A psychologist took aim at one of the central justifications for California's ban on same-sex marriage in federal court Friday, saying researchers overwhelmingly agree that gays and lesbians make just as good parents as heterosexuals.

More than 100 studies have found that "children who are raised by gay and lesbian parents are just as likely to be well-adjusted," Michael Lamb, chairman of the department of social and developmental psychology at Cambridge University in England, testified at the San Francisco trial of a lawsuit seeking to overturn Proposition 8.

Lamb quoted the American Psychological Association's 2004 policy statement that gays and lesbians are "as likely as heterosexual parents to provide supportive and healthy environments for their children." Seven other nationwide professional organizations have taken similar positions, he said.

Prop. 8, a November 2008 initiative, amended the state Constitution to define marriage as the union of a man and a woman. The measure overturned a May 2008 state Supreme Court ruling that allowed gays and lesbians to marry.

The lawsuit by two same-sex couples and the city of San Francisco contends Prop. 8 discriminated unconstitutionally on the basis of sexual orientation and gender and had no justification except prejudice. The nonjury trial, the first in any federal court on same-sex marriage, began Monday before Chief U.S. District Judge Vaughn Walker.

Protect Marriage, the religious coalition that sponsored Prop. 8, argues that the measure was intended to promote traditional marriage because children are better off with their biological parents.

A document on the Protect Marriage Web site, quoted in court, said studies have found that same-sex parents are prone to "gender confusion" and are more likely than heterosexual parents to abuse their children and to raise daughters who become lesbians.

There is no evidence for those assertions, said Lamb, a child development researcher for almost 40 years and former head of a section of the U.S. government's National Institutes of Health.

"No research shows that kids need to be protected from gays or lesbians," he said, and studies also contradict the claim that children are better off with their genetic parents than with adoptive parents.

The only established differences, Lamb said, are that children of same-sex parents are more likely to be bullied by their peers and are less likely to have stereotyped attitudes about gender. For example, he said, they're less prone to thinking that girls should aspire to become nurses while boys should become doctors.

In cross-examination, Protect Marriage lawyer David Thompson labeled Lamb a "committed liberal," citing his membership in such organizations as the American Civil Liberties Union and the National Organization for Women, and his financial contributions to the Public Broadcasting System.

Thompson also said scientific research is not immune from political influence and that "history is littered with scientific theories universally accepted and proven to be wrong."

He said other studies called Lamb's conclusions into question - for example, research showing that children do better with biological parents than with stepparents.

Lamb said such findings wouldn't shake his view that sexual orientation makes no difference in parenting.