---
title: 'We Investigated How Accurate Forensic Methods Are. Here''s What We Found.'
source: 'https://youtube.com/watch?v=mvcesPWvUIc'
video_id: 'mvcesPWvUIc'
date: 2026-06-22
duration_sec: 0
---

# We Investigated How Accurate Forensic Methods Are. Here's What We Found.

> Source: [We Investigated How Accurate Forensic Methods Are. Here's What We Found.](https://youtube.com/watch?v=mvcesPWvUIc)

## Summary

This video investigates the accuracy of five common forensic techniques: microscopic hair analysis, bite mark analysis, bloodstain pattern analysis, fingerprint analysis, and DNA analysis. It reveals that despite their widespread use in court, many of these methods lack rigorous scientific validation and are prone to errors, biases, and misinterpretations.

### Key Points

- **Historical Forensics** [0:00] — The earliest known forensic method is from a 1247 Chinese book, 'The Washing Away of Wrongs', where flies landed on a murderer's sickle, revealing the weapon.
- **NAS Report on Forensics** [1:07] — A 2009 National Academy of Sciences report stated that except for nuclear DNA analysis, no forensic method has been rigorously shown to consistently connect evidence to a specific individual.
- **Microscopic Hair Analysis Flaws** [2:11] — FBI examiners routinely claimed matches between hairs from different people, and sometimes couldn't tell human hair from dog hair. Out of 268 cases, 96% were declared false, with 33 people sentenced to death and 9 executed before errors were found.
- **Bite Mark Analysis Unreliability** [3:44] — Studies show bite marks on skin are not reliable due to skin distortion. Despite this, bite mark evidence is still allowed in courts worldwide, even as recently as 2025.
- **Bloodstain Pattern Analysis Issues** [5:38] — The method assumes straight-line trajectories, ignoring gravity and drag. A 2021 study found analysts disagree on how a stain was made about 8% of the time. New software using fluid dynamics is improving accuracy.
- **Fingerprint Analysis Subjectivity** [8:04] — Fingerprint examiners often disagree on the number of minutiae points, and the same examiner may reach different conclusions 10% of the time. Contextual bias (e.g., knowing the suspect's criminal record) can influence results.
- **DNA Analysis Challenges** [16:41] — Increasing sensitivity leads to issues like trace DNA transfer (e.g., a homeless man's DNA transferred to a murder scene). DNA mixtures from multiple individuals are difficult to interpret, with a 2013 NIST study showing 69% of labs got the analysis wrong.

### Conclusion

While forensic techniques have improved, many still lack rigorous scientific validation and are prone to errors and biases. The video emphasizes the need for continued reassessment and improvement to ensure forensics remains a reliable science.

