Biases in Forensic Science

Some errors in forensic science may be the result of the biases of the examiners:

Though they cannot prove it, Dr Dror and Dr Hampikian suspect the difference in contextual information given to the examiners was the cause of the different results. The original pair may have subliminally interpreted ambiguous information in a way helpful to the prosecution, even though they did not consciously realise what they were doing.


This one example does not prove the existence of a systematic problem. But it does point to a sloppy approach to science. According to Norah Rudin, a forensic-DNA consultant in Mountain View, California, forensic scientists are beginning to accept that cognitive bias exists, but there is still a lot of resistance to the idea, because examiners take the criticism personally and feel they are being accused of doing bad science. According to Dr Rudin, the attitude that cognitive bias can somehow be willed away, by education, training or good intentions, is still pervasive.

Posted on January 31, 2012 at 11:13 AM18 Comments


aikimark January 31, 2012 11:20 AM

We had a really bad problem with our state (SBI) crime lab in NC. The forensic folks were trying to ‘support’ the prosecutors with their findings.

AReader January 31, 2012 12:10 PM

Looks like a script right of CSI. I wouldn’t be surprised if the folks over at SBI in NV watched it religiously.

bill January 31, 2012 12:44 PM

Cognitive bias? Doing this unconsciously? It is like people are assuming the scientists are straying away from baseline of cold objectivity, when in fact, the majority of the time they working at the behest of the prosecution to convict people. There’s an inherent conflict of interest at play. The article would be more shocking if they were moving towards real objective science where the results aren’t gamed or selectively chosen to benefit the prosecution.

John Jenkins January 31, 2012 12:50 PM

Hell, in Oklahoma City, the forensic “scientists” actively fabricated evidence to get convictions. (

The chief problem is that forensic science isn’t science at all, but just a way that prosecutors try to snow judges and juries with pseudo-scientific nonsense (see, e.g., bite analysis, fiber analysis, blood typing “evidence”). The closest thing they have to real science is DNA, and the consistently misrepresent that in such a way as to make it bullshit.

Mark McCutcheon January 31, 2012 12:54 PM

Notorious case: Dr. Charles Smith, former Ontario pediatric pathologist:

“In 20 child autopsies reviewed by outside experts in 2005, he was found to have made major scientific errors, leading to baseless charges of child-killing and 13 subsequent criminal convictions.” – CBC News

bill again January 31, 2012 12:55 PM

Also, it seems kind of ridiculous that the prosecutors will use completely unscientific and falsifiable methods of trying to convict someone (confessions, eye witness testimony) and then fall back on science when these methods fall though. Are we really surprised that the science is bad?

Rory January 31, 2012 1:17 PM

Would the solution to this be to blind the forensics technicians processing the evidence? So you’d have a crime scene unit that documents the evidence in situ and bags it up for transfer to the lab techs, who then examine and report on it without knowing the context. The results can then be made available to the investigating officer. Are there reasons this isn’t feasible?

NobodySpecial January 31, 2012 1:58 PM

@Rory – they already do this, the lab work is mostly contracted out and they not involved at the crime scene. But the lab ‘knows’ the police want a match and it’s business is based on keeping it’s customer happy.

The only way to do it would be to bag all the evidence AND bag identical items from another scene and ask multiple labs to examine all of them.

lazlo January 31, 2012 2:00 PM

“… beginning to accept that cognitive bias exists, but there is still a lot of resistance to the idea, because examiners take the criticism personally and feel they are being accused of doing bad science.”

Wait, you mean that they think there’s a cognitive bias against the acceptance of a cognitive bias in their work? My recursion-sense is tingling…

Randy January 31, 2012 2:10 PM

I’ve always kinda wondered why the coroner is an elected position in many communities. I guess the reason is that it’s a political position.

I also question why a coroner also determines both the cause and manner of death. I believe that they should determine the cause (eg bullet severed artery), but not manner (accident, homicide, suicide). It seems to me that the later depends on the circumstances which this article is saying the coroner should not know.

Randy — kindaramblingoutloud

murray January 31, 2012 3:20 PM

“Wait, you mean that they think there’s a cognitive bias against the acceptance of a cognitive bias in their work? My recursion-sense is tingling…”


NobodySpecial January 31, 2012 3:32 PM

@Randy – I think that’s why it is an elected position.

The coroner’s job is to judge the evidence from the forensic scientists and the police to determine the cause of death.
Ideally it’s more a judicial than technical role.

Gweihir February 1, 2012 4:12 AM

Not a surprise, just human nature. That is why all competent forensics people will refuse to take such information before they have a complete report draft. After that they can still look at the context and see whether it has any influence.

I do the same when reviewing anonymous papers: Review first, write up, then identify the authors (usually easily possible) and look at their other work. In some rare case this changes things, usually because I misunderstood something.

wumpus February 1, 2012 2:04 PM

The prosecution is the customer, and the customer is always right.

Who said customer service is dead?

Peter E Retep February 2, 2012 9:31 PM

A major British forensic pathologist wrote an extensive book
about historically famous cases showing the ways forensics as practiced
is often inherently biased, like the line up, the fingerprint ‘art’,
the calculation of population probability statistics, etc. (Barnes & Nobel).

Most people don’t understand the mathematics of bias,
or fulll field arrays, or of frame induction, and consequent chaining.

Each of these offers bias in calculating population probabilities.

Even worse than such bias,
is willful or negligent action.
A Canadian documentary the New Detectives reviewed
the forensic mis-handling (and false statements made)
about forensic evidence in the O.J,.trial, while being filmed.

Trey February 2, 2012 10:50 PM

As a forensic drug chemist, I can tell you that a great deal of the problem lies in the fact the most crime laboratories are managed by law enforcement. The management are police officers with no science background and usually little more than a high school education. Nothing wrong with that, but none of these people should be running a scientific laboratory. On top of that, their mindset is to use the crime lab to support law enforcement in the sense of obtaining convictions and making a case, rather than to provide objective data to the criminal justice system. I lost my job and was forced to resign from a crime lab in St. Tammany Parish Louisiana because I couldn’t SCIENTIFICALLY prove the presence of marijuana in a charred smoking device, and thus did not and would not support the arresting officer’s belief that his suspect was guilty. This denied the suspect his right to due process because the arresting officer and crime lab director acted like judge, jury, and jailer.

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