Robert Lucas in biology class

Back in August, a bunch of people were talking about Paul Romer and Bob Lucas and history of macro and stuff like that. Somehow I missed this post, where Brad DeLong dug up a Bob Lucas memoir and made fun of Lucas' college biology class exploits. For reference, here's a longer version of Lucas' story:

The only science course I took in college was Natural Sciences II - a biology course. We read a modern anatomy text, and also selections from Darwin, Mendel, and others... 

[T]here was nothing spooky about Mendel’s genetic theories. They were clear, they made some kind of sense (though there was nothing molecular in our Nat Sci II readings), you could work out predictions that would surprise you, and these predictions matched interesting facts. We did a classroom experiment with fruit flies, focused on eyes, and pooled the results. Our assignment was to write up the results in a lab report and compare them to predictions from a Mendelian model. I had not enjoyed the actual lab work but I liked writing the report and spent the better part of my weekend on it. It was the first time I can recall ever working out the predictions of a scientific theory from its basic principles and testing these predictions against experimental evidence. 

On Sunday evening, my friend Mike Schilder asked to copy [my report on the fruit fly experiment]. I agreed...Mike came back in half an hour, and told me: “This is a good report, but you forgot about crossing-over.” “Crossing over” was a term introduced to us to describe a discrepancy between Mendelian theory and certain observations. No doubt there is some underlying biology behind it, but for us it was presented as just a fudge-factor, a label for our ignorance. I was entranced with Mendel’s clean logic, and did not want to see it cluttered up with seemingly arbitrary fudge-factors. “Crossing over is b---s---,” I told Mike. In fact, though, there was a big discrepancy between the Mendelian prediction without crossing over and the proportions we observed in our classroom data, too big to pass over without comment. My report included a long section on experimental error, describing the chaotic scene that generated the data and arguing that errors could have been large enough to reconcile theory and fact. I handed it in as written. Mike, on the other hand, took my report as it stood, except that he replaced my experimental error section with a discussion of crossing over. His report came back with an A. Mine got a C-, with the instructor’s comment: “This is a good report, but you forgot about crossing-over.” 

I don’t think there is anyone who knows me or my work as a mature scientist who would not recognize me in this story. The construction of theoretical models is our way to bring order to the way we think about the world, but the process necessarily involves ignoring some evidence or alternative theories - setting them aside. That can be hard to do - facts are facts - and sometimes my unconscious mind carries out the abstraction for me: I simply fail to see some of the data or some alternative theory. This failing can be costly and embarrassing to me, but I don’t think it has any effect on the advance of knowledge. Others will see the blind spot, as Mike did with crossing-over, keep what is good and correct what is not.

DeLong makes fun of Lucas for rejecting chromosomal crossover. which is indeed a real thing, and the discovery of which won a Nobel in 1933. It does seem kind of lazy, actually. Even before Wikipedia, it wouldn't have been hard to go grab an advanced textbook and look up how chromosomal crossover works. Lucas is unhappy that it's presented as a fudge-factor, but by the time you're an undergrad you should be too old to depend on the teacher for 100% of your knowledge. If something isn't adequately explained to you, go look up how it works! 

Lucas says that this episode demonstrates a professional weakness of his - the tendency to want to over-simplify theory in order to "bring order" to the world. But I think it demonstrates something slightly different and more worrying: selective empiricism.

In his bio class, Lucas did an experiment on fruit fly inheritance. After the results didn't completely agree with the predictions of basic Mendelian theory, he attributed the discrepancies to experimental error - basically, to measurement noise. Fine (if slightly lazy). But then he takes the experimental result as support for the Mendelian theory, despite the presence of all that experimental error!

If the experimental situation was such a "chaotic scene," then any seeming agreement between Mendelian theory and the lab results might well have been an experimental error. So if college-age Lucas had really been an empiricist, he would have said "This experiment was such a chaotic scene that it provides only very weak support for Mendelian theory." Instead, he concludes that the experimental setup was reliable enough to support the theory that makes "some kind of sense" to him, but too unreliable to indicate the presence of additional phenomena like chromosomal crossover.

In other words, Lucas' conclusion from the experiment relied strongly on his own priors. Or if you prefer a frequentist term, he protected the null hypothesis. That has little to do with oversimplification; it's just a manifestation of confirmation bias. You pick the theories that make sense to you, and believe in them until the data decisively refute them.

But hey, who among us didn't have silly ideas in college?