Scientists 64%, Regular Folk 6%

The data, from Religion vs. Science, What Religious People Really Think, by Elaine Howard Ecklund:

The striking result: 64% of scientists are atheist or agnostic, as compared to 6% of the general population of the US (let's call them regular folk).

Effect 1:  Scientists are smarter than regular folk and trained to think critically, and therefore much more likely to see through all the superstitious nonsense.

Effect 2:  God has chosen the foolish things of the world to shame the wise (1 Cor. 1:27).  This produces the same trend as Effect 1, but intentionally and supernaturally. It's a feature, not a bug.

Effect 3:  The percentages for regular folk who are believers is greatly exaggerated by cultural, peer, and familial pressure to affirm Christianity. Scientists, (at least in part because of their egos) are much more immune to these pressures. If everyone were honest regarding their faith, the results for scientists and regular folk would be much, much closer.

I have nothing other than a gut feeling to go by, but I think the answer is an admixture of Effect 2 and Effect 3, with Effect 3 being non-negligible if not dominant.