The growing number of same-sex marriage laws in the United States has had no effect on the marriage rate among heterosexual couples according to new findings by researchers at Portland State University's School of Community Health.

The findings contradict statements by opponents of same-sex marriage arguing that allowing same-sex couples to marry undermines marriage in general and will discourage heterosexual couples from making that commitment.

"This research shows that increasing legal recognition of same-sex marriage has no effect on rates of opposite-sex marriage in states that passed same-sex marriage laws," said Alexis Dinno, assistant professor of Community Health and the lead researcher for the project. "Concerns about potential harm to the rate of opposite-sex marriage resulting from same-sex marriage laws are not borne out by this research."

Dinno and fellow researcher Chelsea Whitney examined heterosexual marriage rates in all 50 states and the District of Columbia from 1989 through 2009. They looked for differences corresponding to years in which Vermont, Massachusetts, California, Iowa and Connecticut passed laws allowing same-sex marriage. They saw no decline in opposite-sex marriage in those states, and in fact found that the marriage rate among heterosexual couples was roughly the same as in states without laws allowing same-sex marriage. They obtained the same results for states allowing same-sex civil unions, concluding that such laws also have no effect on rates of opposite-sex marriage in those states.

The study comes at a time in which two cases regarding same-sex marriage are being considered in the U.S. Supreme Court. Opponents of same-sex marriage in those cases have argued that states have an interest in defining marriage as between one man and one woman. Eleven states currently allow same-sex marriage. Minnesota will become the 12th on Aug. 1. The District of Columbia and three Native American tribes also allow same-sex marriage.

Dinno's article titled "Same Sex Marriage and the Perceived Assault on Opposite Sex Marriage" was published Tuesday, June 11, in the academic journal PLOS ONE, available here.