Why were bite-mark and hair comparisons discredited as forensic evidence?

Because critics, and later several scientific bodies, found that the central claim had never been properly shown, and DNA testing then revealed convictions that were wrong. Bite-mark analysis rests on two ideas: that every person has unique teeth, and that this uniqueness shows up in a bite mark left in skin. Microscopic hair comparison rests on a similar idea, that a hair can be matched to one person. Critics say neither idea has been shown to hold.

For bite marks, critics point out that human front-teeth patterns have not been shown to be unique at the individual level. Bite-mark analysis has been condemned by several scientific bodies, including the National Academy of Sciences, whose 2009 report on forensic science led several courts to restrict bite-mark testimony. In February 2016 the Texas Forensic Science Commission recommended that bite-mark evidence not be used in prosecutions until it had a firmer scientific basis, and that year the President's Council of Advisors on Science and Technology, known as PCAST, declared that bite-mark analysis had no scientific validity.

Hair comparison fell by a different route. Accepted as a forensic science by the 1950s, it was undone by DNA testing, which showed that many claimed hair matches were faulty, even ones made by experts. In a 2015 review of 268 trials, the Department of Justice found that in 257 of them, 95 percent, analysts gave flawed testimony that overstated their findings in favor of the prosecution.

Both techniques were used in court for decades. The National Academy of Sciences said that part of the problem is that many traditional forensic sciences have never been empirically validated.