Jonathan Chait offers a caveat:

I see a serious causation/correlation problem here. I suspect the linkage between knowing somebody who's gay and supporting gay marriage is not a simple case of A causes B, but a third factor causing both. Gays are probably more likely to come out if they live somewhere that's fairly liberal -- that is, the kind of place that would be more likely to support gay marriage (say, New York City). They are probably less likely to come out if they live somewhere that's conservative (say, Smalltown, Arkansas.) So people in New York are more pro-gay marriage because they're more liberal, and more likely to know gays because they're more liberal...I doubt that knowing somebody who's openly gay is really the only determining factor in public support for gay marriage.

It isn't the only factor; but it is the strongest.

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