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Forensic science usually provides the key piece of evidence in true crime shows and podcasts. But what misconceptions about forensic science are making their way into popular culture? What details about forensics do shows like CSI and Law & Order get wrong? Is blood spatter analysis as foolproof as we assume? Has DNA evidence ever steered juries in the wrong direction? Join host Justin Dodd (@juddtoday) as he breaks down some common myths and misconceptions about the investigative world of forensic science. ---------- Mental Floss is the home for all things curious. Subscribe here for new Mental Floss videos every Wednesday at 3pm (and don't forget the bell!): https://www.youtube.com/channel/UCpZ5... Don't miss a video! Subscribe NOW: https://www.youtube.com/@MentalFloss?sub_confirmation=1 About Mental Floss: Mental Floss is where curious people come for trivia-tastic information. Mental Floss produces lists of fun facts, debunks common misconceptions, and tells untold stories from history, science, culture and more. Website: http://www.mentalfloss.com Twitter: http://www.twitter.com/mental_floss Facebook: https://facebook.com/mentalflossmagazine Copyright Notice: This video and YouTube channel contain dialog, music, and images that are property of Mental Floss. You are authorized to share the video link and channel, and embed this video in your website or others as long as a link back to this YouTube Channel is provided. 2025 Mental Floss Misconceptions About Forensic Science | Mental Floss https://www.youtube.com/@MentalFloss
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In January 1981, one of the most sensational murder trials of the late 20th century
0:04
was taking place at the Westchester County Courthouse in New York. Jean Harris, the 57-year-old headmistress of the elite Madera school outside Washington, D.C.
0:14
was accused of killing her ex-partner, Dr. Herman Tarnower, a 69-year-old cardiologist known for his recent bestseller, The Complete Scarsdale Medical Diet
0:23
Prosecutors said that Harris, angry that Tarnower had jilted her for a younger woman
0:27
drove from the D.C. area to Tarnauer's home in Westchester with a loaded gun on the night of
0:33
March 10, 1980. She shot him several times, but then told police she had asked him to kill her
0:40
Her defense attorneys argued that the gun had somehow accidentally gone off during a struggle
0:45
killing Tarnauer in his bedroom. Tarnauer's pajamas, covered in his dried blood
0:49
were presented as evidence in court. The defense called Herbert McDonald, the country's leading expert on blood splatter ysis. In fact, he was responsible for
0:57
reviving what had been a dormant field. McDonald actually put on the pajamas to argue that the lack
1:03
of high-velocity impact splatters on the bed indicated Tarnauer wasn't in the bed when he was
1:09
hit. According to McDonald, the pattern of smears, swipes, and droplets on the bedsheet also indicated
1:14
that some time passed during the crime, perhaps long enough for the shooter to render first aid
1:20
Was it all a tragic accident, as Harris claimed? Hi, I'm Justin Dodd. Today, we're going to look
1:26
some misconceptions about forensic science, from the reliability of blood splatter ysis
1:31
to how TV crime shows affect jury deliberations. Let's get started. Blood splatter evidence is irrefutable. By the time of the Scarsdale Diet Doctor murder
1:49
as an episode of American Justice so memorably put it, McDonnell was already well known for his
1:54
his work on blood splatter. His research, published by the Department of Justice as
1:58
Flight Characteristics and Stain Patterns of Human Blood in 1971, is considered the foundational
2:04
guide for interpreting the spots, splashes, and spatters one might encounter at a crime scene
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He taught 40-hour training courses to police officers, FBI agents, and forensic scientists
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McDonald's methods found their way into law enforcement investigations in legal cases
2:19
across the country. McDonald himself became a sought-after witness at murder trials
2:25
charging $800 for each appearance, like some kind of rock star. And that's why he ended up wearing
2:30
someone else's blood-stained pajamas in a suburban courtroom in January 1981. Despite McDonald's
2:36
testimony for the defense, though, Gene Harris was found guilty of second-degree murder. The jury did
2:41
not find McDonald's ysis that convincing. And maybe it shouldn't have. According to an
2:46
investigation by ProPublica, there's not much conclusive science behind blood spatter ysis
2:52
McDonald devised the experiments and studies in his lab, which is to say in the basement of his
2:57
home in Corning, New York in the 1960s. He was a chemist for the famous glass company in that city
3:01
but had little background in physics, biology, fluid dynamics. Essentially, he was a one-man
3:07
band of speculative forensic experimentation. But with his charisma and enthusiasm, McDonald's sole
3:14
law enforcement and the public on the reliability of blood spatter ysis, and himself as the top
3:20
