Family laughing(Photo: Courtesy of Men's Health)

Patrick Connaro, a 42-year-old robotics engineer living in Colorado Springs, was sitting in the bleachers one warm Saturday afternoon in 2003, watching his son's Little League game, when the ground opened beneath him.

"My little boy was there, he was up at bat, and I started yelling for him, 'Go Matthew [not his real name]! Knock it out of the park!' And another man started screaming for Matthew. Louder than me. I looked over, and I looked at him, and I was like, Who is this guy? And I looked at my son, and I looked at him … and they were identical."

After the ball game, Connaro ordered a paternity test. The results came back 2 weeks later. "I opened up the letter from Labcorp, and it said, ' … 99.9 percent chance you are not the biological father of this child.' I started crying. My head started spinning."

Connaro admits that the possibility had crossed his mind before, given his son's dissimilar facial features, but each time he questioned his wife about it, she vehemently denied the suggestion. Even when he showed her the test results, she still denied it. "She said, 'You forged this,' " Connaro recalls, shaking his head in amazement.

To this day he remembers that game with a kind of nightmarish clarity. Matthew struck out. Connaro had planned on going over and giving him a hug, along with a few words of fatherly consolation, but when he heard the other guy yelling, he just stood up and walked away.

"I was so disillusioned, I just didn't know what to say. It was horrible. I don't think anybody could experience what I experienced there."

Some call this paternity fraud. But a more accurate term is "paternal discrepancy." Paternity fraud emphasizes the financial aspect of the phenomenon, but paternal discrepancy (PD) describes the anomaly itself—the disconnect between what men think is true and the genetic reality. And research shows that it's a lot more common than we might believe.

After recently reviewing 67 studies on the subject, University of Oklahoma researchers found that PD rates tend to be much higher among men who have reason to believe there's been more than one dog in the yard. No surprise there. But leave out these men and you end up with a number that can safely be assumed to represent the rest of us. That number is 3.85 percent. Another review of 19 studies by a group at Liverpool John Moores University backs this up, putting the figure at 3.7 percent of dads. It may not seem like a lot—until you do the math. According to a 2005 U.S. Census Bureau report, there are 27,940,000 fathers nationwide with a child under 18. That means over a million guys out there are taking care of some other man's kid.

Compared with this, infidelity by itself is a mere white lie, a misdemeanor, maybe even forgivable. But this … this lie unravels years of commitment in a single stroke. Does forgiveness even apply here?

There are those who believe that biology shouldn't make a difference, that fatherhood is just a social construct. Connaro himself refuses to let genetics stand between him and his son. "I'm the only man that he knows as his dad," he says. "Why should I lose that bond and that love?"

Many men in Connaro's position might not feel the same way. Studies show that evolution has designed men to care deeply about who their children are. In 2003, for instance, researchers at the State University of New York at Albany recruited 20 men and 20 women, then morphed their facial features with photographs of children. Subsequent testing showed that the women responded equally to photos of kids whose faces resembled theirs and those that resembled the faces of strangers. The men, on the other hand, reacted far more positively to children whose faces resembled their own.

"It's not reproductively beneficial to invest all your resources in a child who is not carrying on your genetic line," says study author Rebecca Burch, Ph.D. "Men throughout the history of the species who have invested all their time and energy in children who weren't theirs no longer have genes in the population."

They are, in other words, extinct.

When the stakes are this high, there is no greater deception than PD. It's a lie that reaches right down to your chromosomes, and when the truth finally comes out, the revelation can be devastating.

Seven years ago, on an otherwise idyllic summer Saturday afternoon, Tony Winbush, 34, was throwing a football with his 5-year-old son in the backyard of his home in Tallahassee, Florida, when he discovered he was not the child's father. The boy himself told Winbush.

"He said, 'My mom told me that I have two dads.' I said, 'Son, you can't have but one dad. I'm the only dad you've got.' He said, 'That's what my mama told me—I have two.' " Within a month, the family ceased to exist. "It got to a point," says Winbush, "where it bothered me so bad, I had to seek psychiatric help."

But while men may be the primary victims of PD, women aren't the only villains. Paternal discrepancy makes liars of everyone, and that includes the people from whom we'd most expect the truth.

Morgan Wise, a 44-year-old train engineer from Big Spring, Texas, might have remained ignorant forever if his fourth child hadn't been diagnosed with cystic fibrosis (CF). Presumed to be a carrier of the CF gene, Wise had his own DNA tested to identify which of the many gene mutations of the disease the boy had inherited. (There are more than 1,000.) The test revealed that Wise was not a carrier, which could only mean one thing: This was not his child.