A 1999 study from from the American Association of Blood Banks showed that 30% of paternity tests came back negative. A subsequent Google search I did showed the number from 32% to 33.6%. It should be noted that these results were only from those who had reason to question whether they were the father, so the numbers could be much higher.

I'm linking a well written 2007 article from Men's Health on the subject of paternity fraud. I read the article a while before I found this sub, so I don't remember exactly what they were, but I remember identifying 4 specific issues that they covered. Probably the most most revealing is the reluctance of medical professionals to volunteer paternity discrepancies to the "father" when discovered during routine testing. Some have even outright lied to them, claiming that it must be due to a previously unknown genetic mutation.

https://www.menshealth.com/trending-news/a19543127/fathers-and-kids-parenting-fraud/?fbclid=IwAR3YjlkPvkBSlC91aOPgm2Ku1WJtHY32kbqBQiB-3_3D50SgntX3ZLxXkuI