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What  Evidence Would You Trust?

Here’s a hypothetical (but not unlikely) situation: You’ve been called for jury duty and chosen for a criminal case. Investigators present a number of different pieces of evidence including fingerprints, burn marks, hair, and DNA. Which of these can chemists use to most accurately determine what happened? You may be surprised to learn that much of the evidence presented in courtrooms as absolute fact is not. Not all experts use hard science, and even conclusions based on chemical analysis are not always 100% certain. (The Innocence Project and similar programs exist to exonerate people on death row who have been wrongfully convicted.) As a potential jury member, it is your responsibility to be informed about how evidence is collected and examined.

In the next episode of #HistChem, airing Wednesday, October 9, at 7 p.m. EDT, forensics experts will discuss this topic and delve into the history of forensic science. To prepare for the show, tweet what type of evidence you would trust most in a courtroom using the #HistChem hashtag: fingerprints, burns, hair, or DNA? You can also tweet questions directly to our guests during the event.

In the meantime, consider the pros and cons of each type of chemical evidence in solving a crime.

Fingerprints

Historically, fingerprints have been the gold standard of crime-scene evidence. If a print is lifted from the trigger of a gun and matched to a suspect, that suspect obviously pulled the trigger. Real life and real crime is more complicated. First is the assumption that all fingerprints are completely unique. Few, if any, scientific studies have been conducted to prove that each person has a different fingerprint to any other person. Fingerprint analysis involves comparing multiple fingerprints while searching for similar patterns, and ultimately comes down to the judgment of the examiner. Recognizing this limitation, today many examiners will say only that a fingerprint is likely to match a suspect, but will not claim to be 100% certain.

Burns

Fires will often leave little behind, making it difficult to investigate how they started. Like fingerprint analysis, fire investigators rely on their judgment in reconstructing the events leading up to a fire. Determining how a fire started can be as much art as science. People have been found guilty of arson and later exonerated through DNA evidence.

Hair

Hair and fiber examiners look at the structure and composition of a suspect’s hair and compare it to evidence found at a crime scene. Before DNA testing was widely available, investigators often had to rely on these examiners when a hair was the only evidence. Some guilty verdicts based on hair analysis have been overturned by DNA testing: in the case of Santae Tribble an expert could not distinguish between a human and canine hair.

DNA

DNA analysis is also misunderstood. Television depicts it as fast and definitive, whereas in real life it takes days to weeks to complete. In addition, many people believe DNA analysis is used primarily to prove the identity of a criminal during trial. In fact, it is more commonly used to advise police investigators in their search for suspects. DNA does not give 100% certainty, though it will rule out a suspect by showing that their genetic signature does not match anything at the crime scene. (Of course, a suspect may have committed the perfect crime by leaving no evidence behind.) Similarly, DNA evidence has been useful for proving the innocence of the wrongfully convicted.

But analyzing the DNA of multiple suspects allows for all kinds of mistakes. DNA can arrive from unexpected places. In the murder case of Raveesh Kumra investigators found that a paramedic transferred Lukis Anderson’s DNA from a hospital–where Anderson had spent the night of the crime–to the crime scene.

#HistChem's “Digging Up the Bodies: Debunking CSI and Other Forensics Myths” will be broadcast on Wednesday October 9 at 7 p.m. ED.T Watch the live webcast at http://www.chemheritage.org/livestream.

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