Kimberly F. Balsam and Theodore P. Beauchaine of the University of Washington, Esther D. Rothblum of San Diego State University and Sondra E. Solomon of the University of Vermont followed-up on a 2002 project that focused on legalized relationships of same-sex couples following civil unions in 2000.

Sixty-five male and 138 female couples who entered into civil unions during the first year were available were asked to provide information. They were compared to 23 male and 61 female couples not in civil unions and 55 heterosexual married couples related to the same-sex couples in civil unions.

The study, published in Developmental Psychology, said same-sex couples not in civil unions were more likely to have ended their relationship than same-sex couples in civil unions or heterosexual married couples in the study.

"Legal couple status may support a relationship -- more family understanding, acceptance by friends and co-workers, greater commitment from a public declaration and enhanced legal protections like healthcare benefits and community property," Robert-Jay Green of Alliant International University in San Francisco said in a statement.