For more than 100 years, law enforcement around the world has been leaning on fingerprint evidence to aid in criminal identification: Everyone has distinct papillary ridges on the pads of their fingers, a key fact for police as they try to link suspects to crime scenes.
Footprint evidence has—by comparison—been left in the dust, a marginalized subset of forensic science relegated to the sidelines of crime solving. But that may be changing: In 2014, forensic podiatrist Dr. Michael Nirenberg’s expert testimony cleared an important legal hurdle (called the Daubert standard) for a homicide case in a Wisconsin court. It was the first such footprint case to pass that test, which helps determine broader legal acceptance of an expert’s scientific testimony nationwide.
A&E Real Crime spoke with Nirenberg to learn about the differences between footprints and fingerprints and why foot evidence can be so valuable to law enforcement.
The idea that police can even find footprints in optimal conditions (e.g. not left in mud) seems hard to fathom. How do investigators find footprints in the first place?
The simplest thing is to just use a light at an oblique angle. [Investigators will] shut the lights off in the room and put a flashlight on the ground at an oblique angle.
There’s also a device [we use]: If there’s a lot of dust in the room, you put down photographic paper and use an electrostatic detector. It shoots electricity through the photographic paper, and the image of a footprint in the dust will be transferred to the photographic paper.
What about when there are footprints through snow? How do you get a record of the footprints before they melt?
You’ll use a chemical to spray the snow to make it more firm. You can use hair spray. That helps firm up the snow, and then you can gently make a cast from the snow.
How distinct is a bare footprint? Is it as easy to match to an individual as a fingerprint is?
A footprint is not unique. Unique would mean there’s no other footprint like it amongst 6 billion people. Footprints have distinctive features… They approach being unique.
There have been various studies, and the largest study was done by the RCMP (Royal Canadian Mounted Police), which looked at 24,000 feet. The RCMP found the chance of a match of a footprint is statistically 1 in 1.2 billion.
What makes them so distinct? Is it like fingerprints, where skin patterns dictate the differences?
We do not generally look at the papillary-ridge patterns the way fingerprint people do. We’re looking at the morphology of the footprint: the shape, the pressure points, imperfections from the skin, lesions on the bottom of the foot. If someone has a callous, that’ll make their foot more distinctive.
There’s 26 bones in the foot, and they shift and change as we age. Also, the motion of the foot is determined by the leg and the body. If someone has a problem with their hip, it’ll change how they walk which will, in turn, change the footprint that’s made. A heavier person will leave a different kind of footprint than their lighter twin. If someone has a laceration because they stepped on a nail 10 years prior, that scar would be more distinctive on their foot.
It sounds like your footprint evolves a lot more over time than your fingerprints do.
Exactly. Over time, our feet change. That works for and against forensic podiatry, because if you get a suspect from a footprint and examine their foot 10 years later, the strength or weight of the evidence may be reduced, because their foot in that 10-year period of time may have changed.
Feet are more in contact with the environment than most parts of our bodies.
You worked on the murder trial of Shymeek Stanfield in Virginia, where he was convicted, in part, because you were able to prove that a shoe left at the crime scene belonged to him. How did you know the shoe was his?
In the case of footwear, where [investigators] send me a shoe, I do what I’ve coined a ‘shoe autopsy,’ where I systematically take apart the shoe and look for impressions on the outside and inside, using a fiber optic camera, and recording any impressions.
In that case, I compared the foot impression on the insole of the crime-scene shoe with the foot impression on the shoe he was wearing when he was arrested. They were both Nike basketball shoes. He was wearing a newer version when he was arrested, so we had an insole impression on the same style of shoe.
When you’re comparing an insole impression, you’re not comparing a snapshot in time. You’re comparing the interaction of the foot with the shoe over time.
It’s hard to believe the wear and tear inside a shoe could really be distinctive enough to serve as strong evidence in a trial.
The strength of the evidence from an insole may be reduced, depending on the circumstances of the case. But in Stanfield’s case, his foot had numerous deformities. He had a bunion, he had hammer toes, he had a flat foot. His foot structure was more distinctive to begin with, and that worked against him.
You’ve also worked on cases where the police inked the bottom of a suspect’s feet, and then had them walk around to compare those footprints to ones taken from a crime scene. But if so much of your footprint is based on the way you walk, couldn’t a suspect change the way they walked for the police to avoid being detected?
They can, and that is a limitation. So you want to try to distract them, talking to them as they walk. And if you have a footprint that’s significantly different from the others, you want to note that.
But at the end of day, that makes the strength of your evidence stronger… because it can’t be falsified the other way. It can only be falsified to the advantage of the suspect. I don’t want innocent people convicted. I like that it goes in that direction. You always want to err on the side of caution.