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Mohamed Nagdy and Max Roser (2017) - "Intelligence". Published online at Retrieved from: '' [Online Resource]

The most common way of assessing intelligence is IQ testing. The Flynn effect describes the phenomenon that over time average IQ scores have been increasing in all countries since the turn of the twentieth century (the earliest point in time for which data is available). The change in IQ scores has been approximately three IQ points per decade. One major implications of this trend is that an average individual alive today would have an IQ of 130 by the standards of 1910, making them more intelligent than 98% of the population at that time. Equivalently, an individual alive in 1910 would have an IQ of 70 by today’s standards, a score that would be low enough to be considered intellectually disabled in the modern world.

I. Empirical View

I.1 IQ Gains Over Time, by world region

This first visualisation shows the gains in IQ that different world regions have made since the first year for which data is available for a particular region.

I.2 IQ Gains Over Time and The Flynn Effect, by country

In a comprehensive study of the Flynn effect, Jakob Pietschnig and Martin Voracek looked at 271 independent samples comprising 3,987,892 participants covering a time span of 105 years (1909–2013).1 They find evidence to support the claim that IQ has been increasing substantially over time.2 The paper discusses several explanations for the observed increases, namely education, exposure to technology and nutrition. For the definitions of the different IQ measures presented below click here.

Domain-specific IQ gain trajectories, 1909-2013 – Pietschnig and Voracek3

Pietschnig and Voracek - Flynn effect

The following charts show the estimated gain in average IQ for a selection of countries and world regions. It is important to note that this is not a reflection of how intelligent a country/region is but instead how quickly advances were being made. Cross country comparisons are of limited usefulness in this context since the data is incomplete, and we would expect developing countries to make larger gains than developed ones. For more information on the drivers and composition of IQ gains, click here.

II. Correlates, Determinants & Consequences

II.1 Composition of IQ Gains

An important and interesting question is, "what has been driving the gains in intelligence?" There are many competing explanations from nutrition and health improvements, greater levels of education to the increasingly abstract nature of human existence. In James Flynn's book What Is Intelligence?: Beyond the Flynn Effect, he decomposes the gains in IQ found for American children in the Weschsler Intelligence Scale for Children (WISC) and finds that much of the gains have come from tests of abstract thinking (similarities test and Raven's progressive matrices). Only a small portion of the gains is due to improvements in information, arithmetic and vocabulary. This observation would support the idea that increases in IQ have been driven by the changing way in which we live.

WISC IQ gains for America, 1947-2002 – Flynn (2007)4

WISC IQ Gains over time - Flynn (2007)

II.2 Alexander Luria's Studies of Reasoning

Alexander Luria, a Russian neuropsychologist, conducted a series of interviews with headmen of villages in rural 1920s Russia as part of a study of reasoning. His research was published in a book titled Cognitive Development: Its Cultural and Social Foundations in 1976. The following extract from James Flynn's What is intelligence? is just one example of the types of responses Luria received from the villagers.

Today we have no difficulty freeing logic from concrete referents and reasoning about purely hypothetical situations. People were not always thus. Christopher Hallpike (1979) and Nick Mackintosh (2006) have drawn my attention to the seminal book on the social foundations of cognitive development by Luria (1976). His interviews with peasants in remote areas of the Soviet Union offer some wonderful examples. The dialogues paraphrased run as follows:

White bears and Novaya Zemlya (pp. 108-109):

Q: All bears are white where there is always snow; in Zovaya Zemlya there is always snow; what color are the bears there?
A: I have seen only black bears and I do not talk of what I have not seen.
Q: But what do my words imply?
A: If a person has not been there he can not say anything on the basis of words. If a man was 60 or 80 and had seen a white bear there and told me about it, he could be believed.

II.3 Population Aging and The Flynn Effect

The effect of an aging population, especially in advanced economies, has an attenuating effect on average cognitive abilities over time. Skirbekk et al. writing in Intelligence create projections of future cognitive abilities and find that if the Flynn effect reaches a saturation point, then average cognitive ability declines into the future. However, if the current Flynn effect persists, average intelligence will continue to rise in spite of an aging population. Consider the following simulations.

Cognitive score in scenario with no cohort effect and constant age variation, 2002-2042 – Skirbekk et al. (2013)5

No cohort effect, Constant age variation. Cognitive score, 2002–2042 - Skirbekk

Projections of cognitive ability, age profile of cognition by cohort in scenario with continued improvement along cohort lines and constant lifespan trajectories – Skirbekk et al. (2013)6

Continued improvement along cohort lines, constant lifespan trajectories - Skirbekk

II.4 Disease Burden and IQ

Disease during pregnancy or early childhood can impair the cognitive development of children permanently. The driving force behind this theory is that if a child becomes seriously ill, the body transfers resources (energy) into fighting off the infection, reducing the amount left for brain development.

