And yet, offered the chance recently to recast a tarnished legacy, Dr. Watson has chosen to reaffirm it, this time on camera. In a new documentary, “American Masters: Decoding Watson," to be broadcast on PBS on Wednesday night, he is asked whether his views about the relationship between race and intelligence have changed.

“No,’’ Dr. Watson said. “Not at all. I would like for them to have changed, that there be new knowledge that says that your nurture is much more important than nature. But I haven’t seen any knowledge. And there’s a difference on the average between blacks and whites on I.Q. tests. I would say the difference is, it’s genetic.’’

Dr. Watson adds that he takes no pleasure in “the difference between blacks and whites” and wishes it didn’t exist. “It’s awful, just like it’s awful for schizophrenics,” he says. (His son Rufus was diagnosed in his teens with schizophrenia.) Dr. Watson continues: “If the difference exists, we have to ask ourselves, how can we try and make it better?”

Dr. Watson’s remarks may well ignite another firestorm of criticism. At the very least, they will pose a challenge for historians when they take the measure of the man: How should such fundamentally unsound views be weighed against his extraordinary scientific contributions?

In response to questions from The Times, Dr. Francis Collins, the director of the National Institutes of Health, said that most experts on intelligence “consider any black-white differences in I.Q. testing to arise primarily from environmental, not genetic, differences.”

Dr. Collins said he was unaware of any credible research on which Dr. Watson’s “profoundly unfortunate" statement would be based.