MICHAEL RUBINKAM
Philly.com

After a 15-year wait and at least $10,000 in child-support payments, Mark Spaid was thrilled to be going in for a DNA test. In 1992, Pennsylvania courts declared him the legal father of his wife's infant daughter even though he knew he was sterile, having had a vasectomy more than two years before the birth.

To the legal system, Spaid's inability to father children meant little when it came to determining who should be responsible for the child's care. In Pennsylvania, like most states, a "presumption of paternity" trumps everything. The doctrine, with roots in English common law, assumes a husband is the legal father of any child born during his marriage. It's designed to preserve marriage and make sure children's financial and emotional needs are provided for.

Source: http://www.philly.com/philly/wires/ap/news/state/pennsylvania/20080427_ap_presumptionofpaternitycantrumpdnatestsfornondads.html