## Transcript

(dramatic music)
- A man has been found lying dead
at the side of the road with oozing,
elongated wounds to his back.
To solve the murder,
a local official calls all
the farmers in the village
and demands they all bring
their tools to the main square.
In the square, each man is
set to lay down his sickle.
And then something
curious starts to happen.
In the rising heat of the
morning, flies begin to descend,
all landing on a single sickle.
Its owner buckles,
falling to his knees and admits to murder.
This is from a 1247 book,
"The Washing Away of Wrongs" by Song Ci.
And it's the first writing we have
of an empirical approach to forensics.
But okay, big deal.
Anyone could have just swapped the sickles
or used their neighbor's.
But forensics today are
much more accurate, right?
Fingerprints support investigations
in over 70% of murders
and DNA evidence in more than 90%.
But then you also read an article like
"Hair sample that put men
into prison for 28 years,
turned out to be dog hair."
- That is not science.
That is not justice.
- See, in 2009, the
National Academy of Sciences
published this 350 page
report stating that,
with the exception of
nuclear DNA analysis,
no forensic method has
been rigorously shown
to consistently demonstrate a connection
between evidence and
a specific individual.
And they also said that some tests
do not meet the fundamental
requirements of science.
So we ask people to rank five
famous forensic techniques,
all of which are still
being used in court.
Okay, five forensic techniques here.
And I just need you to rank them in order
of accuracy from the
least accurate down here
to the most accurate.
So what do you think?
- I don't know if I'm gonna nail these.
- Whoa. I dunno.
- I don't know what
forensic to put first here.
- Okay, hair. What are you thinking?
- I think hair is last, eh?
- I would probably put
the hair on the top two.
- I feel like hair would
probably be the easiest to,
- To fake?
- To fake, if you like.
- I think hair at the bottom,
because if it's not got
the follicle intact,
just hair in general.
- Mm.
- Do you think?
- Yeah.
- Yeah.
(dramatic music)
- Hair has always been
an object of interest
for crime scene investigators.
And before DNA, they would look
for the microscopic
similarities in the surface
and the cross section of
hairs to try and get a match.
For example, analyzing
how rough the fish scale
like structure of the
surface of the hair is,
or how the pigment is distributed
through the hair shaft.
- [Announcer] The examiner's trained eye
can learn many things.
Is it human hair? Of what race?
Animal hair? What family?
This vital information may help
to establish guilt or innocence
- Between the 1970s and 1999,
the FBI used this against
defendants in 268 cases.
But when they later reexamined
this using DNA as evidence,
it completely flipped
the field on its head,
and it turned out that even
the most experienced FBI
examiners would routinely
claim they had a match
between hairs that were
from two different people.
And they sometimes couldn't
even tell the difference
between a human hair and a dog hair.
(dog barking)
Out of those 268 cases that
used microscopic hair analysis,
96% were declared false.
33 people were already sentenced to death,
and nine were already executed
by the time the errors were found.
Today, the FBI only uses hair as evidence
if it can be supported by DNA testing,
which is why microscopic hair analysis
goes to the bottom of this list.
What do you think of bite marks?
- On a crime scene?
- On a crime scene?
- Yeah, yeah.
- It's been...
Dracula, is Dracula there?
- Least accurate is gonna be
pretty close to bite marks, I think.
- Midway.
- Midway?
- Last.
That's last.
That's not realistic, innit?
- Okay. Okay.
Mm.
Bite marks at the top.
The idea behind bite marks is simple.
If you can use dental records
to identify a deceased person,
surely you can identify a perpetrator
by the bite marks left on a victim.
Since the 1950s,
bite mark analysis has been
used in thousands of cases.
But then some experts, like
forensic dentist Mary Bush,
started looking into
the method more deeply.
- There was no scientific exploration
into bite marks before they
were used in the court of law
to say that this was even a feasible
technique that you could do.
- One study from the university at Buffalo
used a set of model teeth to create
89 bite marks in cadavers,
alongside controlled
bite marks made in wax.
And out of those 89
bite marks made in skin,
none of them matched the measurements
of the ones made in wax or
the original model teeth.
In fact, when they compared
the bite marks in skin
to a wider collection of 411 model teeth,
it wasn't even the original
model that made the bite mark
that was the closest match.
After 12 studies, their conclusion,
bite mark transfer to
skin is not reliable.
Skin isn't a good medium
to leave an imprint of
someone's bite mark.
It's soft, it's squishy, and
it distorts under pressure.
But despite all this,
there's still been no ban
and bite mark evidence is still
allowed in courts worldwide.
We asked Dr. Bush for a comment
and she said that while
the scientific method
is happy to discard an
old and disproven theory,
the justice system prefers consistency
in a historical precedent.
So even the new flurry
of evidence wasn't enough
to get bite marks out of court.
In fact, bite marks have
been used as evidence
as recently as 2025,
which is why it gets the
second worst spot on our list.
Blood, if you don't have DNA
and you're just using
like kind of the patterns
that it leaves along the ground.
How they splat.
- Right?
- [Gregor] Like the picture,
that blood on a crime
scene could give you.
- Okay.
- I'm looking,
thinking of putting blood at the end,
but like, I don't know.
- Okay.
Put blood at the end.
- Okay.
- I would go blood then
- Second.
- Second place.
