This forensic science expert has testified in some notorious cases

“I don’t need horror movies,” says Robin Cotton, director of biomedical forensic sciences at Boston University’s School of Medicine. “I already know that people do terrible things to each other.”Pat Greenhouse/Globe Staff

First, it was crime lab chemist Annie Dookhan falsifying evidence. And then Amherst drug lab analyst Sonja Farak was found to be siphoning off drugs intended as evidence in criminal cases. Veteran forensic expert Robin Cotton says she was horrified — but not entirely surprised – as she watched the two Massachusetts lab scandals unfold. “Both new and longtime chemists can go astray, and maybe more than we realize,” she says. Cotton knows the pressures scientists can face from heavy caseloads, law enforcement, and exposure to horrifyingly violent crime scenes. “A lab chemist can somehow get it into their head that they are there to facilitate the prosecution. But really we are here to come to the best and just answer, which is not always clear cut,” says Cotton, who has testified in more than 200 criminal cases, most revolving around DNA identification.

As director of Boston University’s biomedical forensic sciences program, she teaches her students to be unequivocally ethical, and exacting when it comes to protocol. “Forensic means the application of science to the law. Your science has to be good, and your ability to testify has to be good. If only one of these works, that’s a breakdown of the system,” says Cotton, who has been working in the field for more than three decades — she started before molecular biology was used as evidence in crime scenes. She began her career at the National Institutes of Health, researching genes on the Y chromosome. Then she happened upon a research paper on how DNA could be used for sexual assault evidence. “I thought, ‘That’s a really great thing to be able to do,’ ” says Cotton, who was one of the first to apply DNA testing at a private crime lab — a practice soon employed by the FBI.

She was involved in several high-profile cases, including the O.J. Simpson saga, and the case of Gary Ridgway, the “Green River Killer” serial murderer from Washington state.

Cotton spoke to the Globe about her long experience as a forensic scientist.

“It didn’t take me very long to get hooked on the idea of using molecular biology to solve crimes — instead of having an esoteric research goal that lasted for years. Forensic science is an applied science, and I find that fascinating. Although forensic science changed crime solving, not all DNA evidence is definitive. DNA testing is very sensitive and needs proper procedures so there’s no contamination. There has to be enough DNA, and in good enough condition, whether it’s from bodily fluids, semen, saliva, or blood. These differences matter in being able to obtain a viable result. It’s just one example of how a forensic scientist has to understand what they’re doing at the bench because it’s not cookbook. What is the scientific result obtained from the evidence tested? What do the results mean in the context of the case?

“When I was testifying around the country, investigators used to pick me up from the airport and promptly start telling me all the horrific details. I’d say, ‘Don’t tell me any more.’ I’m not sure I could stay balanced if I heard what someone else thought had happened. Like the case where a serial killer was chopping up bodies in his apartment and there were all these tiny blood stains that needed to be tested individually. I don’t need horror movies — I already know that people do terrible things to each other.

“You could be the best scientist ever, but you also need to be able to go into court and properly explain what you’ve done and what it means – and in an unbiased manner. I’ve been an expert witness in 35 states, including [during] the O.J. Simpson trial in California. Some of the items [in that case] were sent to a lab I was at in Delaware. I didn’t actually perform the analysis but was the technical lead and testified to the lab results. I was on the witness stand for five and a half days. That was a media circus, but usually there’s very few people in the courtroom.

“In many cases, my testimony is critical. I’ll give my qualifications, then the DNA testing results. Often, my credibility is questioned, and I’ve had attorneys scream at me, which is unnerving. But my singular job [is] to speak to the data and give accurate information about the results I’ve obtained.

“I don’t testify that much anymore — I mostly consult now, and also teach aspiring forensic scientists. I’ve found that it’s common for my students to say that they’re keen to solve crimes and ‘put away the bad guy,’ as if you could easily identify who the bad guy was.

“But I’m struck by how the defendants, charged with unimaginable crimes, look so ordinary, just a regular person in a suit. The rapist, the murderer, the serial killer — they look like someone you’d run into at the corner market. That was a personal revelation to me. And it’s always sad in court. What happened before is never a good thing.”