As many as one in twenty-five men are not the fathers of the children they think are theirs.
Could be that dad is not real father, report shows (Thursday August 11, 2005)
Paternity Study Finds Daddy Discrepancies (Thursday, August 11, 2005), By Miranda Hitti. SOURCE: Bellis, M. Journal of Epidemiology and Community Health, Sept. 2005; vol 59: pp 749-754. News release, BMJ.
Even if a man proves he's a child's biological father, it doesn't necessarily trump the rights of the legal or social father. Here's one example from The Waukesha Freeman written by BRIAN HUBER
"Supreme Court denies paternity to biological father; Child born of adultery remains with her marital dad." April 8, 2004, Waukesha, Wisconsin, U.S.A.