decade | Conscript IQ | Dutton et al. (2016)[19] |
| UK | 1980s | 2-4 points/generation | Raven's matrices | Dutton & Lynn (2016) |
| USA | 2006-2018 | 0.3-0.5 points/year (select domains) | Fluid reasoning, memory | Dworak et al. (2023) |