How reliable is fingerprint evidence? - Theodore E. Yeshion

On October 17th, 1902, Paris detectives arrived at a grisly scene.

Joseph Reibel had been murdered at his workplace.

With no eyewitnesses, the officers had little to work from,

until they discovered a shard of broken glass

with several bloody fingerprints.

An investigator manually searched the police station's fingerprint records

and eventually found a match.

Henri Scheffer, who had been arrested for theft the year before,

was detained and later confessed to the murder.

This marked the first time in Europe

that investigators solved a crime using fingerprints alone.

Over a century later,

fingerprints remain one of the most common types of evidence

in criminal courts.

But just how reliable are they?

Humans, like other climbing species,

are born with patterns of raised ridges and recessed furrows

not only on their fingers but also along their hands and feet.

These friction ridges help provide a firmer grip,

especially on wet surfaces, and increase our sensitivity to touch.

They begin to develop in the womb at about seven weeks,

when the skin on the hands and feet swell to form smooth raised pads.

At around ten weeks, deeper layers begin to grow faster than the upper layers,

causing the pads to buckle and fold.

This folding is guided by genetics and environmental factors

and continues over the next several months.

Scientists have yet to fully understand the exact environmental factors at play,

but it’s their randomness that leads to the formation

of unique fingerprint patterns.

In fact, many experts believe it’s improbable that any two fingerprints,

past or present, are exactly alike.

They're even different among identical twins.

People have likely known about the individual nature of fingerprints

for centuries,

but it wasn’t until the late 1800s that scientists began studying and classifying

their different features.

By the early 1900s, prosecutors began utilizing fingerprints in courts,

forever changing how detectives approach and analyze crime scenes.

Today, investigators often begin by searching for visible prints.

These include patent prints,

left by transferable substances like blood or dirt,

and plastic prints, which are impressions in soft, malleable materials,

like caulking, wax, or even a newly painted fence.

However, most fingerprints aren't visible to the naked eye.

These are called latent prints,

and they’re composed of the water, oils, proteins, and salts

that ridge patterns leave on surfaces.

They can be revealed by dusting with a fine powder,

which sticks to the water and oil trail.

If surfaces are porous or difficult to dust,

investigators use chemical developers like Ninhydrin,

which reacts with proteins left by fingers.

Forensic scientists continue to develop more sensitive and surface-specific tools.

For example, one experimental technique uses an electrical charge

to capture the corrosion that fingerprint salts leave on metals,

even if they have been wiped down.

Once investigators have collected prints,

then they get to work matching them with potential suspects.

In certain cases, investigators will use automated computing systems

that can narrow down potential matches

found in national fingerprint databases.

Certified experts then compare minor fingerprint details,

such as how the ridges branch and intersect,

and the spacing of oil pores.

Any result must be blindly verified by a second expert

before information is handed over to justice departments.

The fact that fingerprint patterns are unique for every individual

makes them particularly strong evidence in criminal cases.

However, the system isn't infallible.

Prints are often smudged, distorted, or overlapped with other prints,

which can make matching more challenging.

And it’s prone to human error—

one 2011 study found that fingerprint experts falsely identified

two different fingerprints as a match 0.1% of the time.

While this may seem low, the stakes are high for a wrongly accused defendant.

Further, there’s no universal standard on the number of ridge details

that experts must analyze to make a match.

Many investigators stress that no conviction should be made

on fingerprint evidence alone.

For these reasons, forensic experts continue to work to improve

and standardize the fingerprinting process.

After all, when it comes to investigating crime,

fingerprints have certainly left their mark.