with choice of major accounting for half of the variation in ROI between majors. College degrees with the highest ROI are in engineering, medicine, business, and other sciences. While nearly 40% of degree programs do not deliver a financial return, a bachelor's degree can also have social benefits that can increase ROI, which is often not accounted for in typical ROI calculations. Effects on productivity Economy-wide, the effect of human capital on incomes has been estimated to be rather significant: 65% of wages paid in developed countries is payments to human capital and only 35% to raw labor. The higher productivity of well-educated workers is one of the factors that explain higher GDPs and, therefore, higher incomes in developed countries. A strong correlation between GDP and education is clearly visible among the countries of the world, as is shown by the upper left figure. Of course, correlation does not imply causation: It's possible that richer countries choose to spend more on education. However, Hanushek found that scores on internationally standardized tests of student achievement do better in explaining economic growth than years of schooling, as discussed further below. Multiple studies have found that investing in the education of poor children on average substantially reduces their risk of poverty as adults and increases their life expectancy. Children in the 1962 Perry Preschool program and matched controls have been followed for decades since. The Perry Preschool participants had substantially fewer teenage pregnancies, fewer high school dropouts, less crime and higher incomes on average as adults. And the results have been intergenerational: The children of the Perry Preschool children have similarly had fewer school suspensions, higher levels of education and employment, and lower levels of participation in crime, compared with the children of those in the control group. To distinguish the part of GDP explained with education from other causes, Weil has calculated how much one would expect each country's GDP to be higher based on the data on average schooling. This was based on the above-mentioned calculations of Hall and Jones on the returns on education. GDPs predicted by Weil's calculations can be plotted against actual GDPs, as is done in the figure on the left, demonstrating that the variation in education explains some, but not all, of the variation in GDP. Finally, the matter of externalities should be considered. Usually when speaking of externalities one thinks of the negative effects of