Instant Genius - How forensic scientists gather vital evidence to solve crimes

Episode Date: August 30, 2026

Whenever a crime is committed, tiny traces of evidence are left behind at the scene, which, if pieced together correctly, can tell us exactly what happened, when it happened and who was responsible. This is the job of forensic scientists. But how exactly does this process work, how accurate are the portrayals of crime scene investigators we see on film and TV, and what happens if when mistakes are made? In this episode we’re joined by science journalist and author Sanjukta Mondal, to talk about her latest book, Forensic Science. She talks through the fascinating history of forensic science, explains how we all leave traces of our presence wherever we go, and tells us about some of the common myths and cliches we often see in TV crime shows. Take your curiosity further with a subscription to BBC Science Focus magazine. Every issue is packed with fascinating insights into the science behind everyday life, the latest breakthroughs and expert analysis, delivered straight to your door. Receive an extra £5 when you subscribe using the code SF5OFF, (minimum spend £20, see full terms and conditions on our website). https://www.ourmediashop.com/bbc-science-focus-magazine-pod30 Learn more about your ad choices. Visit podcastchoices.com/adchoices

Two and five Canadians will hear the words, you have cancer. That's why every step and dollar raised matters. On September 19th, join thousands in Toronto for the Princess Margaret Cancer Foundation Walk. Challenge yourself, friends, and family to walk 21 kilometers in support of life-saving research. Together, we can carry the fire and help create a world free from the fear of cancer. Register today at pmcfwalk.ca. Hello and welcome to Instant Genius, a bite-sized master class in podcast form. Every Monday and Friday, you're here of world-leading scientists and experts

talking about the most fascinating ideas in science and technology today. I'm Jason Goodyear, commissioning editor at BBC Science Focus. Whenever a crime is committed, tiny traces of evidence are left behind at the scene, which, if pieced together correctly, can tell us exactly what happened, when it happened, and who was responsible. This is the job of forensic scientists. But how exactly does this process work? How accurate are the portrayals of crime scene investigators we see on film and TV

and what happens if mistakes are made? In this episode, we're joined by science journalist and author Sanjokta Mondal to talk about her latest book, Forensic Science. She talks us through the fascinating history of forensic science, explains how we all leave traces of our presence wherever we go, and tells us about some of the common myths and cliches we often see in TV crime shows. So welcome to the podcast. Thanks so much for joining us. Hello, thank you for having me here.

So today we're talking about your new book, Forensic Science. So everyone will likely have heard this term. But first off, you know, what exactly do we mean by forensic science? When we use the word forensics, it has a very interesting history. stray like the etymology of it. But when it comes to a definition that we commonly use now is use of scientific method as an evidence used during a criminal trial to solve investigation and to solve crimes. So yeah, that's what forensic science means. So we use forensics and forensic sciences interchangeably. Yeah. So how and sort of when did forensic science as we

know it now get started? Because it has quite an interesting history. Criminal investigation until the 17th or 18th century was mostly based on eyewitnesses and that usually created a lot of problem because we do know that as society we do have a lot of biases, whether it's a class-based bias or whether it's a socioeconomic bias. So when there were cases in court during 17 or 18th century, it's mostly he said she said. So we didn't have a solid proof that would allow you to present evidence that was not a human speaking or that was not unless you have a weapon like present presenting weapons was always a thing like if that is a weapon and somebody can prove it's related to you that is like we don't need to look any further but if there was no one present there was no eyewitness present

and even when eyewitnesses were present they could be bought out they could be biased about you so there was no framework that made sure that whenever there is a criminal investigation happening that you need to present so and so proof so um not in the modern sense of forensics, but the Chinese had their own way. Like the Chinese had their own way of finding out who the criminal was, even when there were no witnesses. Even India had that thing where they would ask, like imagine their theft has happened. So the village, wise men would gather all the people who could have been possible suspects. And then they would tell them that, hey, why don't each one of you just hold on to like a handful of rice in your

mouth. So what they knew was when somebody is nervous. Like whenever we are nervous about something, we are parched, right? Like we don't produce enough saliva in our mouth. So they would ask everyone to hold some rice in their mouth. And then after a while, they would just ask them to spit it out. So if your rice came out wet, that means you're not worried about something. You created enough saliva for it to be wet. But it came out dry, which meant you were lying about something. So that was how they used the deductions to find out a possible. murderer or a possible killer. But right now we have even more efficient methods that can tell us exactly who has done it, like with molecular precision. Yeah, so let's have a look at some of those.

