UPDATE: I’d forgotten, but perhaps not completely, that John Brockman edited a book in 2009 in which he asked Science Heavy Hitters the exact question above.  This was based on a 2008 Edge Question, and you can find a lot of the answers here. I don’t think I contributed to that annual question (I’ll be horrified if I did), but the question may have been bubbling in my subconscious.
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All of us like to pride ourselves on our open-mindedness and receptivity to new ideas. After all, that’s one of the main reasons to favor freedom of speech, for the assumption behind that stand is that people will eventually come to the truth, or to the best solutions, via the clash of ideas. That is, people can be persuaded to change their minds. The tacit rider is that we can change our minds as well.

But we all know that we’re less open-minded than we like to believe, and the amount of evidence required to do so is likely to be more than we’d think would be necessary to change our opponents’ minds.

In science, of course, changing your mind is supposed to be a virtue, and in principle it is; scientists change their minds more readily than do others in, say, the humanities. After all, evidence is evidence, and humanities is not so evidence-driven. And religion isn’t evidence-driven at all. Richard Dawkins gives a nice anecdote about this in The God Delusion:

I have previously told the story of a respected elder statesman of the Zoology Department at Oxford when I was an undergraduate. For years he had passionately believed, and taught, that the Golgi Apparatus (a microscopic feature of the interior of cells) was not real: an artifact, an illusion. Every Monday afternoon it was the custom for the whole department to listen to a research talk by a visiting lecturer. One Monday, the visitor was an American cell biologist who presented completely convincing evidence that the Golgi Apparatus was real. At the end of the lecture, the old man strode to the front of the hall, shook the American by the hand and said–with passion–“My dear fellow, I wish to thank you. I have been wrong these fifteen years.” We clapped our hands red. No fundamentalist would ever say that. In practice, not all scientists would. But all scientists pay lip service to it as an ideal–unlike, say, politicians who would probably condemn it as flip-flopping. The memory of the incident I have described still brings a lump to my throat.

As Richard notes, that’s not always the way it works. For one thing, public admissions of error are rare. Usually scientists just shut up and stop espousing their erroneous views while incorporating better ones into their work. For another, scientists are human, and thus loath to relinquish their pet theories. If you’re strongly identified with a theory, it makes it harder to give up, because it becomes part of your reputation and your persona.

By way of asking readers to let us know what they’ve changed our mind about, I’ll give a list of where I’ve veered away from earlier views:

1. I was initially in favor of Richard Nixon in his 1960 Presidential race against Kennedy. To partly exculpate myself, I’ll add that I was only 11 years old (and thus unable to vote), and was good friends with a guy who turned out to be a Republican, and who had a lot of influence on me. By 1965, however, I had become a committed Democrat, though my father remained a fan of Nixon. That became a source of friction with my dad.
2. I believed in God, without really thinking about it, until I had my “conversion experience” in 1967.  Religious people still make fun of me because I had a flash of insight while listening to the Beatles’ Sergeant Pepper album, that there was no good evidence for God. It is as if, to these mockers, music cannot be a catalyst of—or even a background for—thought.
3. I was once in favor of the second Iraq war, buying the bogus evidence that Iraq had weapons of mass destruction. I eventually realized that I was not only duped, but was not particularly skeptical, and I am ashamed of that. I now think that invading Iraq was the wrong thing to do, but I also decry those who say that there’s no discussion to be had about the war and no good argument for deposing Saddam Hussein. I think it’s at least worth discussing, which is what Nick Cohen does in his excellent book What’s Left: How the Left Lost its Way. Cohen thinks starting the war was wrong, but that the Left showed distressingly little sympathy for the horrible evils Saddam Hussein and the Baath party inflicted on the people of Iraq.
4. I haven’t changed my mind about that much in science, simply because I’ve tried not to adopt pet theories to which I’ve become wedded.  I’ve certainly accepted new evidence in areas where I previously was a doubter (e.g., Homo erectus having gone extinct without issue), but haven’t often been a strong proponent of theories that have later been shown to be wrong. One of them, though, is the possibility of sympatric speciation: that new species can form without the need for geographic separation, and that the populations destined to become new species can exchange genes during the process. Adhering to Ernst Mayr’s views on this, I once thought there was no good evidence for such speciation. Now I think there is, though I still don’t see it as a major form of speciation in nature.
5. I once was a strong opponent of the notion of “species selection”: that patterns of biological diversity could reflect the differential extinction and speciation of different species. When writing Speciation with Allen Orr, however, I realized that there was indeed evidence that some patterns, like the number of sexually dimorphic versus sexually monomorphic bird species, could indeed reflect a form of species selection. I discuss this in the very last part of the book. Let me hasten to add, though, that my belief that species selection sometimes goes on does not mean I endorse Steve Gould’s view of punctuated equilibrium (in which species selection played a major role), for I think the process he proposed as part of that theory is completely wrong.

Your turn. What have you been wrong about, or changed your mind about?