Dr.CardioMD said:

If two explanations both fit, the useful question is which one predicts something the other does not.

Coming at Dr.CardioMD’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.

The figures, for anyone assembling their own picture. If you are comparing against somebody else’s result, check that you are comparing the same measurement taken the same way. Most of the apparent contradictions in these threads dissolve at that step.

A narrower follow-up, since the general answer is now clear:

How would you tell the difference between that and the alternative explanation?

VendorMark said:

If you are comparing against somebody else’s result, check that you are comparing the same measurement taken the same way.

This is where I part company with the consensus forming above. The objection nobody has made: this only works if the measurement is doing what we assume it is doing, and that assumption has not been tested here.

VendorMark said:

If you are comparing against somebody else’s result, check that you are comparing the same measurement taken the same way.

That is right for the common case. Whether it is right for the case being asked about depends on a detail that has not been given.