Children raised by gay and lesbian couples develop just as well as those brought up by traditional couples, a British child psychologist today told a US federal court considering whether a California ban on gay marriage denies constitutional rights.

Lawyer David Thompson, defending the ban, jousted with Michael Lamb, head of the Social and Developmental Psychology Department at Cambridge University, about whether a child growing up without a father or without a mother would face developmental problems.

Two gay men and two lesbian women are asking the federal court to rule the right to marry has no exceptions under the US Constitution, a fight they hope to take all the way to the US Supreme Court in a bid to overturn bans on same-sex marriage in 40 states.

A key question in the case is whether government, and US voters, have a reasonable justification for denying same-sex couples the right to marry, such as promoting healthier families, or if the bans reflect discrimination and hate.

The record of evidence gathered by Federal District Chief Judge Vaughn Walker, including the issue of whether gays are good parents, may be relied on heavily by appeals courts, which generally do not have as much time to review a case as in the original trial.

Lamb, a witness for the same-sex couples, argued that the gender of parents was not the main criterion of success for raising children.

The quality of the relationship between the child and the parents, the relationship between the parents, and the economic resources available to the family, were the top issues for healthy children, he said.

Kids had no trouble with their own sexual identity or other development due to growing up with same-sex parents, he argued, and the ways fathers and mothers interacted with kids was not as important as having two parents, he said.

"Children clearly benefit when they have two parents, both of them actively involved," said Lamb. Asked if mothers and fathers interacted differently with children, he replied, "It is now quite clear that those differences in and of themselves do not significantly affect children's adjustment," he said.

Studies reject the conclusion that children are abused more when raised by same-sex couples.

"There is no evidence that gays or lesbians are more likely to sexually abuse children," he said. "This is one of those fairly old canards."

Later he conceded that stepfathers often created problems in families. "Girls are at greater risk of abuse by stepfathers," he agreed.

Proponents of the gay marriage ban will produce their own witnesses later in the trial and attempt to show that there is good reason to believe that homosexual parents endanger children.

The passage of the ban known as Proposition 8, in November 2008, stunned gay advocates in the United States and ended same-sex marriage in California, whose Supreme Court had legalised such unions early that summer.

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