Thursday, during the federal trial to decide the constitutionality of Proposition 8, a judge heard that gay marriage bans send the message that gay relationships are not respected, and that this message creates emotional stress for gay couples.

On the fourth day of the trial in San Francisco, Chief U.S. District Judge Vaughn R. Walker heard testimony from Columbia University Professor Ilan Meyer, a social scientist who spoke on the mental health impact of discrimination against gay men and lesbians.

The trial began on Monday with the emotional testimony of a gay and a lesbian couple, each of which has been denied the right to marry in California because of Proposition 8, the gay marriage ban approved by voters in 2008.

When asked about the impact of the measure on gay couples, Meyer responded: “It certainly doesn't send a message, 'It's OK, you can be who you want to be.' It sends the opposite message.”

Such measures erect a barrier to a “desirable and respected” institution, he said, adding to the social stigma that leaves gay people more susceptible to mental health problems, including depression, suicide and substance abuse.

“People in our society have goals that are cherished by all people, that are part of the social convention,” Meyer said. “We are all raised to think there are certain things we want to achieve in life, and this Proposition 8 says if you are gay or lesbian, you cannot achieve this particular goal.”

Upon cross-examination Meyer was forced to admit that he is a “gay-affirmative advocate” and gave to the campaign against Proposition 8 twice.

Lawyers for the defense took their time looking for vulnerabilities in Meyer's testimony. They insisted the professor's research on minority stress is hopelessly flawed, they read excerpts from conflicting scholarly papers, and Howard Nielson Jr. finally pushed Meyer to concede that no research is perfect.

Earlier Thursday, Edmund Egan, an economist for the city of San Francisco, testified that the gay marriage ban cost the city millions of dollars a year in lost revenue.

“To put it simply … married individuals tend to accumulate more wealth than single individuals,” Egan said, then added that married people are healthier, which increases overall productivity, which in turn boosts payroll tax revenues.

The trial resumes Friday with the testimony of William Tam, a controversial Proposition 8 proponent who alleged that gay marriage advocates also supported the legalization of sex with children, and Helen Zia, a lesbian who married her partner during the brief window when gay marriage was legal in California.