Over the past decade or so, divorce has gradually become more uncommon in the United States. Since 2003, however, the decline in divorce rates has been largely confined to states which have not passed a state constitutional ban on gay marriage. These states saw their divorce rates decrease by an average of 8 percent between 2003 and 2008. States which had passed a same-sex marriage ban as of January 1, 2008, however, saw their divorce rates rise by about 1 percent over the same period.
The table below details the divorce rates for the 43 states that reported their divorce statistics to the CDC in both 2003 and 2008. It is calculated by taking the total number of divorces in the state that year, and dividing it by the number of married persons, as reported by the Census Bureau. The result is then multiplied by two, since each divorce involves two people. This is different than how the divorce rate is sometimes calculated, which may be as a share of the overall population rather than the number of married persons; I prefer my approach because it will not penalize a state for having a lot of marriages (and therefore more opportunities for divorce). However, there are also more complicated versions of the divorce rate calculation that account for the age of the married couples, and so forth; these are probably superior, but mine is intended to be a simple approach. The table also lists the percentage change in the divorce rate between 2003 and 2008, and the current status of gay marriage and domestic partnerships within each state.
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.
Overall, the states which had enacted a constitutional ban on same-sex marriage as of 1/1/08 saw their divorce rates rise by 0.9 percent over the five-year interval. States which had not adopted a constitutional ban, on the other hand, experienced an 8.0 percent decline, on average, in their divorce rates. Eleven of the 24 states (46 percent) to have altered their constitutions by 1/1/08 to ban gay marriage experienced an overall decline in their divorce rates, but 13 of the 19 which hadn't did (68 percent).
The differences are highly statistically significant. Nevertheless, they do not necessarily imply causation. The decision to ban same-sex marriage does not occur randomly throughout the states, but instead is strongly correlated with other factors, such as religiosity and political ideology, which we have made no attempt to account for. Nor do we know in which way the causal arrow might point. It could be that voters who have more marital problems of their own are more inclined to deny the right of marriage to same-sex couples.
There is, however, probably now enough data on this subject to engage in more sophisticated longitudinal studies on this subject (more sophisticated than I have engaged in here), which might produce more robust conclusions. Although only Massachusetts has affirmed gay marriage for any length of time, the difference between the states which have banned it constitutionally versus statutorily may be worth examining, as the former represents a significantly more confident assertion about the nature of state-sanctioned marriage. At the very least, I would be surprised if there were any statistical evidence that interpreting the right of marriage to apply to same-sex couples would be injurious to heterosexual couples in any material way.