"The High Cost of Low Educational Performance: the long-run economic impact of improving PISA outcomes", Hanushek & Woessmann 2010:

This report uses recent economic modelling to relate cognitive skills – as measured by PISA and other international instruments – to economic growth. The relationship indicates that relatively small improvements in the skills of a nation’s labour force can have very large impacts on future well-being.

...A modest goal of having all OECD countries boost their average PISA scores by 25 points over the next 20 years – which is less than the most rapidly improving education system in the OECD, Poland, achieved between 2000 and 2006 alone – implies an aggregate gain of OECD GDP of USD 115 trillion over the lifetime of the generation born in 2010 (as evaluated at the start of reform in terms of real present value of future improvements in GDP) (Figure 1). Bringing all countries up to the average performance of Finland, OECD’s best performing education system in PISA, would result in gains in the order of USD 260 trillion (Figure 4). The report also shows that it is the quality of learning outcomes, not the length of schooling, which makes the difference. Other aggressive goals, such as bringing all students to a level of minimal proficiency for the OECD (i.e. reaching a PISA score of 400), would imply aggregate GDP increases of close to USD 200 trillion according to historical growth relationships (Figure 2).

...Using data from international student achievement tests, Hanushek and Kimko (2000) demonstrate a statistically and economically significant positive effect of cognitive skills on economic growth in 1960-90. Their estimates suggest that one country-level standard deviation higher test performance would yield around one percentage point higher annual growth rates. The country-level standard deviation is equivalent to 47 test-score points in the PISA 2000 mathematics assessment. Again, in terms of the PISA 2000 mathematics scores, 47 points would be roughly the average difference between Sweden and Japan (the best performer among OECD countries in 2000) or between the average Greek student and the OECD average score. One percentage point difference in growth is itself a very large value, because the average annual growth of OECD countries has been roughly 1.5%.

Their estimate stems from a statistical model that relates annual growth rates of real GDP per capita to the measure of cognitive skills, years of schooling, the initial level of income and a wide variety of other variables that might affect growth including in different specifications the population growth rates, political measures, or openness of the economies.

...The relationship between cognitive skills and economic growth has now been demonstrated in a range of studies. As reviewed in Hanushek and Woessmann (2008), these studies employ measures of cognitive skills that draw upon the international testing of PISA and of TIMSS (Trends in International Mathematics and Science Study) (along with earlier versions of these).7 The uniform result is that the international achievement measures provide an accurate measure of the skills of the labour force in different countries and that these skills are closely tied to economic outcomes.8

...While the PISA tests are now well-known throughout the OECD, the history of testing is less understood. Between 1964 and 2003, 12 different international tests of mathematics, science, or reading were administered to a voluntarily participating group of countries (see Annex Tables A1 and A2). These include 36 different possible scores for year-age-test combinations (e.g. science for students of grade 8 in 1972 as part of the First International Science Study or mathematics of 15-year-olds in 2000 as a part of the Programme on International Student Assessment). Only the United States participated in all possible tests. The assessments are designed to identify a common set of expected skills, which were then tested in the local language. It is easier to do this in mathematics and science than in reading, and a majority of the international testing has focused on mathematics and science. Each test is newly constructed, until recently with no effort to link to any of the other tests. While the analysis here focuses on mathematics and science, these scores are highly correlated with reading test scores and employing just mathematics and science performance does not distort the growth relationship that is estimated; see Hanushek and Woessmann (2009). The goal here is construction of consistent measures at the national level that will allow comparing performance across countries, even when they did not each participate in a common assessment.

...The simplest overview of the relationship is found in Figure 6 that plots regional growth in real per capita GDP between 1960 and 2000 against average test scores after allowing for differences in initial GDP per capita in 1960.14 Regional annual growth rates, which vary from 1.4% in Sub-Saharan Africa to 4.5% in East Asia, fall on a straight line.15 But school attainment, when added to this regression, is unrelated to growth- rate differences. Figure 6 suggests that, conditional on initial income levels, regional growth over the last four decades is completely described by differences in cognitive skills.

Second, to tackle the most obvious reverse-causality issues, Hanushek and Woessmann (2009) separate the timing of the analysis by estimating the effect of scores on tests conducted until the early 1980s on economic growth in 1980-2000. In this analysis, available for a smaller sample of countries only, test scores pre-date the growth period. The estimate shows a significant positive effect that is about twice as large as the coefficient used in the simulations here.

Needless to say, "cognitive skills" here is essentially an euphemism for intelligence/IQ.