expert on it. Which reminds me, check out my new book, How to Tell if Your Chicken Nuggets Are Ready
3:25
Tales from a Toaster of an Expert. In 2009, the government's National Research Council
3:29
formally questioned the reliability of forensic methods like McDonald's. In a landmark paper
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it revealed that much of forensic science was developed without objective standards
3:39
and that the chance of mischaracterizing forensic evidence existed on practically every level of investigation. The council called for standardized methodology for the use and
3:48
interpretation of forensic techniques. It wrote, scientific studies support some aspects of blood
3:54
stain pattern ysis but some experts extrapolate far beyond what can be supported Legal cases still rely on bloodstained pattern ysis to reconstruct crimes but as Ralph
4:04
Ristenbott, a forensic science professor at Penn State, told ProPublica, There's this belief out there that you can look at the patterns of blood at a crime scene
4:12
and it's the be-all, end-all, when in reality, bloodstained pattern ysis is just one tool
4:17
in the toolbox. Bite marks can match only one person's teeth. You've seen it on a hundred crime shows
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A murder victim's skin reveals the telltale evidence of a human bite down to the individual teeth
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A forensic odontologist, otherwise known as a bite mark yst, matches the pattern to that of the suspect's dental mold
4:36
Case closed. Except it's not that simple. Bite mark comparison is a pattern-matching method
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An expert looks at two pieces of evidence and, based on his or her experience
4:46
says they are or are not a match. According to the Innocence Project, one fundamental reason bite mark ysis is
4:53
unreliable is that scientists are not actually sure that each bite mark is unique to an individual
4:58
Another key reason is that human skin is flexible and can show contusions in many different ways
5:03
and is prone to decomposition after death. So it's not the best medium for recording precise
5:09
teeth marks. As Adam Freeman, a forensic dentist whose research has demonstrated the fallibility of
5:14
bite-mark ysis told the Innocence Project, what we're looking at isn't actually a bite-mark
5:18
indentation. We're looking at the bruise that's left over. And a bruise doesn't exactly approximate
5:24
the teeth that made it because bruises are diffuse areas of blood under the skin. So how did this
5:30
rather grim type of evidence become so important to forensic investigation? Bite-mark ysis is
5:35
actually a pretty recent development. Until the mid-1970s, dental forensics focused on identifying
5:41
victims of natural disasters or other catastrophic events by matching the human remains to existing
5:46
dental records, which is still a widely used and scientifically valid practice. Prosecutors
5:51
in the 1974 case People v. Marks, however, introduced bite marks as evidence in a California
5:56
murder trial. Walter Edgar Marks was accused of killing a 75-year-old woman named Lovie
6:01
Bonofsky. During the investigation, her body was exhumed six weeks after burial, at which
6:06
time forensic scientists discovered a well-defined elliptical bruise on her nose. Three forensic
6:12
dentists broke protocol and testified for the prosecution because they all agreed the
6:16
bruise was a human bite mark made by a distinctive set of teeth—teeth virtually identical to
6:22
those of the defendant. They also noted that the skin of the nose is thin and taut, which
6:26
preserved the bite impressions very clearly. The defense argued that there was little scientific
6:31
basis for the ysis, but because this particular bite was so uniquely well-preserved and the
6:36
teeth that made it so obviously atypical, the evidence was judged admissible. Marx was
6:41
found guilty of voluntary manslaughter. That case and the precedent it's set opened the floodgates to using bite-mark ysis
6:48
in criminal cases. It's been used to support cases against clearly guilty defendants
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like that of serial killer Ted Bundy in 1979, who apparently also had quite atypical teeth
6:58
But it's also been used to convict numerous defendants who have later been exonerated
7:02
by other forensic evidence. Some bite-mark ysts have made careers out of appearing
7:06
as witnesses despite the fact that the validity of their technique has never been scientifically
7:11
established. Oh, and by the way, look out for my follow-up bestseller, Who Took a Bite
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Out of My Nuggets? The Story of a Food Thief Specialist. The 2009 National Research Council
7:21
report came down hard on bite-mark ysis, calling it a practice with no evidence of
7:26
an existing scientific basis for identifying an individual to the exclusion of all others
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That echoed the conclusion of a 2001 review in the journal Science and Justice, which found that the literature revealed a lack of valid evidence to support many of the assumptions made by forensic dentists during bite-mark comparisons
