From a compelling story in the New York Times Magazine:
In an age of DNA, when biological relationships can be identified with certainty, it can seem absurd to hew so closely to a centuries-old idea of paternity. And yet basing paternity decisions solely on genetics places the nonbiological father’s welfare above the child’s. Phil Reilly, a lawyer who is also a clinical geneticist, has been wrestling with the policy implications of DNA testing for years, and even he is stumped about how society should manage the problem that men like Mike face. “We’re at a point in our society where the DNA molecule is ascendant, and it’s very much in the public’s consciousness that this is a powerful way to identify relationships,” Reilly says. “Yet at the same time, more people than ever are adopting children, showing that parents can very much love a child who is not their own. The difference here for many men is the combination of hurt and rage over the deceit, the fact that they’re twice beaten. I can see both sides of this argument. As a nation, we’re still in search of what the most ethical policy should be. Every solution is imperfect.”
The Personal Genome Project includes disclosure of nonpaternity as one of the explicit risks of participation. That said, having been warned is probably not much consolation to people who discover certain surprising things about their families.