A Nobel for Natural Experimenters
Why do natural experiments matter? One of the toughest problems in economic research is figuring out whether a relationship between two variables is causal or coincidental. So, for example, economists find that the lifetime earnings of people who go to school for 12 years are higher than those of people who go to school for 11 years. But what if those who stay in school longer are more motivated or smarter than those who are in for only 11 years? Then the earnings of the more-schooled would be higher even if schooling per se doesn’t add much to earnings. What one would ideally like is to compare the earnings of people whose motivations and intelligence don’t differ.
Enter compulsory schooling. In 1991, Mr. Angrist and the late Alan Krueger noted that under compulsory-schooling laws, students born in the first quarter of the calendar year would be able to leave school earlier than students born in the fourth quarter. Sure enough, they found, those born in the fourth quarter had an average of 0.15 year more in school. And the earnings of those in the fourth quarter were 1.4% higher than the earnings of those born in the first quarter. Extrapolate that to a full-year difference in schooling, and you can conclude that one extra year of schooling raises earnings by about 9%.
This is from David R. Henderson, “‘Natural Experiments’ Lead to an Economics Nobel,” Wall Street Journal, October 11, 2021. (The print edition will be tomorrow, October 12.)
I would quote more, especially about the controversial Card/Krueger finding on the minimum wage, but my contract with the Journal allows me to quote only 2 paragraphs until 30 days is up. I’ll post the whole thing on November 11.
Thanks to Alex Tabarrok for taking a look at the draft, although the section he wisely suggested I add, on Card’s study on the Mariel Boatlift, was cut.
I’ll publish a couple of the deleted sections tomorrow.