of human capital; countries that rely on natural resource exports may tend to neglect education because they see no immediate need for it. Resource-poor economies like Singapore, Taiwan or South Korea, by contrast, spent enormous efforts on education, and this contributed in part to their economic success (see East Asian Tigers). Other researchers, however, dispute this conclusion; they argue that natural resources generate easily taxable rents that can result in increased spending on education. However, the evidence for whether this increased spending translates to better education outcomes is mixed. A study on Brazil found that oil revenues are associated with sizable increases in education spending, but only with small improvements in education provision. Similarly, an analysis of early-20th century oil booms in Texas and neighboring states found no effect of oil discoveries on student teacher ratios or school attendance. However, oil-rich regions participated more intensively in the Rosenwald schoolbuilding program. A 2021 study found that European regions with a history of coal mining had 10% smaller per-capita GDP than comparable regions. The authors attribute this to lower investments in human capital. Adverse effects of natural resources on human capital formation might come through several channels. High wages in the resource extraction industry could induce young workers to drop out of school earlier in order to find employment. Evidence for this has been found for coal or fracking booms. In addition, resource booms can lower the wages of teachers relative to other workers, increasing turnover and impairing students' learning. Incomes and employment A study on coal mining in Appalachia suggests that "the presence of coal in the Appalachian region has played a significant part in its slow pace of economic development. Our best estimates indicate that an increase of 0.5 units in the ratio of coal revenues to personal income in a county is associated with a 0.7 percentage point decrease in income growth rates. No doubt, coal mining provides opportunities for relatively high-wage employment in the region, but its effect on prosperity appears to be negative in the longer run." Another example was the Spanish Empire which obtained enormous wealth from its resource-rich colonies in South America in the sixteenth century. The large cash inflows from silver reduced incentives for industrial development in Spain. Innovation and investment in education were therefore neglected, so that the prerequisites for successful future development were given up. Thus, Spain soon lost its economic strength