A British professor testified in a same-sex marriage trial in San Francisco today that children aren’t hurt by having gay parents and would benefit if those parents could be married.

Michael Lamb, who heads the Department of Social and Developmental Psychology at Cambridge University, testified on the fifth day of a trial on a challenge to Proposition 8, California’s ban on same-sex marriage.

“We have a substantial body of evidence documenting that children being raised by same-sex parents are just as likely to be well-adjusted as children raised by heterosexual parents,” he told U.S. District Judge Vaughn Walker.

The trial before Walker is the nation’s first federal trial on a U.S. constitutional challenge to restrictions on same-sex marriage.

Two same-sex couples who sued to overturn Proposition 8 claim it violates their rights to due process and equal treatment. Walker will decide the case without a jury.

Lamb said current research suggests the most important factor in children’s well-being is not their parents’ genders but rather “the quality of their relationship with their parents, the quality of the relationship between parents, and the social and economic resources available to the family.”

He told the judge he believes the adjustment of a significant number of children of same-sex parents “would be promoted were their parents able to get married.”

Children prosper when their parents are married because being part of “a recognized social institution” is beneficial, Lamb testified.

Under cross-examination by David Thompson, a lawyer for the sponsors of Proposition 8, Lamb agreed there are some differences between men and women, including that men are more likely to drop out of high school, can’t breastfeed and die sooner, while women earn less money.

He also agreed some studies have shown there are differences in the ways mothers and fathers raise children.

But he insisted, “It’s now quite clear that these differences in themselves don’t appear to affect the children’s adjustment.”

The trial before Walker will resume Tuesday and is expected to continue for one or two more weeks.

The plaintiffs have several more witnesses to present before the supporters of Proposition 8 begin their side of the case.

Plaintiffs’ spokesman Yusef Robb said San Diego Mayor Jerry Sanders is likely to be among the witnesses called by the plaintiffs on Tuesday and that he will be questioned by San Francisco City Attorney Dennis Herrera.

Sanders, who has a lesbian daughter, is slated to testify about why he as mayor supported a legal brief favoring same-sex marriage.

The Proposition 8 attorneys originally planned six expert witnesses but notified Walker earlier this week that they have pared their list down to two.

The two experts are David Blankenhorn, president of the New York-based Institute for American Values and the author of a book entitled “Fatherless America,” and Kenneth Miller, an associate professor of government at Claremont McKenna College.

The Proposition 8 lawyers have also used lengthy cross-examination of the plaintiffs’ witnesses to support their arguments that limiting marriage to the union of a man and woman is justified because it is rooted in tradition and is better for children.

The only other witness called to the stand by the plaintiffs today was author Helen Zia of San Francisco.

Zia testified that when she married her lesbian domestic partner, “It changed things on a very huge level” in the way their families related to them as a couple.

Zia said, “That’s because, as I’ve always read and am just beginning to understand, marriage is the joining of two families.”

Zia and her partner married in June 2008, during a five-month period in which a California Supreme Court decision holding that the state Constitution provides a right to same-sex marriage was in effect.

Although Proposition 8, enacted by voters on Nov. 4, 2008, overturned the state high court ruling, the California court later said the 18,000 marriages like Zia’s that were performed during the five-month period remain valid.

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