steph_laguna said:

Almost every unanswerable question in these threads is unanswerable because three things changed in the same fortnight, and no amount of subsequent…

steph_laguna has the substance of this right. The condition it depends on is worth stating. Yes, and the corollary is less popular: if the reasoning holds, it also holds where it produces an answer we do not like.

Closing the loop on my own question.

Closing the loop. The answer turned out to depend on the detail I had left out of the opening post, which is a lesson about how to ask rather than about the subject.

Adding the clinical framing, because it changes how the question reads. Worth answering the question that was asked rather than the one behind it. The narrow version usually has an answer; the broad version usually does not, and answering the broad one is how a thread stops being useful.

I would rather be corrected than agreed with, if it comes to it.

Found this through search years later and it is still the best discussion of the question, so adding one thing. For anyone reading later: the numbers in this thread are worth checking against a primary source before you act on them, including mine. Half the figures circulating in this community trace back to a secondary summary that dropped a qualifier.

carl_compliance said:

Worth answering the question that was asked rather than the one behind it.

Coming at carl_compliance’s question from a different direction. The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either. Those three things can all be true at once, and most arguments here are two people holding different parts of that.