Social stratification is present in all modern societies. Do income differences simply reflect inherited differences in individual abilities and resources? If not, why does not everyone choose strategies that lead to high income? This article shows that the psychological phenomenon known as temporal discounting will lead to differences in educational attainment and social stratification in any society where the demand for workers with a higher level of education is higher than for those with a lower level. The model is used to predict income differences between people with and without college education in seven developed countries, based only on official statistics of the cost and length of college education. The model explains 93% of the variance, strongly suggesting that temporal discounting is a major factor behind income differences.