One of the most used arguments against gay marriage is that it devalues the institution of marriage itself. The obvious counters- how does what two other people you've never even met live affect your marriage, and how can gays do any more damage to the state of marriage than Hollywood stars have done- are rejected out of hand by opponents of gay marriage. But neither side has been convincing, because they were opinions; there weren't any real numbers or hard evidence to support either position. Until now.
The most obvious test of the health of the state of marriage is the rate of divorce, and at the national level, it's been improving the last decade or so; divorce rates are down. But that's the national average; individual states vary wildly, with many showing an increase in divorce. Could gay marriage be having an effect? Nate Silver at the blog FiveThirtyEight has put numbers to it. 538 used marriage and divorce rates from the CDC - 43 states reporting, from the 2004-2008 period- and correlated them against the level of marriage equality, using a four-stage rating: Legal, Not Performed, Forbidden By Statute, Forbidden By State Constitution, displayed as a chart.
The results? "As is somewhat visually apparent, those states which have tended to take more liberal policies toward gay marriage have tended also to have larger declines in their divorce rates. In Massachusetts, which legalized gay marriage in 2004, the divorce rate has declined by 21 percent and is the lowest in the country by some margin. It is joined at the top of the list by Rhode Island and New Mexico, which do not perform same-sex marriages but idiosyncratically also have no statute or constitutional provision expressly forbidding them, as well as Maine, whose legislature approved same-sex marriage only to have it overturned (although not banned constitutionally) by its voters.
On the other hand, the seven states at the bottom of the chart all had constitutional prohibitions on same-sex marriage in place throughout 2008. The state which experienced the highest increase in its divorce rate over the period (Alaska, at 17.2 percent) also happens to be the first one to have altered its constitution to prohibit same-sex marriage, in 1998."
In fact, looking at his chart, it's immediately obvious from the chart that all 24 states in which divorce rates declined had either become more liberal or remained the same through the reporting period. While this does not imply a cause and effect- there's no reason to believe that liberalizing marriage will reduce divorce rates- it does in fact demolish the claim that liberalizing marriage laws harm marriages.