I have been perplexed at Heckman's positions and have wondered why he is coming to such different conclusions than would be supported by the body of research available. On the other hand, he is a Nobel economist. I had assumed that there must be something he was seeing that I wasn't but could not imagine what that was.
Apparently I am not the only one to have noticed this disconnect. From How does a Nobel-prize-winning economist become a victim of bog-standard selection bias? by Andrew Gelman. On a paper co-authored by Heckman and dealing with the statistical treatment used.
This whole thing is unfortunate but it is consistent with the other writings of Heckman and his colleagues in this area: huge selection bias and zero acknowledgement of the problem. It makes me sad because Heckman’s fame came from models of selection bias, but he doesn’t see it when it’s right in front of his face.Both of us may be wrong and Heckman right but it is nice to have company in my skepticism.
First, Heckman is a renowned scholar and he is evidently careful about what he writes. We’re not talking about Brian Wansink or Satoshi Kanazawa here. Heckman works on important topics, his studies are not done on the cheap, and he’s eminently reasonable in his economic discussions. He’s just making a statistical error, over and over again. It’s a subtle error, though, that has taken us (the statistics profession) something like a decade to fully process. Making this mistake doesn’t make Heckman a bad guy, and that’s part of the problem: When you tell a quantitative researcher that they made a statistical error, you often get a defensive reaction, as if you accused them of being stupid, or cheating. But lots of smart, honest people have made this mistake. That’s one of the reasons we have formal statistical methods in the first place: people get lots of things wrong when relying on instinct. Probability and statistics are important, but they’re not quite natural to our ways of thinking.
And, remember, selection for statistical significance is not just about the “file drawer” and it’s not just about “p-hacking.” It’s about researcher degrees of freedom and forking paths that researchers themselves don’t always realize until they try to replicate their own studies. I don’t think Heckman and his colleagues have dozens of unpublished papers hiding in their file drawers, and I don’t think they’re running their data through dozens of specifications until they find statistical significance. So it’s not the file drawer and it’s not p-hacking as is often understood. But these researchers do have nearly unlimited degrees of freedom in their data coding and analysis, they do interpret “non-significant” differences as null and “significant” differences at face value, they have forking paths all over the place, and their estimates of magnitudes of effects are biased in the positive direction. It’s kinda funny but also kinda sad, that there’s so much concern for rigor in the design of these studies and in the statistical estimators used in the analysis, but lots of messiness in between, lots of motivation on the part of the researchers to find success after success after success, and lots of motivation for scholarly journals and the news media to publicize the results uncritically. These motivations are not universal—there’s clearly a role in the ecosystem for critics within academia, the news media, and in the policy community—but I think there are enough incentives for success within Heckman’s world to keep him and his colleagues from seeing what’s going wrong.