Amid pleas by “Lost Boys” actor Jason Patric and other sperm donors to receive the greater legal rights afforded to fathers, California lawmakers on Tuesday began wrestling with this sensitive issue: Who can claim fatherhood of a child conceived in a lab, and under what circumstances?

Mr. Patric – whose ex-girlfriend last year won custody of a son, now 3, conceived through in vitro fertilization – spoke at a hearing where state lawmakers are considering a bill that would give judges in child-custody cases greater latitude to take into account the interests of the men involved.

Whether and how much to adjust the balance of maternal and paternal rights is a longtime question, made more complicated by modern advances in reproductive medicine that allow for "test tube babies," pregnancies through surrogates, and other innovations. The stakes are high for the men and women involved, but also for the children growing up in a society in which lineage can be confusing, if not outright mysterious, say some family studies experts.

“We’re at an unusual point for families in general,” says Randal Day, a family studies professor at Brigham Young University, in Salt Lake City. “We’re racing ahead at a breakneck pace of change without really knowing how it’s going to affect us down the road.”

Patric is currently appealing a judge’s decision that gave full custody of Gus, his son with Danielle Schreiber, to the boy’s mother, in large part because the pair had no written agreement as to the parental arrangement. Patric assumed he would stay involved with the boy; Ms. Schreiber said that was never part of the deal.

Either way, Patric was involved in the boy’s life until Schreiber decided to fight for sole custody. Turned aside by the courts, the actor appealed to the California legislature to address what he sees as a parental injustice and institutional prejudice against men.

“The fact is, I had a little modicum of fame to bring light to a problem that’s going to affect in such a horrific manner children and families,” he said. “I didn’t donate my sperm; I gave my sperm to have a child.… I want my son back,” he said during an interview with ABC's Katie Couric in June.

In emotional but measured testimony Tuesday, Patric said he went to "great lengths," including surgery, to become a father. He said both he and Schreiber signed an "intended parent" document, but that current California law prevented him from making his case before a judge. Other men have complained about the same thing, he said.

The emergence of sperm banks and the high success rate for in vitro fertilization have created markets where would-be moms can shop for the right attributes in a sperm donor without having to deal with the actual man. They have opened new options for older unmarried women, as well as for same-sex couples who want to raise children who are biologically connected to one of them.

But the effects of those advances concern some researchers, including Mr. Day.

“There’s a fundamental contribution that men make,” but it's being tested by an emerging view among some mothers, or would-be mothers, that “’I want your sperm because you’re good-looking, you have a high IQ, and you do OK on personality tests, but basically I don’t want you around," he says.

Day cites the unknown effects of what he calls “intergenerational confusion,” in which traditional roles such as fathers, mothers, aunts, uncles, and grandparents are upended in part by government policies such as welfare but also by changing cultural mores in which women hold increased power over child-bearing and in determining family structure.

Worry that women would see their parental rights erode is why the National Organization for Women and some other feminist groups oppose the bill considered Tuesday by a panel of the California Assembly. Giving judges greater discretion on custody cases involving in vitro fertilization, they argue, would whittle away at women’s autonomy by ceding some power to sperm donors who suddenly want a greater-than-planned role in raising the child.

"Parents, reliant on a sperm donor's agreement not to parent, could have allowed the donor to develop a close relationship with the child, without thinking that he could later come in and demand paternal rights," warned Assemblyman Tom Ammiano in a letter to the Assembly Judiciary Committee in June.

Assemblyman Jerry Hill, who introduced the measure in the lower chamber after it passed the state Senate without opposition, says the tweak is succinct enough to give judges options only in cases where fathers have made a significant and involved contribution to the rearing of the child.

"This is truly about the modern family, and it has raised questions and issues for the courts that haven't kept up with changing times," he told the Associated Press. Last month, Mr. Hill told Ms. Couric that some groups fear “sperm donors will be coming out of the woodwork, flooding the courts to test parenting rights with children that they have created.” But the reality, he suggested, “is that there’s only a small universe of individuals that this [California bill] really affects.”

The committee voted 5-to-2 Tuesday to hold the bill in committee for further discussion. Hill said after the hearing that he will continue to work with various parties to reach agreement on the legislation.

Anthropologist Peter Gray of the University of Nevada, Las Vegas, author of “Fatherhood: “Evolution and Human Paternal Behavior, agrees that the number of men affected by the suggested tweak in California law is probably small. But he argues that the issue is nonetheless important. “This [bill] is conceptually important, because it’s trying in a sense to lay out a framework that will accommodate men who want to be involved in quite variable genetic or social backdrops.”

The case also resonates because it contradicts social and even biological stereotypes about men, says Mr. Gray.

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“More broadly, we’re often surprised if men want to stick around beyond sperm donations when they have the opportunity to leave, which is part of what makes this [Patric-Schreiber] case so interesting,” he says. “This guy wants to be involved, even though it’s an uphill battle.”

Material from Associated Press was used in this report.