THE Australian Psychological Society has urged federal parliamentarians to change the law to allow same-sex marriage on the grounds of mental health and wellbeing.

The body, which represents 20,000 psychologists across the country, yesterday urged a change in the Marriage Act, citing evidence that marriage is beneficial for an individual's mental health and that harm is caused by social exclusion and discrimination.

The APS is the latest entrant into the political debate over same-sex marriage, which will resume when Labor MP Stephen Jones introduces a private member's bill into parliament in February for marriage equality.

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Labor's national conference this month controversially passed Julia Gillard's motion to allow a conscience vote on same-sex marriage, with the focus of the debate now shifting to the Coalition.

Tony Abbott is expected to come under pressure over the summer, after he rejected calls by frontbenchers Malcolm Turnbull and Simon Birmingham for a conscience vote on the issue.

The frontbenchers say the Coalition has always been allowed to participate in conscience votes in the past, but the Opposition Leader says the Coalition will not change its election policy, which stipulates that marriage is between a man and a woman.

APS president Simon Crowe said the society was following in the footsteps of its US counterpart, the American Psychological Society, which backed same-sex marriage in August.

"Decades of psychological research provides the evidence linking marriage to mental health benefits, and highlighting the harm to individuals' mental health of social exclusion," he said. "The APS supports the full recognition of same-sex relationships, on the basis of this evidence."

Australian Marriage Equality spokesperson Rodney Croome welcomed the statement.

"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," he said.

"It shows marriage equality is not an abstract issue but a reform that will have concrete benefits for a significant number of Australians."