7:43
According to Washington Post opinion journalist Radley Balco, who published a four-part expose on the technique in 2015, and then a book based on the series
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there is no better example of the pitfalls of allowing junk science into the criminal justice system than bite ysis Junk science science is officially entering my vocabulary for new put Thank you very much For what it worth a 2017 article in the Journal of Law and the Biosciences argued
8:08
that it's really bite mark comparison that's specious as opposed to ysis. The latter
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practice tries to identify purportedly more objective facts, like how long ago a bite mark
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was left and whether it comes from a human or an animal. The Innocence Project cast doubt
8:22
on even those claims, though, pointing to an incident in which a crawfish bite may have been
8:27
erroneously identified as coming from a human. I mean, we've all been there. We'll probably have
8:31
to see how this debate develops in and out of the court system. For now, though, all sides seem to
8:36
agree that bite mark comparison is not a valid science. Junk science. Nice. DNA is a foolproof
8:43
way to identify perpetrators. In 1987, a man with the suspiciously evil name of Colin Pitchfork
8:50
was arrested and soon found guilty of murdering 15-year-olds Don Ashworth and Linda Mann in
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Lestershire in the UK. He was caught by his own DNA, a first for a murder case. During the
9:00
investigation of Ashworth's death, police arrested a suspect who was not Pitchfork. I'm gonna guess
9:06
Randy Cloak and Dagger, but don't quote me on that. The suspect confessed to murdering Ashworth
9:11
but denied killing Mann. Police, though, were sure that both crimes had been committed by the same
9:17
person. They asked Alec Jeffries from the University of Leicester to examine the DNA evidence
9:22
A few years earlier, Jeffries had found that patterns of a person's DNA were unique to that
9:27
person. Now he compared the genetic patterns of the suspect to evidence from the crime scenes
9:32
The suspect's DNA was not a match. Though he had confessed to the crime, he was ultimately
9:37
exonerated. The killer was still at large. There was no database of DNA profiles back then
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So police created a dragnet collecting DNA from more than 5,000 area men who volunteered their samples
9:49
There was no match to the perpetrator, but the police did have a clue. One of the volunteers told a friend that a man had paid him to give a sample in his name
9:58
The man who paid was Colin Pitchfork. Jeffrey soon found that Pitchfork's DNA matched the samples from the two crime scenes
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In 1988, he was sentenced to life in prison. Decades later, he was paroled but was soon recalled back into the prison
10:11
and as of filming this video, he's waiting for another parole hearing
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Oh my gosh, I just felt like a real true crime podcast host there. That was really fun
10:19
Pitchfork's conviction demonstrated a revolutionary new forensic method. In the 35 years since that trial, DNA evidence has helped solve plenty of cold cases
10:27
and led to the convictions of notorious serial killers, such as the Southside Strangler, Timothy Spencer
10:33
the grim sleeper, Lonnie Franklin Jr., and the Golden State Killer, Joseph James D'Angelo
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But even DNA ysis is not a foolproof forensic technique. The risk of errors comes not from the underlying science guiding the practice, which is sound
10:47
but from the methods of collecting and interpreting samples. The mistakes can lead investigators down the wrong path, such as in the case of the Phantom of Heilbronn
10:56
Oh my god, now I'm Rod Serling. This is awesome. In the early 90s, an apparent serial killer began a rampage of attacks across Europe, lasting nearly 20 years
11:04
Police collected the same DNA from each crime scene that identified the perp as a woman
11:09
But beyond that, they couldn't pinpoint a pattern to the assaults. People convicted of some of the attacks denied that they had a female accomplice
11:16
If she did exist, she had never been caught on security cameras. Despite the string of more than 40 burglaries, break-ins, and murders
11:23
bearing the same genetic signature, police were stumped. In January 2009, the reward for information leading to the Phantom's capture
11:31
was increased to 300,000 euros. But just two months later, investigators cracked the case. The phantom's DNA collected at the crime
11:39
scenes didn't originate at the crime scenes. The cotton swabs used to collect samples had been
11:45
contaminated with rogue DNA at the factory where they were made. The so-called phantom turned out
11:50
to be an elderly female worker at the manufacturer plant Such cases of transfer DNA can cloud investigations that otherwise seem conclusive The Marshall Project reported the case of Lucas Anderson a man experiencing homelessness in Santa Clara County California who was arrested for the murder of Silicon Valley
