I've found a flaw in my analysis.

It turns out that I ignored most posts with zero comments. The results including those posts significantly change the picture. I'm not sure that my theory is wrong, but I need to do some more analysis in the database. There's a really good chance I was thinking too fast.

Showing comments costs a post votes.

I believe that comments are most distracting to people who are reading a Q&A page. Comments are most useful for people who are authors or editors of individual posts. It's pretty easy to measure the output of authors and editors by looking at how often they create or revise posts. It seems harder to measure the output of readers on most sites. But on sites with voting (such as ours) the output is easy to measure: readers contribute by voting (up or down) on posts.

My guess is that when readers get to the bottom of a post, they tend to forget to go back to the top of the post and vote if there are comments to read. As a compulsive reader, I can say, anecdotally, that I'm far more likely to keep reading if there's more text. It's possible that comment clutter prevents people from going back to vote on posts without extra mental effort. If so, displaying comments might cost the authors of posts reputation.

That's a good story, but how can we test it? A split test where group A sees comments as they are now and for group B all comments are hidden would probably do the trick. If people in the test group vote (up or down) more often that people in the control group, we can be pretty certain that displaying comments is a drag on our reputation-based economy.