“The law needs to adapt to the reality of children’s lives, and if children are being raised by three parents, the law should not arbitrarily select two of them and say these are the legal parents, this other person is a stranger,” says Nancy Polikoff, a family-law professor at American University’s Washington College of Law.

Now a few family-law scholars have begun to argue that there is nothing special about the number two — if three or four or five adults have a parental relationship with a child, the law should recognize them all as parents. Going beyond two, these scholars argue, would better reflect the dynamics of the modern family, and also protect the children in such families. It would ensure that, even in the event of a split or major disagreement between the adults in question, the children would not be deprived of the affection, care, and financial resources of any of the people they have grown up regarding as their mothers and fathers.

Still, even in a time of changing attitudes about who can be a parent, the legal and social definition of a family still has certain rules — a family can be run by a single mom or a single dad and, increasingly, by two moms or two dads, but it can’t have three parents, or four. For a long, long time — going back to when the English common law first started codifying such things — the law has set the maximum number of parents a child can have as two. Only two people, in other words, can enjoy the unique set of rights to determine a child’s life — and the unique set of responsibilities for the child’s welfare — that legal parenthood entails. That matches how most people think about parenthood: Two people, after all, are how many it usually takes to make a baby in the first place.

The film’s family is indeed unconventional, but it is not unique. In the age of assisted reproductive technology, the increasing acceptance of same-sex partnerships, and a steady growth in “blended” families, more parents and more children are finding that traditional notions of the nuclear family don’t accurately reflect their lives and relationships.

“To an unconventional family.” That’s what Paul, the roguish restaurateur and sperm donor, raises his glass to in this summer’s movie “The Kids Are All Right.” Paul is, he has recently discovered, the biological father of two teenage children, one by each partner in a long-term lesbian couple. Contacted by the kids, he has come into their lives and begun to compete for the affections of various members of the family he unknowingly helped create. Complications — funny, then sad — ensue.

Legally, however, Sharon and Bill were Jesse’s parents, and that put Matty in a potentially precarious position. “Let’s say I died in some terrible car crash or whatever and Matty had no legal rights, and let’s say she and Billy had a falling out or one of my parents or brother wanted to take care of Jesse,” Sharon says. In that case, Matty could have had Jesse taken away from her altogether.

Sharon, Matty, and Bill agreed that Bill would be more than just a source of genetic material — they wanted him to be a father. When Sharon had a son, Jesse, in 1994, the boy lived with Sharon and Matty, but growing up he spent one day a week with Bill and Bill’s same-sex partner, Thompson. In addition, the whole family would gather once a week for dinner.

When Sharon Tanenbaum and Matty Person, a married lesbian couple in San Francisco, decided to have a child together, it wasn’t hard to figure out who they wanted the sperm donor to be. Bill Hirsh was one of Sharon’s oldest friends, they had known each other, Sharon says, “since we were born, more or less.” Their fathers had been best friends in college, and Sharon and Bill had grown up spending summers together and calling each other’s parents aunts and uncles.

And as supporters of revising the definition of parenthood point out, there’s nothing tidy or biologically preordained about today’s prevailing notion of parentage, one that often has to shoehorn families jumbled and reassembled by divorce, adoption, and reproductive technology into one standard model, in ways that can prove disruptive to the families in question.

Ultimately, the legal definition of parenthood is part of a broader philosophical question: What is a family? And what is it for? While some scholars have focused on expanding the number of parents, others argue that the law needs to do more to recognize the social context in which families exist, and the extent to which child care is actually performed by people who aren’t part of the nuclear family at all.

In a few recent cases, courts seem to have agreed with the calls for multiple parents. But critics argue that tinkering with the definition of parenthood in this way threatens to dilute the sense of obligation that being a parent has always carried, and that increasing the number of legal parents only raises the likelihood that family disputes will arise and get messy and find their way into court. Not to mention that having judges routinely declare that Heather has two mommies and three daddies would represent a radical cultural shift, and one that, like gay marriage, many will find threatening.

At the same time, no one in the family wanted to force Bill to give up his parental status. So, when Jesse was 4, their lawyer persuaded the San Francisco Superior Court to allow Matty to do a third-parent adoption. The move, which had little precedent, gave Jesse three parents, three people who, in the event of a split, could demand custody or visitation rights and would be responsible for paying child support.

Asked why it was so important to recognize all three of them in the eyes of the law, Sharon responds, “When you look back on your life, there’s a big difference between your father and your uncle and your parents’ best friends. There are certain rights and responsibilities that also come with being a parent, and those rights and responsibilities only come with being a parent.”

Third-parent adoptions remain extremely rare, and only a handful have been done, mostly in Massachusetts and California. But some legal scholars see in them the seeds of a larger shift in how the law defines parenthood. These advocates point to a few recent court decisions that suggest a willingness to recognize more than two parents.

It would not be the first time that American law has changed the rules of parenthood. According to Polikoff, in the English common law from which American law is derived, children born out of wedlock before the 19th century had, legally speaking, no parents at all. They were filius nullius. By the 1800s, however, their status had changed — legal parentage was automatically assigned to the mother. If she was unmarried, she was the sole parent; if she was married, her husband was the father, regardless of whether he was biologically related.

In the 20th century, the most significant change in parenting law was erasing the distinction between legitimate and illegitimate children. Until the 1960s, the law regularly denied rights to children born out of wedlock: the right to collect worker’s compensation benefits or Social Security survivor benefits for a dead parent, for example, or sue for a parent’s wrongful death or inherit in the absence of a will (so-called intestate succession). With the sexual revolution, of course, popular attitudes about marriage changed, and the law changed with them. In decisions in 1968 and 1972, the Supreme Court struck down state statutes penalizing children born to unmarried mothers. The states claimed the laws encouraged marriage, but the justices focused on the fact that the penalties were largely aimed at the children.