- Yeah, yeah, yeah, yeah.
- So bloodstain pattern analysis.
When someone is injured,
the way their blood pools
and drips into the environment
is in a distinct pattern.
And in 2020, "Wired"
actually did an episode
with a crime scene analyst
on how these different
stains can be interpreted.
You can use physics and
biology to rewind the clock
and extrapolate where in space
the blood must have come from.
This all started in 1971 when chemist,
Herbert Leon MacDonell, published
"Flight Characteristics and
Stain Patterns of Human Blood."
In this book, MacDonell
described how you can use
the width and length of a stain
to calculate the angle of impact,
and then using trigonometry,
trace that back to the point
of origin of the spatter.
The book quickly became the foundation
of bloodstain pattern analysis,
and everyone was using it.
In fact, MacDonell even
established a Bloodstain Institute
where thousands of US police
officers would enroll.
The Supreme Court of Iowa
actually referred to the practice
as relatively uncomplicated.
So they didn't require
proof of its reliability.
They were happy to go
ahead with MacDonell's tests
that he did alone in his basement.
And soon enough,
bloodstain pattern analysis
was used in courts all over the states.
But here's the problem.
Plotting the flight
path of each bloodstain
using trigonometry
assumes the trajectories
were straight lines and gives
this common origin point,
which suggests the victim was standing up.
The issue is,
MacDonell and then many forensic
investigators after him
and even "Wired," failed to account
for the effects of gravity and drag
as obvious as this sounds.
When you include these
in your calculations,
the common origin point lowers,
and you can see that in this example.
It's more likely that the victim
was actually sitting down.
The first study to measure
the baseline reliability
of bloodstain pattern analysis
was only done in 2014.
That's over 50 years
after it started being admitted in court.
And a large scale study from
2021 found that analysts
come to different conclusions
about a way a stain was made,
about 8% of the time.
And that's because the same stain
can come from many different mechanisms.
And the fact that blood
differs from person to person,
depending on how many red blood cells
versus the plasma you have.
Men actually tend to have
a 15% higher concentration
of blood cells than women,
which makes their blood more viscous.
Nowadays, investigators have new software
which uses fluid dynamics
to help them map these complex 3D seams.
So bloodstain pattern analysis
is improving in accuracy,
which is why it gets this
middle spot in our list.
Okay.
But surely fingerprints
must have a lot of
value in identification.
How unique do you think fingerprints are?
- I think they're pretty accurate.
- They're fingerprints.
- Fingerprints.
- That's probably one.
- That's number one.
- Yeah.
- Okay. Okay.
I mean, yeah, you use
fingerprints on your phone,
you unlock it, you pay stuff, right?
- 'Cause fingerprints are unique.
Even when you have a passport,
like they scan you when
you enter a country.
- On March 11th, 2004,
a terrorist organization
operating in Madrid
set off explosions on
four commuter trains.
The attack killed 193 people
and injured thousands more.
Quickly after, the police
found a finger mark
on a detonator bag left behind,
and they matched it to Brandon Mayfield,
a lawyer in Oregon.
There's only one problem,
there are no records of
Mayfield even leaving the US.
He doesn't even own a passport.
So how could he have pulled this off?
- I honestly felt like I was being framed
because I hadn't been outta
the country for over 10 years,
- But despite that, the
fingerprint evidence against him
was considered so damning that
Mayfield was incarcerated.
But this wasn't a glitch.
It was built into the
method from the start.
(groovy music)
In 1890s Kolkata,
the city's rich career
criminals were avoiding jail
by paying people to
serve sentences for them.
It was a good tactic since, at the time,
there was no practical
way to identify a person.
So the criminals were
getting away with it.
Three officers, Edward Henry, Azizul Haque
and Hem Chandra Bose
started taking the prints
of everyone who came through the station,
but they quickly ran into a problem.
When they got a suspect in,
how could they crosscheck their prints
against their own database
when there were 10 prints per person
and thousands of people coming through?
They needed a classification system.
Haque proposed that they should look at
whether a fingerprint has a whirl.
It's the spiral pattern
that a person might have on their fingers.
And because each of your 10 digits
can either have a whirl or not,
there are a total of
two to the power of 10
or 1024 ways your whirls can be arranged.
So the officers just built
1024 wooden pigeonholes
to organize these fingerprints.
Okay.
But what if like me,
you don't have a whirl?
Well, two thirds of the
population actually don't.
So the system had to go further.
Once you have a pigeonhole number,
you go for additional
layers of identification.
Some of the additional categories
that you could assign included
whether you had a plain arch,
which was denoted with the letter A,
or whether you had a tented arch,
which was denoted with a T.
Similarly, you could look at which side
of the finger this loop structure is on.
And with a few more
identifiers like these,
we get a much more
in-depth classification,
one that would go on to be
used for the next 100 years.
Famously, the world's
first fingerprint bureau,
which was established in London
in the New Scotland Yard,
used this fingerprinting system.
- I started in 1988
with Metropolitan Police
in forensic services.
- Could you paint a
picture of what the room
would've looked like with
all these pigeonholes?
How big was this space?
- So we're looking at a space of about