12:07
investor Ravish Kumra. Anderson said he had never met Kumra, but Anderson's DNA was found under Kumra's fingernails
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Prosecutors thought they had a slam dunk. The presence of Anderson's DNA on the victim seemed incontrovertible
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But the defense discovered Anderson had been in the hospital at the time of the murder
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taken there by paramedics after he collapsed a few hours earlier. Later that night, the same paramedics examined Kumrah's body
12:32
They may have slipped the same pulse exometer onto both Anderson's and Kumrah's fingers
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transferring the former's DNA to the latter. Or maybe the DNA hitched a ride on a paramedic's uniform
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Anderson was released from custody, and three other people were eventually convicted for
12:46
the murder. According to the Marshall Project, it's the best-known case of an innocent person
12:51
nearly being implicated by transferred DNA. Forensic evidence ensures criminals are convicted
12:57
There's no such thing as committing the perfect crime. Perpetrators always leave something behind, like hair, spit, or even fingerprints
13:04
that can prove their culpability. Forensic science is so advanced now that with just a few lab tests
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prosecutors can win convictions in the time it takes to watch an episode of Dayline
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Or so many people think. This misconception is considered so common, there's even a name for it, the CSI effect
13:22
The Cornell Law School describes it as the impact of crime-solving shows like CSI and Law & Order on jurors
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It may take several forms. Jurors who binge crime shows may view forensic evidence as more important than other types of evidence in a trial
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They also expect forensic evidence to be presented in every case and perceive forensic conclusions as foolproof
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They might also demand more testing for DNA, gunshot residue, or blood spatters that aren't relevant to the case
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If a trial lacks those elements, they may be less likely to convict a defendant. In one of the first trials to demonstrate the CSI effect, according to a 2004 USA Today story
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wealthy real estate heir Robert Durst was tried for the 2001 murder and dismemberment of his neighbor Morris Black in Galveston, Texas
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Black's head was never found. Durst's defense team believed that the missing head could be the key to establishing reasonable doubt
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if the jury could be convinced that the head could have revealed forensic clues supporting Durst's case
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The team hired a consultant to find potential jurors who watched CSI Crime Scene Investigation
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which was then the number one show on TV, whom they believed would demand more forensic proof of Durst's guilt and be more likely to acquit
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As the consultant told USA Today, Today, talking about science in the courtroom used to be like talking about geometry, a
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real jury turnoff. Now that there's this almost obsession with the TV shows, you can
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talk to jurors about scientific evidence and just see from the looks on their faces that
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they find it fascinating. The tactic worked. Durst beat the charge. Famously, in later trials, he was convicted
14:52
of a different murder along with several other offenses. But the Morris Black case suggested
14:57
that juries were beginning to change their expectations of forensic evidence in criminal
15:01
trials, thanks in part to the popularity of crime dramas. The problem is that CSI and shows like it
15:07
don't always present forensic science accurately, not to mention that the forensic methods themselves
15:12
aren't always based on actual science. Some scholars suggest that the CSI effect is more of
15:17
a phenomenon in the media based on anecdotes and speculation than a legal conundrum backed up by
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empirical evidence. It's hard to pin down how each television-educated juror applies the knowledge
15:28
they glean from TV to actual court proceedings. But if you see a juror intensely putting on a pair of sunglasses and delivering a punny
15:35
one-liner, you might want to question their impartiality
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