Correlation between national average IQ and the disease burden – Eppig et al. (2010)7

Correlation between IQ and disease burden

II.5 Nutrition and Prosperity: Evidence from Aberdeen

An examination of the differences in IQ between two cohorts, one group born in 1921 and the other 15 years apart in 1936, finds substantial differences in IQ over their lifetimes. The study conducted by Staff et al. uses panel (longitudinal) data on the same groups of individuals.8 All students born in either 1921 or 1936 and attending school in Scotland on June 1, 1932 or June 4, 1947 were made to sit intelligence examinations. The authors report that scores on the Raven's Progressive Matrices (RPM) test decreases annually by over one-half point. At age 77 (where there is an overlap in data) there is an estimated difference of 16.5 IQ points between the two cohorts, which is roughly three times larger than expected.

Dr Roger Staff explains that "those born in 1936 were children during the war and experienced food rationing. Although rationing meant that the food was not particularly appetising it was nutritious and probably superior to the older group. In addition, post-war political changes such as the introduction of the welfare state and a greater emphasis on education probably ensured better health and greater opportunities. Finally, in their thirties and forties the 1936 group experienced the oil boom which brought them and the city prosperity. Taken together, good nutrition, education and occupational opportunities have resulted in this life long improvement in their intelligence. Aberdeen has been good for their IQ!" More information on this research can be found here.

II.6 Genetics and Intelligence

The influence of genetics in determining intelligence is both an immensely important and controversial question. The main reason for the controversy surrounding this question is the connection between race, genetics and intelligence, as well as the policy implications for development and equality. Whilst there exists a broad consensus that intelligence is jointly pinned down by genetics and environmental factors, the precise mixture is uncertain.

In 1994 Richard J. Herrnstein and Charles Murray published The Bell Curve, a highly controversial book on intelligence that explored the race gap in IQ that exists in the United States. The authors' belief that IQ is mostly inherited leads them to the following policy recommendations to prevent a decline in national intelligence:

  • The US government should eliminate any welfare incentives for poor (low-IQ) women to have children, since richer (high-IQ) couples have less children.
  • Funding for educational programmes aimed at raising the achievement of poor and disadvantaged students should be stopped.
  • Reducing immigration into the US.
  • The removal of affirmative action programmes as under-representation of particular racial groups is a reflection of ability and intelligence.

The major flaws in the research of Hernstein and Murray rest with their assertions about intelligence, in particular, that intelligence is mostly inherited and therefore invariant to time and environment. This central belief would generate a causal relationship between IQ and socioeconomic indicators such as high school dropout, incarceration, and poverty.9 Research looking into the effects of environment on intelligence expose serious short-comings in the arguments and conclusions of The Bell Curve. Turkheimer et al. (2003) find that the relative importance of genetics and environment in the IQ of genetically identical twins is linked to socioeconomic status.10 They report that "in impoverished families, 60% of the variance in IQ is accounted for by the shared environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse."

II.7 Global Differences in IQ

International comparisons of IQ measures reveal large disparities between developed and developing countries. Richard Lynn and Tatu Vanhanen's controversial research explores this area.11 Lynn and Vanhanen subscribe to the view that IQ is mainly inherited and argue that the lower IQ of these nations is what holds back development. To support this claim they make use of the correlations between standard of living measures and IQ. This conclusion rests on the direction of causality; if low economic development has held back the Flynn effect and prevented intelligence growth, the results of the study are flawed.

The methodology of their research has been heavily criticized for not having consistent measures of IQ across countries and for the way estimates were generated. For example, the average IQ of countries without data is reported as the average of neighboring or comparable countries. In addition to this, the way in which samples were either included or excluded in their analysis is unsystematic, potentially biasing their results. Thomas Nechyba's review in the Journal of Economic Literature points out the following flaw:12

The degree of IQ heritability, however, has no logical bearing on this issue. Even if we take the authors’ reading of the literature as given and assume that 80 percent of IQ is heritable, the entire observed cross country difference in IQ may be environmentally rather than genetically driven. Consider, for instance, a plant variety whose size is known to be 80 percent heritable. If we grow genetically identical seeds of this plant in two plots under the same initial soil conditions and then fertilize only one plot, the dramatic differences in plant height that emerge will be entirely driven by environmental factors despite the high heritability of plant height. The fact that some countries score lower on IQ tests than others thus has nothing to say about the degree to which IQ scores are genetically predetermined or the degree to which they will change under different circumstances. Developed countries may simply be like the plants that received fertilizer earlier for reasons having nothing to do with IQ.

III. Data Quality & Definitions

Different IQ measures

The different measures of IQ displayed in the figure below correspond to different types of intelligence.

  • Fullscale IQ: overall intelligence
  • Crystallized IQ: the ability of an individual to deploy skills, knowledge and experience to answer questions
  • Fluid IQ: logic and problem solving skills in novel situations independent of acquired knowledge
  • Spatial IQ: spatial judgement and the ability to visualise information to solve problems

IV. Data Sources

One Century of Global IQ Gains - Jakob Pietschnig and Martin Voracek

Pietschnig, Jakob, and Martin Voracek. "One Century of Global IQ Gains: A Formal Meta-Analysis of the Flynn Effect (1909-2010)." Perspectives on Psychological Science, 2015, 282-306. doi:10.1177/1745691615577701.

Intelligence and the Wealth and Poverty of Nations - Richard Lynn and Tatu Vanhanen

Lynn, Richard, and Tatu Vanhanen. "Intelligence and the Wealth and Poverty of Nations." (2009).