So I think that probably the one that people's minds will turn to first is fingerprinting. So you're talking the book about Sir Francis Galton and his work, early work on fingerprinting. So can you tell us a little bit about that, please? Yes. Sir Francis Galton, was a eugenicist. So he thought that people could be like, like we could improve the human race by eliminating certain group of people or whatever. So when he started collecting the fingerprints and thumbprints, one of his first things was maybe he could distinguish people based on their fingerprint,

like what race they were, whether they were more intelligent. But while doing that, he discovered he saw that fingerprints are quite unique. Like even though he couldn't tell what the race of that person was, or whether that person was smart or where they were from, he saw that every fingerprint was unique. At the same time, India was a British colony. And Sir Edward Henry, who was the commissioner of the police of metropolis in London, he was also coming up with the Henry classification system,

which was also supported by the Calcutta, like the Calcutta, which we now call Kolkata, Calcutta Anthropometric Bureau, who were also collecting fingerprints from a lot of people over here and trying to see if they could find patterns in those fingerprints. So Francis Galton was like when he saw the Henry classification system, he was like, okay, this is quite interesting. Maybe I can dig deeper into this. So it was almost like different people came up with a similar idea

that what if we used fingerprints as a form of distinction? Because fingerprints have also been used by the British offices in India as a binding contract. So whether they were giving tenders for building, for construction of railways or whether it was labourers who were working on a certain project. So even now, people who are not literate use thumbprint as their unique identifier. Now, why are fingerprints unique? Because fingerprints are one of the first unique features that we form because it's formed during the first few weeks of gestation, like around 10 to 12 weeks.

And it is unique to us because it's the environment inside the placenta and all. Also, there are genetic proteins that decide how the skin on our fingers are going to fold. So there are different worlds and worlds that make all of our fingerprints very unique. Like, even though recent AI advances have seen, even though fingerprints of different people are very different, like our fingerprints, like, our fingerprints, like, if we look at the fingerprints of all our fingers and thumbs, they do share certain patterns. Okay. like scientists believe that if we are able to develop on this, like imagine there is a crime scene where you have found the, where the CSI have found the right thumbprint of a person, like maybe one thumbprint and half a finger print. But what they have is the left impression of that person, like in the fullest records. So now using AI and using AI, they might be able to tally if the left fingerprint and the right fingerprint are similar because of the shared features that we have.

have. So you mentioned there like CSI. So we've all seen these television shows. So often in these shows, there'll be a crime scene. And sometimes the first thing they'll say is sweep the room for prints. You know, check the room for prints. So what do they mean by that? How's that done? This takes us back to Edward Locard's principle, which is every contact leaves a trace. Like we might not be able to see it but at this moment if I touch my bottle I'm leaving behind my fingerprints because our skin has oil glands that leaves behind prints on everything that we touch so every contact leaves a trace or maybe if you're touching something that has dust on it you might take away that dust from that surface so it's a given take thing so when they say sweep the room they mean

to say all the different places where the criminal must have left behind their print or they could have left behind different fluids in their body because we sweat, right? Like we sweat, we might not be able to see where we are sweating. And if there is an altercation, you might even shout. And when we shout, we know that we expel saliva. Coming back to the fingerprint thing, it's much easier for the experts to spot the fingerprint on smooth surfaces. Because it's way more prominent. Like, we must have also seen that if we have greasy hand and if we touch our phone,

it's way more visible than if we touch light. a handkerchief or a cloth because those are porous things. So smooth surfaces that are not porous are the first thing that they would go for because that's where you get to get very clear fingerprints. And then there are places where you leave which are malleable, for instance, a bar of soap or maybe a wall that is freshly painted or molten wax. So there are different places that we might leave our fingerprint and they have different techniques for it. Many of us must have seen the CSI is using a powder with a makeup brush and dusting it on different places. So that creates a contrast that helps them see the fingerprint and sometimes also lift the

fingerprints. So they have a special kind of tape, like a cello tape that they use to lift off the prints. And if it's an impression, for instance, if it's in a bar of soap or a freshly painted wall, they might use moulds to stick it into that place so that they can get that impression and tally it with the records that they have. But another myth that we usually see is they always get a hit. That, oh, this is that person, we found their fingerprints and this person is so and so, and he or she is a criminal.

