Forensic Scientists on TV do not utilize forensic thinking.

On television shows, forensic scientists generally fall into two types. Let's call the first Sherlock. Relying more on instinct and experience than any scientific skill, Sherlock only uses science to confirm what he or she already knows. It's almost as if Sherlock solved the crime at first glance, and the science is merely an amusing afterthought. Denzel Washington's character in "The Bone Collector" is a Sherlock. He sees things that no one else sees, and is always one step ahead of the science. He picks as his successor a beat cop with zero forensic experience who just happens to be born with the same instincts as Sherlock Washington.

The second type we can call the Deep Space Probe. The Deep Space Probe works in the lab and through miraculous techniques can determine the exact substance that was on the suspect's shoe and link it to the exact place of the crime. Results are produced immediately and eliminate all other possible explanations leaving only one possible culprit. CSI is a big fan of the Deep Space Probe forensic scientist, often paired up with a Sherlock working in the field. Sherlock's initial hunches are only disproved when the Deep Space Probe extracts miraculous (and impossible) amounts of information from the evidence.

Neither the Sherlock nor the Deep Space Probe utilize forensic thinking.

Confirmation Bias

Sherlock suffers from what is commonly known as confirmation bias.

Imagine a crime where thieves break into a donut shop and steal money from the register. Examination of the scene reveals fingerprints not belonging to the owner or current employees. Some prints at the scene match a man on parole for a felony burglary. Should we focus only our examination on this one man, or is it a mere coincidence?

Confirmation bias is a pernicious hound, stalking the intellect and striking when we least anticipate falling into its clutches. In its most basic form, confirmation bias is a form of bigotry. A very clear sign of confirmation bias are cases where a theory is proven regardless of the results of our investigation. If fingerprints matching our suspect are found, that means the suspect committed the crime. If fingerprints were not found, that means that only an experienced burglar such as our suspect committed the crime.

If you've seen the film In the Heat of the Night one of the opening scenes has a classic rendition of confirmation bias. A man is found murdered. A short distance a way, a black man is found trying to leave town on the next train with a large amount of cash in his wallet. The police are certain that they have their man because he fits their expectations and experience. They're horribly wrong, yet it takes great effort to convince them otherwise.

Politics are rife with confirmation bias. When the public votes for our favorite candidate, that means that the candidate is the right person for the job. When the public votes against our favorite candidate, the candidate is still the right person for the job, but the public is too stupid to recognize this.

Overzealous Interpretation of Evidence

The Deep Space Probe suffers from overzealous interpretation of evidence. A single hair or mote of dust is expanded into the entire fabric of the crime. In reality, minute traces of evidence found by the Deep Space Probe can rarely point to only one source. For example, a brown hair from the head of a Caucasian will not be easily confused with a hair from someone of African descent. However, one cannot look at a hair under a microscope and uniquely determine which person the hair came from.

DNA analysis can in many cases determine which person left a hair, bloodstain, or other biological evidence at a crime, but that does not mean the person committed the crime. Forensic DNA analysis is an excellent way to determine identity, but we cannot determine when the evidence was left at the scene. If a person sneezes on a tissue and leaves in a trashcan where someone is assaulted later on, overzealous interpretation of the DNA profile obtained from the tissue may unfairly implicate the sneezer in the crime.