It seems pretty clear that Jan Brewer’s decision to veto Arizona’s “religious freedom” bill on Wednesday was less about courage than political common sense. Among those urging Brewer to block the proposal were Arizona’s Republican delegation in Washington, the host committee for next year’s Super Bowl in Phoenix, and a number of prominent corporate leaders.

The campaign undoubtedly reflected some genuine moral outrage at what the bill would have done: allowing businesses to refuse service to LGBT clients. But it was also recognition of how significantly attitudes about homosexuality have changed in this country. Overt discrimination against gays is no longer acceptable in polite society, making these sorts of laws toxic to politicians and business leaders alike. As Alexander Burns and M.J. Lee observe in Politico, “What Arizona proved, as much as any other in recent American politics, is that there’s currently no more powerful constituency for gay rights than the Fortune 500 list.”

But that raises an interesting question: Why have attitudes changed? The transformation may seem inevitable now, but it didn’t always. And it’s happened with breathtaking speed. As my colleague Marc Tracy noted the other day, it was just ten years ago that President Bush and the Republicans unabashedly used the threat of gay marriage to whip conservative voters into a frenzy. Since that time, support for same-sex marriage has increased 21 percent, according to a just-released poll from the Public Religion Research Institute (PPRI). A majority now express support for same-sex marriage, up from about a third in 2003. Other polls show similar shifts.

It’s easy to assume the change represents nothing more than a generational shift. And, superficially, the survey data would seem to support that: In virtually every demographic category, support for same-sex marriage is higher among younger generations. But that’s not the whole story—not by a longshot. Pollsters have found that, over time, support for same-sex marriage has risen even within generations. “We estimated that support for marriage equality rose by 16 percentage points between 2004 and 2011, and that only one-quarter of it can be explained by cohort replacement,” says Gregory Lewis, a professor at Georgia State University, referring to report he co-authored for the group Third Way. “The remainder is people changing their minds.” (See the graph below, which comes from that report and shows how approval of same-sex marriage has risen over time, within each birth cohort.)