He or the who are the criminal. But the point is, unless a person is completely living in isolation, there is a very high chance that anyone visiting them, anyone who has been there for the past week, who has been there for the past month, must have left back their fingerprint because we are always touching multiple things. And also, not everyone's fingerprints are in the record.

So what are they comparing it against? So that's sort of one of the persistent myths that these television shows keep perpetuating? That we have an instant match. That's not always possible. Like, yes, if there is a suspect who has, a criminal past who has a record, it's much easier for them to find a match or else they would have to call in all the people they suspect and get their fingerprints and then tally it with

the other person. And it's not just one machine doing it. Like in science, we believe in having redundancy. So there is one expert who will be doing it and then another expert who does the same thing. Two and five Canadians will hear the words you have cancer. That's why everyone. Every step and dollar raised matters. On September 19th, join thousands in Toronto for the Princess Margaret Cancer Foundation Walk. Challenge yourself, friends, and family

to walk 21 kilometers in support of life-saving research. Together, we can carry the fire and help create a world free from the fear of cancer. Register today at pmcf walk.ca.ca. Another sort of trace that we can leave is things like saliva. And so it's this sort of like DNA analysis. So this is another thing, isn't it? People, crime scene investigators look for traces of DNA and which we're just, as you say, like our fingerprints and we're touching things. We're just leaving DNA sort of everywhere, aren't we?

So what are some of the common sources of DNA that forensic science takes advantage of? Dead skin cells? Because we are shedding, I'm not sure how it is in your case, in India, it's quite a dusty country, right? But they say that the majority of the dust that we collect every morning when we are cleaning, like a significant portion of it is also dead cells. So that's there. Then we have hair. We do shed hair and we do not see where we are leaving that.

Then if it's a crime scene where there has been altercation between two people, there could be bloodshed in both the ways. Or maybe you must have, they served you something, you drank water from a glass, so you leave back your saliva. So, like, there are many sources of DNA that we can leave behind. Having said that, like you said with the fingerprints, this must introduce like a huge potential for making mistakes because everybody that's been in a certain area will have left some of their DNA there, right?

So, yeah, that's like one of the things that the forensic experts assert on again and again is that we need to interpret the results based on the context. So without context, the forensic clue does not make any sense. Because as I said, there is a very high chance that number of people have visited that particular place. Like one example that I mentioned in the book is that of Amanda Knox's case, like where her roommate is murdered. And when all the forensic evidence, they show that, oh, there's blood and there is their footprint and they have left their DNA on a knife, they have left their DNA in the kitchen.

but they do share a room right we need to take that thing into context like yes they are going to leave their DNA because they do share the room with that person if they have left the fingerprint on a knife because they because it's a shared kitchen they do share the same knife so without knowing what the context of that entire case is it's very difficult for the investigators to put the things together but also the interpretation as in the the results are black and white right like whether it matches with a certain person or it doesn't match with a certain person or it doesn't match with a certain So when it's a DNA expert who is matching all of these things, their job ends at finding the results, whether it's a match or whether it's not a match. Now, it's up to the investigators and other expert who make sense of that result. Because if the DNA analyst or if the fingerprint analyst is given the context of the entire case, it can create a form of cognitive bias that can also derail the entire case. like there was a case in US where a forensic analyst thought she was being a savior for criminals. Their lab received a lot of materials from drug trials. Like whether they had to test whether they are drugs or whether they are just a random white powder. So what she did was she used to follow through all the cases and listen to, okay,

so they want to catch criminals, I'm going to clean the street. So that's how she manipulated a lot of the results. And because of that, when she got caught, a lot of, they had to go back and check all the verdicts that were given. Using forensic clues without, like interpreting forensic clues without the context can lead to a lot of blunders. Yeah, absolutely. So you mentioned bloods there. So another really interesting one, like especially for people who've seen the television show, Dexter, is a blood splatter analysis. So, you know, how did this get going? How was it initially done? And, you know, how has that developed? And, you know, what are we looking for?

