It's a flawed sample. Notice the fine print: one in five DNA tests sought by fathers doubting their paternity are negative. That's actually not too bad. Means that 80% of fathers who doubt their paternity are just jealous, obsessive a-holes, and only 20% have their suspicions confirmed.
My guess is if you tested the population at large, you'd get an even smaller number. But we'll never know unless there's a randomized trial, because fathers who don't doubt their paternity tend not to get DNA tests.
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