Nature or nurture? This familiar question has been given a poignant twist with the marriage of an old issue and a recent genetic technology. DNA fingerprinting is increasingly being used in family disputes1. Women use it to seek support from the biological father of their child, and men in divorce disputes are being advised by their lawyers to have their children tested. Any child found not to be their biological offspring becomes a card to be played in the divorce settlement. In some cases, men who have raised a child for many years are being urged to cut off all contact.

Anglo-American law has long embodied the presumption that any child born to a married couple was the husband's. This legal rule was probably less motivated by any tender regard for the health of the marriage or the well-being of the child than by the determination of the state to make certain that someone — other than the state — be financially responsible for the child2. Exceptions were rare. Impotence or sterility could overcome the presumption. An absent husband could invoke the 'four seas' rule. But in general, despite evidence indicating that a non-trivial number of children are not the biological offspring of the husband of their mother, law has assigned paternal rights and obligations to him. The paternity blood test, developed by Landsteiner in 1901, could exclude some men as possible fathers, but it could not show that any particular man was, indeed, the father3.

DNA relationship testing changed that. It is now possible to establish beyond reasonable doubt whether a first-degree biological relationship exists. The ethical question is, so what?

When a man has raised a child as his own for one, five or fifteen years, are they not parent and child? The Romans had a practice of raising children who were not their offspring. They were called 'alumna' or 'alumnus' and the relationship, being voluntary, was regarded as superior to mere ties of biology4. Whether the Romans got the details correct is not important; they understood that a relationship between adults and children nurtured over years of continuing care and growing mutual affection mattered, with or without a biological connection. Today, adoptive parents and the children that they raise and love understand this well.

When a technology — such as DNA relationship testing — undermines the settled social policy that ties legal parenthood to rearing parenthood, the law must adapt. Our best ethical understanding of what it means to be a parent should guide the law — not the anger of 'duped dads' or a simple-minded conception in which a genetic test trumps a decade of diapers, bedtime stories and trust. After all, it is in our nature to nurture, and law should affirm, not undermine, that truth.