It comes down to the physics of blood. So unlike water, it's a non-neutonian fluid. So if you mix cornstarch and water together, like most of us have done this experiment, we have mixed corn starch and water together and tried to punch it and then hits you back really hard because it turns quite solid. So that's one example of a non-neutonian fluid. On the other spectrum, we have blood. So as soon as there is an impact, since blood is a viscous liquidate, on impact it becomes quite thinner. And whenever there is an injury based on trauma or impact, it creates a splatter pattern. So like if it is done with a hammer, it is going to create a specific pattern where like, or when it's done with a knife, it is going to leave a trail of blood splatters. Or if it's a bullet wound, it can tell the forensic experts where exactly where the person might be standing. or what was the range of firing. That is, Fon think experts can find that on it.

This all happened because this is a pattern analysis, right? So they have had to conduct a lot of experiments to see that what kind of impact creates what kind of pattern. So this had quite a gory start where the person who thought they should check how blood's pattern analysis works, tried doing that by badgering the head of a rabbit with. different blunt and sharp instruments to see how the blood splatters. Thankfully, we do in a much more humane way right now. They use a fake blood that are collected from different banks and use it to see.

They also have, sometimes they also have to simulate. Like, imagine that they have a hypothesis. Like the victim was killed using a baseball bat and the perpetrator was standing behind them and they hit it with so and so force and that's how we see the blood splatter. what they do is they also try to simulate that in a lab and see if it matches something, if it looks closer. And since there has been a lot of simulations over the time, there have

been a lot of experiments over the time. So now scientists also have data to look back upon and tally that if things match. So another sort of less gruesome area that you discuss in the book that I thought was really interesting is art forgeries. So kind of traditionally you'd have kind of experts or art historians, analyzing things like the composition of a painting, you know, the colours used, the brush techniques, etc. to determine if it was genuine. But now we can employ some of these more scientific techniques to help us identify forgeries. So, you know, what can you tell us about that? So not detecting art forgery was, is still subjective to a certain level.

Like the first line of difference are art historians because they can look at a painting and tell if the brush strokes match or if the color vibrance is the same as what it was and so and so. So the first sign of defense is definitely art historians and art experts. But if they are puzzled and then certain buyers who have spent millions on an auction would also want to double check and know that whether the thing that they bought is objectively real or not. So then they send it to forensic labs. So what forensic experts can do is they can remove like a minuscule amount of paint from that particular painting and check if the composition, if the paint is relatively new.

Because during the Renaissance time, there were no petroleum derived products. So if your paint has something that is petroleum derived, if your paint has something which is a relatively new compound that has been invented in the past 50 to 100 years, then this is definitely a fake. Then we also have carbon dating. So the painting is done on a canvas, which comes from plant material. and all living creatures exchange carbon-14 isotope with the atmosphere with every breath. The CO2 that we get, some of it has the normal carbon C-12, some of it has the isotope C-14.

So as long as we are alive, there is a fixed ratio of carbon exchange between living creatures and the environment and the atmosphere. But as soon as the thing or a human is dead, that stops, right? and the degree at which this drops is constant. Like there is a fixed rate to which this happens. So what experts can do is they can take a piece of the canvas and then date the amount of C-14 that is present and then using comparison they can tell, okay, this is 50 years old or this is 200 years old.

And also they use x-rays. So many forgers have found ways to do the experts. So what they do is they buy old canvases and paint on them. that there are existing old paintings that you can find in thrift shops or you can find in antique stores. So they buy them and then they paint on it. So now experts also have X-T machines that allow them to look through the painting and see if there is any painting underneath or if there are any underdrawings. Another thing is when forgers do it, they know exactly what they want to draw. so they're under sketches

like many painters start with under sketches so if like a fojour knows exactly what to draw but a creative person who was creating that painting for the first time might have certain corrections might have scribbles here and there so it's a combination of

your understanding of how art works and also tools such as mass spectroscopy or Raman spectroscopy that give us exact fingerprints of the element chemical fingerprint of the paint that is used or the varnish that is used. Thank you for listening to this episode of Instant Genius,

brought you from the team behind BBC Science Focus. That was Sanjokta Mondal. To discover more about the topics we've just discussed, check out her book, Forensic Science. If you liked why you just heard, then please do consider subscribing to Instant Genius on your preferred podcast platform. If you'd like to see our guests and hosts in person,

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Two and five Canadians will hear the words you have cancer. That's why every step and dollar raised matters. On September 19th, join thousands in Toronto for the Princess Margaret Cancer Foundation Walk. Challenge yourself, friends, and family to walk 21 kilometers in support of life-saving research. Together, we can carry the fire and help create a world free from the fear of cancer. Register today at

PMCF walk.ca.