Discriminating against gay people, stigmatising them and depriving them of social support, leads to shame and loss of self-esteem and can make them more vulnerable to mental illness, Ms Justice Elizabeth Dunne in the High Court has been told.
Dr Harry Kennedy, clinical director at the Central Mental Hospital and a consultant psychiatrist, was called as an expert witness by a lesbian couple who are challenging the State's failure to recognise their Canadian marriage.
Dr Kennedy said homosexuality today was a condition considered to be no more abnormal than childhood, old age or pregnancy.
In cultures where social stigma, discrimination and adversity were attached to homosexuality, he said, it may be associated with increased rates of mental illnesses. This was an indirect consequence of homosexuality in such discriminating cultures but not in others.
He said the indirect effects of mental illness arose from stigma and denial of identity. Stigma led to shame and loss of self-esteem which was a vulnerability factor for mental illnesses.
Denial of identity by others and active denial by the individual of his or her identity through the necessity of disguising it, or presenting a false identity, also represented a vulnerability factor for mental illness because of the effort involved in maintaining the false identity and the fear of discovery.
He said lack of social support was also established by research evidence as a vulnerability factor for mental illnesses, particularly depression.
Dr Kennedy was giving evidence in the continuing action by Dr Katherine Zappone, a public policy consultant, and Dr Ann Louise Gilligan, an academic, against the Revenue Commissioners and the State.
They claim the failure by the authorities to recognise their Canadian marriage breaches their right to marry under the Constitution, the European Convention on Human Rights and the European Charter of Fundamental Freedoms. They also argue that if their Canadian marriage is not recognised, they should have the right to marry within the State.
The State denies their claims.
Dr Kennedy noted that public attitudes to homosexuality began to change in the second half of the 20th century. In England in the 19th century, there were more executions for homosexual behaviour than for murder. Organisational recognition of homophobia had been important in changing attitudes about homosexuality.
In the 1950s attempts were made to treat or cure homosexuality and to change one's preference, but this had not succeeded because there was "nothing to treat", Dr Kennedy said. Today, homosexuality was held to be no more an abnormal condition than childhood, old age or pregnancy.
Prof Daniel C Maguire, a Catholic theologian from the US, said civil marriage was not controlled by the church during the first thousand years.
There was no systematic regulation of marriage until the 10th century and marriage was not called a sacrament until 1215, he said. The 14th century marked the beginning of extreme hostility to homosexuality.
The case continues today.