30 to 40 meters in length
and about 10 to 20 meters in width.
The collection at the time was about
3 to 4 million I think.
- That is intense.
Yeah, that's wow.
- It's a lot of fingerprints.
- Now, today's system takes into account
these little details along the ridges.
It's called friction ridge analysis.
And well, it's no surprise that
combing through pigeon holes
for these would've taken weeks,
but online databases today can
basically do it in minutes.
(phone ringing)
No way this phone works.
Hello?
- Howdy, Gregor.
Just trying to get a hold of you.
- Why are you calling
me through this thing?
(caller speaks faintly)
You have no data?
Just get Saily.
- You know, I travel a lot for filming.
I actually just got to London.
I always wanna be able to
connect to the internet
as soon as I land,
and with the help of
today's sponsor, Saily,
it couldn't be easier.
First, you just scan the QR
code and download the Saily app,
then you search your
destination, the UK, for example.
And next you just pick a data plan.
And finally, during checkout,
apply the coupon code Veritasium
to get your exclusive 15% discount.
No more dodgy cafe wifi or
unexpected roaming charges.
All you need to do is just
switch on your mobile data when you land,
and that's it.
Saily works with all
iPhone and Android devices.
And once an eSIM is installed,
you can keep adding plans in
the app for each new trip.
And in the rare case that your phone
isn't compatible with Saily,
you can get a full refund
at no questions asked.
I've gone with the Saily Ultra Plan,
which gets you unlimited data globally
with access to airport
lounges, fast track services,
and NordVPN, NordPass,
NordLocker and Incogni on top.
So to stay connected on the
go for an affordable price,
you can scan this QR code, download Saily,
and get 15% off your plan with
the coupon code Veritasium.
So I wanna thank Saily for sponsoring
this part of the video,
and now back to our case.
So if all the details
are painstakingly placed
into these databases,
then where's the problem?
How is Brandon Mayfield falsely accused?
Well, say you have a mark from a scene
and you wanna run it through
the US fingerprint database,
you can't actually search the
database using the whole mark
because you first need to identify
a number of features on the print.
These are called minutiae.
It's where the ridge line split or end.
- To even search the database,
you need a minimum number
of minutiae points.
It's very efficiently sorted,
hundreds of million prints,
but it cannot make an ID.
So what it does, it pulls
out the most similar ones,
at least 10, 20 up to
40 in some jurisdiction.
And then the human
examiner has to compare.
- [Gregor] This works in
theory, but the problem is
how these minutiae are
identified in the first place.
- What is interesting in my research
published scientific reviewed research,
the same thing one examiner
see only three minutiae
and one sees 10 minutiae.
So 10 minutiae is enough
to run the database.
However, if the other are
only the three minutiae,
they couldn't run it.
So whether you find a criminal or not
depends on the luck of the draw.
Even if you give the same pair of prints
to the same examiner
twice, 10% of the time,
they will reach a different decision.
- And these disagreements
are from controlled studies
where the examiners had no
context about the crime,
but in reality, they often do.
As many as 42% of the requests
that fingerprint experts process
state whether a suspect
has a criminal record.
- In a recent case that I was involved
where a firearm was involved
and a forensic firearm expert
had to decide whether a
bullet was actually fired
on the firearm, on the suspect.
On the former submission form
that was given to the forensic scientist,
it said that it's homicide,
it said the age of the
suspect and the victim,
and it even said the race.
It said the suspect is black,
and the person who died
the victim was white.
- For over a century,
experts have held so much
trust in fingerprints
that in the FBI case for Mayfield,
the examiners claimed with 100% certainty
that that was his mark.
This likely happened because
when another fingerprint expert
was brought in to verify the match,
they probably already knew the verdict
of the original analyst,
and they probably also knew the stakes
of what a match could mean for the case,
which made it all the easier to agree
with the initial verdict.
Even the fingerprint expert
working for the defense
agreed that the mark matched
Mayfield's fingerprint
when in reality, it belonged to a man
who had links to terrorist
organizations in Spain.
This is the impact of conformity bias.
- So all of this pressure,
all of this context and intervention
in the forensic science,
I say to the police and the prosecutor,
leave the forensic examiners alone.
Give them independence of mind
to make decision based on the
relevant scientific evidence
and do the work rather than
telling them about the case.
- Well, if there were issues
with all these other techniques,
why don't we just use,
- DNA?
- DNA first?
- Yeah.
- DNA has got to be surely,
- DNA should be top.
- It has to be DNA because,
- Okay, okay.
- You already know everyone
have their own DNA.
- But even DNA analysis can
fall prey to the same problems.
Back in the 80s,
you had to do this with a sample the size
of a dime of blood or saliva.
But today you can do it with
less than a pin hair's worth.
In fact, just this year
a 61-year-old cold case
was solved with just 0.4 nanograms of DNA.
But the paradox is, the
increasing sensitivity
of DNA analysis could
also be its downfall.
In 2012, paramedics in
California responded to a call
about a severely intoxicated homeless man.
They treated him and
took him to the hospital,
but along the way, they
also picked up traces
of his DNA on their equipment and gloves.
Then later that night,
