The courtroom’s atmosphere is electric as the prosecutor strides to the front. Holding up a transparent bag, she reveals her trump card: a hair from the accused found at the scene of the crime. One look at the jury’s faces and it’s clear this case is closed. Who can argue with the damning proof of forensic science, right?

Wrong, says Ruth Morgan, professor of crime and forensic sciences at University College London and director of the UCL Centre for Forensic Sciences. While forensic science plays an important role in the reconstruction of a crime, she says, it’s dangerous to place undue emphasis upon a strand of hair: “It can be possible to put more weight on a piece of evidence than we’ve got the data to support.”

Forensic science is predicated on the principle that every...