The world’s largest body of psychologists, the American Psychological Association, has called for state and federal officials to lift barriers to marriage equality.

Members at the group’s annual meeting in Washington voted unanimously on Wednesday to back “full marriage equality for same-sex couples”.

Its policy-making panel cited evidence which shows that marriage has the same health benefits for gay people as it does for straight people and that barring gay couples from marrying “may have negative effects on their psychological well-being.”

Clinton Anderson, director of APA’s Office on Lesbian, Gay, Bisexual and Transgender Concerns, told USA Today: “Now as the country has really begun to have experience with gay marriage, our position is much clearer and more straightforward – that marriage equity is the policy that the country should be moving toward.”

States which allow gay marriage are New York, Vermont, Iowa, Massachusetts, Connecticut and New Hampshire, as well as Washington DC. A small number of states permit civil unions, which many gay rights campaigners see as inadequate.

The resolution passed by the APA says that “many gay men and lesbians, like their heterosexual counterparts, desire to form stable, long-lasting and committed intimate relationships and are successful in doing so”.

It adds: “Emerging evidence suggests that statewide campaigns to deny same-sex couples legal access to civil marriage are a significant source of stress to the lesbian, gay and bisexual residents of those states and may have negative effects on their psychological well-being.”

Evan Wolfson, the founder and president of Freedom to Marry, said: “The American Psychological Association represents over 100,000 medical professionals who see day in and day out the real harms gay and lesbian people and their families experience when denied the freedom to marry.

“With the freedom to marry in 12 countries on four continents, and most recently New York joining five other states plus the District of Columbia in ending exclusion from marriage, there is a mountain of unrefuted evidence and experience showing that extending the freedom to marry to loving, committed same-sex couples helps them and their families while hurting no one.

“Based on the evidence, the APA, like every other professional medical, scientific, child-welfare, and social science organisation, is calling for an end to the unfair exclusion of same-sex couples and their families from marriage and its crucial meanings to families, including the safety-net it brings to couples and their kids.”

In 2009, the APA announced its formal opposition to ‘reparative therapy’, which claims to be able to ‘cure’ gay people.