The years indicated are those by which a gay marriage ban would be defeated by voters in a given state, according to a regression model designed by Silver. (Again, all the math and hard work is Silver's; I just made the map.)
How did Silver come up with these results? Here's the explanation:
I looked at the 30 instances in which a state has attempted to pass a constitutional ban on gay marriage by voter initiative. The list includes Arizona twice, which voted on different versions of such an amendment in 2006 and 2008, and excludes Hawaii, which voted to permit the legislature to ban gay marriage but did not actually alter the state's constitution. I then built a regression model that looked at a series of political and demographic variables in each of these states and attempted to predict the percentage of the vote that the marriage ban would receive.The more religious a state is, and the more white evangelicals it has, the higher the percentage of voters who would be likely to support a gay marriage ban. However, according to Silver marriage bans "are losing ground at a rate of slightly less than 2 points per year. So, for example, we'd project that a state in which a marriage ban passed with 60 percent of the vote last year would only have 58 percent of its voters approve the ban this year." So it's possible to extrapolate, given the current religious demographics of a state and the trend of decreasing support for bans, when a gay marriage ban would fail.
It turns out that you can build a very effective model by including just three variables:
1. The year in which the amendment was voted upon;
2. The percentage of adults in 2008 Gallup tracking surveys who said that religion was an important part of their daily lives;
3. The percentage of white evangelicals in the state.
These variables collectively account for about three-quarters of the variance in the performance of marriage bans in different states. The model predicts, for example, that a marriage ban in California in 2008 would have passed with 52.1 percent of the vote, almost exactly the fraction actually received by Proposition 8.
There are 11 states where a marriage ban would already be expected to fail: all of New England and New York, plus several states in the West (all of which are among the least religious states in the country). California wouldn't be likely to reject a ban until next year. (Side note: despite California's status as a sort of poster boy for social liberalism, most of the Northeast tends to be more liberal on these kinds of social indicators.)
Over the next couple of years, majority opposition to gay marriage bans will spread quickly through the Northeastern and Western states, then through the Midwest - claiming a majority of all states by 2013 - and finally through the South, with Mississippi bringing up the rear in 2024.
Of course, history rarely moves in a straight line, and there's a big element of speculation in simply extrapolating from current trends. As Silver notes, a backlash against gay marriage might mount, delaying or reversing the trends that have been evident over the past few years; or by a sudden gestalt shift gay marriage might find broad acceptance. What seems much more likely than the precise dates given here, though, is the chronology of the geographical spread of acceptance. Same-sex marriages are already legal in the New England states of Massachusetts and Connecticut, with the Vermont legislature recently voting overwhelmingly to allow it (though the Governor may veto the bill [UPDATE: the legislature overrode the veto, so Vermont is the fourth state to make gay marriage legal, and the first to do so legislatively rather than judicially]). And even among states where gay marriage bans have passed, some of the most narrow margins were in Western states like Oregon, Colorado, Arizona and, of course, California. Meanwhile, it is certain that the South will be the last region in the country to become amenable to gay marriage.
As for Iowa? According to Silver, an amendment to the state constitution to ban same-sex marriage in that state would require passage in two consecutive sessions of the state legislature, and then would have to be ratified by the voters. So it couldn't come up for a vote at the ballot box until at least 2012. According to Silver's regression model, such a ban might pass in Iowa until 2013 - but who knows; maybe 3 years of being neighbors with happily married gay couples without having their social fabric torn asunder will cause Iowans' tolerance clock to speed up just a bit.
5/19/09 UPDATE: This map was never intended as a prediction of when gay marriage would actually become legal. But it is interesting that, as of a month and a half later, gay marriage is now legal in 4 states (MA, CT, VT, ME), will soon be legal in a 5th (NH; just waiting for a technicality in the bill to be worked out), and is making progress in a 6th state (NY) which, according to Nate S., would vote against a hypothetical gay marriage ban as of this year. In other words, gay marriage may actually be legal in at least half of these 11 states by the time the year is out, and will definitely be legal in at least 5 - plus Iowa, of course. Again, the point was never that these would be the dates by which gay marriage would actually be legal - but it almost seems to be turning out that way, to some extent.