they responded to a murder
of a businessman and they
accidentally transferred
the homeless man's DNA to
the victim's fingernails.
Because of that, the
homeless man, Lucas Anderson,
was charged with murder and
he faced the death penalty
despite being hospitalized at
the time the crime occurred.
He spent five months in jail
before charges were finally dropped.
- So this is trace DNA and touch DNA,
where you can get this kind
of DNA transfer that happens.
- I see how that could be abused easily.
So my DNA could be in a
place I've never been before.
- Yeah.
- And if DNA is deposited
under the right conditions,
say a dark dry place, it can
persist for hundreds of years.
- And if, for example,
somebody touches a door handle,
well, loads of people are gonna
be touching a door handle,
so you're definitely going to get
more than one profile off of there.
- [Gregor] These samples
with multiple DNA profiles
are called DNA mixtures,
and they have been found to be
the most common source of
error in DNA interpretation.
- Some people shed more cells than others,
so the majority of the
DNA that you're getting
off of that door handle may not be
from the last person who touched it.
- The problem in interpreting a mixture
comes down to how a sample gets analyzed.
The most common way to do it
uses short tandem repeats or STRs.
DNA is made up of four nucleotides,
G, A, T and C.
And the SDR method looks
at how chunks of DNA,
usually three to five nucleotides long
repeat in your genome.
For example, G-A-T-A, G-A-T-A and so on.
For one person, they might
have G-A-T-A repeat six times,
whereas another individual
might have nine repetitions.
A standard SDR test will look at around
20 locations on your genome
for where these repeats can occur,
and it will count how many repeats
you have in each location.
- So if you are looking
at 20 of these markers
and you've got one from
mom and one from dad,
you're actually getting 40 different
genetic markers coming back.
So the chances of somebody
having the same combination
of genetic markers as you
is around one in a billion.
- Here's what a crime
scene sample containing
just one contributor
looks like on an STR test.
You can see how it's easy to compare this
to a sample from a suspect.
But if the crime scene sample contains DNA
from multiple individuals,
then their profiles will begin to overlap,
all with varying signal strengths.
And the more individuals in the mixture,
the more difficult these
results are to interpret.
With four or more individuals,
it becomes increasingly hard to compare
a clean DNA profile from
a suspect to the mixture.
And now it's hard to
say whether the suspect
was actually in the sample.
You just can't tell which
peak belongs to who.
It's kind of like having
five people talk to you
at the same time.
You can hear all the noise,
but it's really hard to single one out.
Now, some labs claim they
can reliably separate samples
of up to five individuals.
And to test this,
the National Institute of
Standards and Technology
ran a controlled study in 2013.
They've sent out a DNA mixture
from a fictional crime scene
to labs all across the US.
Their aim to see how different facilities
interpret the same mixture of
four people's DNA profiles.
69% of the labs got the analysis wrong.
And despite the sample
being deliberately complex,
only 21% of the labs
deemed the mix inconclusive
and not possible to give a comparison on.
After NIST published the study,
new checks have been imposed
to address some of the issues,
but there's still no lower limit
on the quality or the
quantity of a DNA sample
that labs are permitted to analyze.
And labs themselves still decide
if something is too mixed or too partial.
Now, you might think that
using the entire sequence
genome would be better,
but that can actually introduce
different kinds of problems.
Now, your analyst has
access to hair color,
eye color, ethnicity,
and that could introduce discrimination
in the way the sample is being analyzed.
- This is where the guidelines
and the guardrails have to come in here.
What are the genetic markers
that are being used for this?
This is where the really interesting
ethical questions come into it.
People think that DNA is
like the silver bullet
that will answer everything.
And it is true.
DNA evidence is incredibly powerful,
it's amazing for identifying individuals,
but DNA can never be taken out of context.
- Now, the point of this
video isn't to bash forensics.
I think it's still better
for us to live in a world
where forensics exist rather
than one where they don't.
But if we want to keep
calling forensics a science,
we need to continue the work of the people
who actually made this video possible,
who've dedicated their lives
to already reassessing the field
and making it as accurate as it is today.
Do you think it's fair calling
this a forensic science?
- I still think there's science in it
because you're trying to do the best
with the information you have,
which is often times what
science is trying to do is,
with the information you have,
this is the conclusions you make
until you know something better.
(gentle music)
- Hey, one last thing.
Last year we launched the
official Veritasium game,
Elements of Truth.
It's a trivia game with 800 questions
covering science and technology,
and we on the team get quite competitive.
So every time we play
things get very saucy.
See, there's this mechanic
where you can't only guess an answer,
you have to put down a
number between one and 10
to gauge how confident you
are that your answer is right.
So it gets very fun very quickly.
The game is coming out later this year,
so if you're interested in pre-ordering,
you can click the link in the description,
which will take you to the
Elements of Truth website.
Thank you so much for your
support in this project.
And as always, thanks for watching.