Mr P and gay marriage

Andrew Gelman has a terrific post, Gay Marriage: A Tipping Point? over on Statistical Modeling, Causal Inference and Social Science on a study by Jeff Lax and Justin Phillips on changes in attitudes to gay marriage in the different states.

AG:

Here I'll focus on the coolest thing Lax and Phillips found, which is a graph of state-by-state trends in public support for gay marriage. In the past fifteen years, gay marriage has increased in popularity in all fifty states. No news there, but what was a surprise to me is where the largest changes have occurred. The popularity of gay marriage has increased fastest in the states where gay rights were already relatively popular in the 1990s.

In 1995, support for gay marriage exceeded 30% in only six states: New York, Rhode Island, Connecticut, Massachusetts, California, and Vermont. In these states, support for gay marriage has increased by an average of almost 20 percentage points. In contrast, support has increased by less than 10 percentage points in the six states that in 1995 were most anti-gay-marriage--Utah, Oklahoma, Alabama, Mississippi, Arkansas, and Idaho.

(AG displays this excellent graph, so I shall not haul it over here.)

I was stunned when I saw this picture. I generally expect to see uniform swing, or maybe even some "regression to the mean," with the lowest values increasing the most and the highest values declining, relative to the average. But that's not what's happening at all. What's going on?

I have not found a way to link to this specific post, but AG's analysis is well worth reading. (Update, thanks, anon - OK, the rest here.)