In addition, there is the question of how representative the Perry population is of the general African-American population. Those who advocate access to universal early childhood programs often appeal to the evidence from the Perry study, even though the project only targeted a disadvantaged segment of the population.

We present some suggestive evidence that the limited effect of the Perry program on the education of males was due to the peculiarities of the Michigan economy. High school degrees were not required to work in well-paying manufacturing jobs. Perry treatment males appear to have adjusted to the decline in manufacturing that occurred in Michigan better than the controls. This accounts for the statistically significant treatment effects in employment and earnings found formales at age 40.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3524308/