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HomeCRIMEAre footprints, tyres or tool marks reliable evidence?

Are footprints, tyres or tool marks reliable evidence?

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Forensics: exact science or dramatised fiction? #13

footprints

Trying to nab a criminal by footprints, tyre-marks or teeth-marks (collectively known as impression evidence) is such an outrageous hoax that the law enforcement and the forensic community must be ashamed of themselves for resorting to such things.

Relic of a bygone era

The business of footprints is nothing but a relic of a bygone era when thieves used to be so poor that they could not afford shoes. Most of the time, they operated in rural areas where they scaled walls, etc. In this process, particularly on unpaved ground surfaces found in rural areas, they could leave muddy footprints on hard ground or foot impressions on soft ground. In that era, forensics very pompously used to make Plaster of Paris cast of those impressions. Thirty-five years ago, when I was in the SVP National Police Academy, Hyderabad, the practices of this farce of lifting the cast used to be a part of the curriculum.

Granted, all of us have different body structures including our feet, but it stretches credulity to even think for a moment that a person could be positively identified by his footprint. All those footprints can tell is that someone was there.  

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There is simply no science behind it. No analysis has ever been done that such and such are the parameters which characterize the imprint of a foot and that they are adequate for ‘individualizing’ a person. As we stated in an earlier article, ‘individualization of evidence’ to the exclusion of all other possible sources, necessarily requires measurements of object attributes, data collected on the population frequency of variation in these attributes, testing of attribute independence, and calculations of the probability that different objects share a common set of observable attributes. No such study has ever been done nor is ever likely to throw up any meaningful result even if it is undertaken.

footprints 2

We are aware that everybody walks a bit differently. Some people dig their heels as they walk; some people do not. Some people walk fast; some people take deliberate steps. Some people lay more stress on one foot; some people on the other. 

When shoes are worn, the obvious distinctions of barefoot of the type mentioned above disappear. However, if you observe closely, you may get some idea of the shoe type, shape, brand, and size, and the person’s general height, weight, ambulatory difficulties, loads being carried, and whether the footwear is new, etc. You can also look for individual wear characteristics such as distinct wear, etc. For some people, the outer side of the shoes wear more than the inner side; for others, it may be different.

However, the point to be noted is that none of these has any ‘uniqueness’ to them and they cannot identify a person. There may be millions of people with similar characteristics.

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Most importantly, it should be kept in mind that wear characteristics appear only in old shoes. Is there a law, which says that a criminal cannot wear a new shoe for each crime? Shoes used to be a little different from each other when in the 19th century, they were handmade. If the criminal was so poor as to continue wearing the same shoe for years, perhaps a pattern could be noted. Now, with modern shoes being made on machines by the millions, there is practically no difference amongst all of them.

Also Read: Fingerprint analysis- science or ‘medical palmistry’?

In People v. Campbell, the Illinois Supreme Court took note of the fact that most shoes today are designed by computers and mass-produced on machines.

Thus, for such productions, there is practically no difference between two sets of new shoes and identical shoes could very well have been sold to many people.

tyre marks

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Tyre impressions

Tyre impressions are made on soft ground. Since tyres of different companies have different treads, the tyre marks are different. Tyre impressions are also made when on a hard road surface, the vehicle brakes hard. But the problem is that tyres of a given type are sold by the millions. Any one of those could have formed those impressions.

In principle, tyre impressions analyses are very similar to footprints or shoe impressions analyses and suffer from the same inherent weaknesses. It is not possible to positively exclude all other tyres as there may be a large number of such tyres on the road. New tyres would lack any individual characteristics such as nicks, cuts, or gouges. For old tyres, such individual characteristics might provide points for comparison but cannot make for positive identification.

This line of logic was accepted by the Illinois Supreme Court in People v. Sutherland.

Teeth marks of forensic odontology

Tooth bite marks are often found in sexual offences on the body of the victim. They can also be found on the body of the accused. Once again, the forensic community, in their misplaced enthusiasm of discovering new techniques, hoped that since everybody has a different design of his dentures, the teeth marks would also be different. But, there is no scientific method of processing those differences in a manner that would yield a positive identification. Moreover, as C. M. Bowers has shown, in numerous instances, experts diverge widely in their evaluations of the same bite mark evidence.

There are such obvious loopholes in this business that it becomes outrageous. For example, bite marks on a victim could be consistent with a chipped tooth or missing tooth. The defendant can, however, argue that he got the chip only recently, that is, after the alleged incident. What is there to prove that he had the chipping earlier also. Or, if it comes to that, he can very well get that tooth extracted.

Lip and ear print impressions

Lip and ear prints are on a much weaker foundation. Both are impressions of a three-dimensional object which are malleable. Lips in fact could be manipulated in complex ways when they make a print. It is extremely difficult for the suspect to be ‘made’ to replicate exactly that sort of manipulation for comparison. There are no friction ridges as in fingerprints. Different pressures may cause different results with the same lip/ear or different lips/ears to have similar ear prints. Furthermore, there are no studies concerning the comparisons of lip prints to establish their reliability and validity as a means of identification. Finally, there is a significant difference between comparing actual lips/ears and photographs of lips/ears and comparing lip/ear prints to each.

All against the scientific opinion

For footprints, tyre and teeth mark all of them, that is, impression evidence in general, the National Academy of Sciences Committee concludes that there is no scientific basis for identifying an individual to the exclusion of all others by these marks. Thus, no positive identification is possible and no large population studies have been conducted.

They also pointed out that there is no consensus regarding the number of individual characteristics needed to make a positive identification. Moreover, there are no data about the variability of class or individual characteristics or about the validity or reliability of the method. Without such population studies, it is impossible to assess the number of characteristics that must match in order to have any particular degree of confidence about the source of the impression.

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Dr N C Asthana IPS (Retd)
Dr N C Asthana IPS (Retd)
Dr. N. C. Asthana, IPS (Retd) is a former DGP of Kerala and ADG BSF/CRPF. Of the 51 books that he has authored, 20 are on terrorism, counter-terrorism, defense, strategic studies, military science, and internal security, etc. They have been reviewed at very high levels in the world and are regularly cited for authority in the research works at some of the most prestigious professional institutions of the world such as the US Army Command & General Staff College and Frunze Military Academy, Russia. The views expressed are his own.

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