The Australian Psychological Society has officially given its support to full marriage equality for gay couples, adding the voice of its 20,000 members to the debate.

The group’s board approved an endorsement of the American Psychological Association’s resolution this summer, which gave its unequivocal support to the introduction of equal marriage rights for gay couples on behalf of its 100,000 professional members.

The resolution was based on both the positive effects on mental health which marriage equality has, and the negative effects which result from campaigns to deny couples equal marriage rights.

Professor Simon Crowe, President of the APS, said: “Decades of psychological research provides the evidence linking marriage to mental health benefits, and highlighting the harm to individuals’ mental health of social exclusion.

“The APS supports the full recognition of same-sex relationships, on the basis of this evidence.”

Dr Damien Riggs, convenor of the APS Gay and Lesbian Issues in Psychology Interest Group, said: “Marriage discrimination has a flow-on effect on same-sex attracted Australians, their loved ones, and the wider community. Psychologists must work to ensure that all Australians are supported to achieve positive mental health and full social inclusion.”

In August, the American Psychological Association called on state governments “to repeal all measures that deny same-sex couples the right to civil marriage and to enact laws to provide full marriage equality to same-sex couples”.

Rodney Croome, Australian Marriage Equality’s campaign director, said there was a “mountain of scientific evidence that same-sex partners and their families experience less stigma and stress when they can marry, leading directly to better mental health outcomes”.

He added: “It should be no surprise that those people excluded from marriage simply because of their partner’s gender can experience long-term psychological problems, given that marriage is a key legal and social institution from which many people derive great meaning and purpose in their lives.

“The APS statement sends a clear message to all federal MPs – if you support better mental health outcomes for Australian families then you must support marriage equality.”

Paul Martin, a psychologist from the Centre for Human Potential in Brisbane, said: “The APS statement illustrates what I’ve seen in my practice for over 25 years, namely that legislative discrimination against gays and lesbians fosters the kind of negative stereotypes and prejudice that lead directly to psychological stress and self-loathing”, Mr Martin said.

“The negative impact of legislated inequality is particularly bad when we’re dealing with exclusion from marriage because marriage is all about affirming and strengthening relationships and strong relationships are a key to good mental health outcomes.”