Econometric exercises show that these differences are not simply due to e.g. cohort effects. The U-shape in education persists when regressing the probability of being an entrepreneur on a set of demographics using discrete choice models. This is found both by Blanchflower (2000) in data across 19 OECD countries, and by Schjerning and Le Maire (2007) in Danish data, using very fine education categories. Combining data from Eurobarometer Surveys and General Social Surveys for 1975 to 1996 for individuals aged 16–64, Blanchflower (2000) finds that controlling for age, education, gender, household size, the number of children under the age of 15 in the household and the gender-specific country unemployment rate, “the least educated (age left school 22 years) have the highest probabilities of being self-employed” (p. 488). This pattern is statistically significant.5 A similar pattern arises in results reported in Section 5 of this paper. Similarly, Schjerning and Le Maire (2007) controlling for age, wealth, number of children by age, marital status, immigrant status and origin, and the spouse's self-employment status still find that the probability of being self-employed is lowest for the intermediate education categories of post secondary education and a short cycle of higher education, and higher at the extremes. A linear specification for education would not be able to pick this up. Evans and Leighton (1989) for instance, using years of schooling as a measure of education, do not find it to be significant when controlling for urban vs rural, experience, unemployment status, father's occupation, and some sectors.
As far as the evidence goes, the U-shaped relationship between entrepreneurship and educational attainment hence emerges robustly. In spite of this, it has received no systematic attention in the previous literature. The lower end of the U carries substantial weight: