How bite marks can lead to convictions.
by Liz Porter
Criminals are regularly nabbed because they make stupid and careless mistakes. The burglar who robbed a safe at the up-market Haigh’s chocolate factory in the South Australian capital city of Adelaide is a perfect example. He was caught by his own bad manners. On his way to burgle the establishment’s safe, he toured the factory’s display area, helping himself to samples and biting into several chocolate bars. He then threw the uneaten leftovers on the floor – from where police collected them. If he had taken his uneaten chocolate home – or neatly disposed of it in a rubbish bin – he might never have been caught.
The robbery took place on a weeknight in early February 1996. Office staff arriving for work at the inner suburban factory the next morning discovered that an intruder had broken in through a skylight. He had used an axe and jemmy to bash the alarm system off the wall and had then overturned the office safe, making a hole in its back and removing the $1,800 inside. The workers also spotted the partially eaten chocolate bars the thief had dropped on the floor, and noticed that he had also stolen chocolate from a display area nearby.
Police recovered the uneaten chocolate, noting that clear tooth marks were visible in a 40 by 55-millimetre piece of chocolate frog, a 27 by 29-millimetre portion of chocolate honey nougat, and a 40 by 77-milllimetre bar of plain chocolate.
The chocolate evidence was stored carefully, while police closed in on a man they believed to have been responsible for a recent series of similar night-time break-ins. A gourmet deli had been robbed on January 14, and bus tickets, blank checks, phone cards and $400 worth of cakes were stolen. On January 22, a travel agency had been robbed of foreign currency. Then, a week later, and only three days before the Haigh’s heist, a robber had climbed through a window of another travel agency and stolen $4,850 in cash. Possibly emboldened by his success in previous break-ins, he left a handwritten note for police or, as he called them, the “boys” in the “Beuro”(bureau) telling them they had “better be considering a new line of work,” and signing it “Traffick.”
The police suspected that the person responsible for all these burglaries was a 24-year-old man from the Adelaide suburb of Northfield. When they raided his home, they seized property allegedly taken in the previous cake shop and travel agency robberies. They also took a Valentine’s Day card with the man’s handwriting on it. But their suspect continued to deny involvement in any of the robberies.
Police were able to use their powers under Section 81 of South Australia’s Summary Offences Act to compel the man to have impressions of his teeth taken with special dental gel. These impressions, along with the chocolate samples, were sent to Adelaide University’s Forensic Odontology Unit, where Dr. Kenneth Brown made casts from the impressions of the suspect’s teeth and used the same gel to make casts of the tooth marks in the chocolate surfaces. The casts were then examined and compared and photographed microscopically.
DNA analysis wasn’t considered as an investigative tool in this case. Although DNA profiling was already being used at Adelaide crime scenes by 1996, the technology available at that time wasn’t up to detecting traces on chocolate, and in separating saliva from chocolate. By 1998, new technology was introduced that revolutionized the sensitivity and reliability and speed of DNA analysis. Even if this case were to happen now, the bite-mark examination would still include forensic odontology, which would be used, with DNA, to add weight to the evidence presented in court.
The suspect’s teeth, Brown noted, had certain distinctive features. He had a 1.5-millimetre gap between his upper left central incisor and his lateral incisor. (The central incisors are the two front teeth, top and bottom, while the lateral incisors are the teeth either side of them.) The upper left lateral incisor was rotated at an angle of about 20 degrees off center. There was a recognizable notch on the biting edge of the upper right lateral incisor and noticeable facets of wear on both the upper right central incisor and the lower right lateral incisor.
Types of Bite Marks
Brown’s next task was to examine the kind of bite marks that had been left in the chocolate samples. Broadly speaking, bite marks in foodstuffs vary according to the kind of food being bitten, and have been classified as Types 1, 2 and 3. Type 1, typically seen in chocolate, fractures the foodstuff, but allows only limited tooth penetration, so that the bite marks will only record the most prominent edges of the incisors and some of the characteristics of the outer edges of the upper and lower front teeth. Type 2, typically seen in apples, requires extensive pressure by the teeth, and sufficient penetration to create a fracture of the surface. Type 3, usually seen in cheese, fractures the food surface, but allows complete or near complete penetration by the teeth.
Fortunately for the investigators, the honey nougat bar, with its layer of softer nougat below the chocolate surface, displayed a Type 3 bite mark, retaining marks made by the scraping of the teeth and recording a more detailed impression of the suspect’s upper and lower front teeth. In particular, the notch on the upper right lateral incisor was recorded in the cast made of the bite marks, along with the distinctive facets of wear on the outside biting edge of the suspect’s lower right lateral incisor. In fact, the entire outline and dimensions of the suspect’s tooth was reproduced exactly in the cast taken from the nougat.
The bite marks on the chocolate frog and the chocolate bar provided extra details, but the nougat evidence was the most useful source of corresponding details between the bite marks and the suspect’s teeth.
After seeing Brown’s results, police had enough evidence to charge their 24-year-old suspect. But, in May 1997, when the man faced the Adelaide Magistrate’s Court on six charges of breaking and entering between November 1995 and February 1996, he continued to deny any involvement in the burglaries, pleading not guilty to all charges.
He may well have been cheered by the news that the chocolate samples, having been returned to police custody after Brown had cast the bite marks in them, had significantly deteriorated during Adelaide’s February 1997 heat wave. But while the original chocolates themselves couldn’t be produced, police tendered the casts made of them as evidence, along with the casts of the suspect’s teeth.
Brown told the hearing that the likelihood of two individuals having an identical dental imprint was “so remote that it is difficult for us to comprehend.” The bite marks recorded in all three chocolate samples bore “remarkable similarity” to the accused man’s teeth, the expert said. The accused, he concluded, had bitten into the chocolates.
The police weren’t the only ones impressed by the forensic odontologist’s evidence. After hearing it, the accused changed his plea to guilty. He also pleaded guilty to the robbery of the travel agency where he had left a note, after he realized that a handwriting expert had matched the writing on a Valentine’s Day card he had sent to his girlfriend to the note he had left for police.
The magistrate, displeased that the accused had pleaded guilty only when “the writing was on the wall, so to speak,” sentenced him to two years in prison.
Not All Bite Marks Are Equal
Evidence of bite marks in foodstuffs have been able offer police more certainty than evidence of bite marks on skin, which has resulted in many unsafe and unjust convictions – all based on the false belief that a bite mark was “as identifiable as a fingerprint.”
Chewing gum, for example, has been proven to be a useful medium for the recording of tooth marks. In the world’s first reported case, involving a 1976 murder in the United States city of San Diego, a wad of red cinnamon-flavored chewing gum enabled local homicide detectives to place a suspect at the murder scene and clinch a conviction.
The male victim had been shot and stabbed in his bedroom and there were two suspects, only one of whose fingerprints were found in the room. Impressions of the victim’s and the two suspects’ teeth were taken to compare with a silicon reproduction of the chewing gum found at the scene. Under magnification, it could be seen that the chewing gum had been wedged against the inside surface of the upper and lower incisor (front) teeth of an unknown person.
Comparisons ruled out the victim and the suspect whose fingerprints had been found. The other suspect had an opening drilled in the inside surface of her upper incisor (as access for root canal therapy). There was also a defect – either a missing filling or some decay – on its mesial surface (the side towards the centre of the mouth). These defects were also apparent on the wad of gum. A blood typing test showed that this suspect and the chewing gum user had the same blood group.
The world’s second chewing gum/tooth marks case was reported in South Australia, in May 1990. A local policeman, Detective Constable Geoff Carson, was investigating a burglary at a physiotherapy clinic in the South Australian town of Murray Bridge when he found a wad of pink strawberry-scented chewing gum on the floor. It was in an area of the clinic that was a private dwelling, not a place where a patient could possibly have dropped it. Noticing that it bore distinct tooth mark-like indentations, Carson asked the crime scene examiner to collect the gum. He arranged for it to be stored in the fridge so that it wouldn’t soften before he got it to a forensic expert.
The break-in had been the latest in a spate of burglaries from local houses and businesses, all involving entry via a smashed window and the theft of food. The size of the windows and the kind of food taken suggested that the offender was both young and of a slight stature. Meanwhile, fingerprints found at some of the scenes had been identified as those of a local 15-year-old. The youth was arrested and questioned, but he denied any involvement in any of the robberies, including the physiotherapy clinic break-in. But if the chewing gum were proved to be his, it would show that he had been in an area of the clinic which he could have no legitimate reason for visiting.
Using Section 81 of the South Australian Summary Offences Act, police ordered that impressions of the youth’s teeth be taken by a local dentist, who pressed a special gel around his upper and lower teeth, creating a shape which then set as a mould. In the meantime Carson contacted Dr. Kenneth Brown of Adelaide University’s Forensic Odontology Unit, and sent him the chewing gum and the dentist’s impressions of the suspect’s teeth.
Brown prepared casts of the suspect’s teeth by pouring a dental stone plaster mixture into the impression moulds. When they had set hard, and the impression moulds were removed, these casts represented the shape of the suspect’s upper and lower teeth. Brown then examined the oval-shaped wad of chewing gum, which measured about 29 by 12-millimetres. After photographing both sides of it, he made replicas of the tooth impressions, using a special polyvinyl siloxane mixture. Comparing the teeth on the casts with the impressions on the chewing gum replica, he noted that seven specific features of upper teeth numbers 23, 24, 25, 26 had been reproduced on one side of the chewing gum. The other side of the gum had produced impressions of teeth that corresponded to the lower teeth numbered 34, 35 and 36. There was insufficient detail of teeth 34 and 36 on the chewing gum, but the impression of tooth 35 on its own mirrored six different features of tooth 35 on the cast.
When the youth was confronted with this forensic evidence, he confessed to both the physiotherapy clinic break-in and the other burglaries where he had left his fingerprints.
This case was so remarkable that Malaysian forensic odontologist Dr. Phrabhkaran Nambiar eventually wrote it up for publication in the June 2001 edition of the Journal of Forensic Odonto-Stomatology. Nambiar, who had assisted Professor Brown with the lab work on this case when he was a postgraduate student at Adelaide University, noted that, while some foods were better than others for bite-mark registration, chewing gum was in a class of its own. Chewing gum, for example, is the only “food” which will show the biting surfaces of teeth towards the back of the mouth, providing information not produced when people bite into other kinds of foods.
Bite marks left on surfaces such as food raise a different set of scientific issues. Food has different properties from skin (such as not moving when it is being bitten) and some foods can record better teeth impressions than skin. In the preface to his paper on the Murray Bridge case, Nambiar documented a variety of food bite-mark cases. In one, a lamington (a small sponge cake covered in chocolate) had been bitten into during a night arson attack on a coffee shop in the South Australian town of Mt. Gambier. The offender was convicted on the basis of evidence provided by a computer-generated image of his dental arch. When this image was superimposed over the bite mark in the lamington, it was found to match.
Forensic science academic literature abounds with examples of criminals convicted after leaving bite marks in foods. In one of his many papers on the subject, British expert David Whittaker reported that one of the earliest bite-mark convictions dated back to 1906, when a burglar was convicted after leaving tooth marks in a piece of cheese.
In 1955 the tooth marks left by an alleged rapist in a cucumber helped to secure his conviction, while in 1971 the tooth marks left by a murder suspect in the pastry of a meat pie were also vital in obtaining his conviction.
British forensic dentist Dr. Colin Bamford has also given evidence about bite marks in a series of different inanimate materials, including cheese and apples. He once testified in a murder case involving a bite mark on a Mars Bar, and in a murder and armed robbery case where the defendant had been chewing the plastic butt on a car key and had left tooth marks.
Junk Forensics
The history of cases involving bite marks on skin is not as impressive. In general, there are too many variables involved in the examination of a bite mark on the skin, where the compression factors vary according to the location on the body of the bitten skin. There may also be problems related to the way the bite mark has been recorded. If a bite mark is very fresh, there will be indentations in the skin. If an impression is taken with synthetic silicone rubber material, these impressions may be reproduced. Otherwise, if the expert is relying on photos, he or she is examining two-dimensional images of a three-dimensional construction.
In fact, the history of forensic evidence in U.S. courts is stained with examples of death sentences unjustly imposed as a result of bite-mark evidence given by dentists. Air force veteran and postal worker Ray Krone, for example, spent four years on Arizona’s death row and another seven in prison on a life sentence after being convicted, in two trials, of the 1991 murder of Phoenix cocktail waitress Kim Ancona. Testimony that his teeth matched bite marks on Ancona’s breast and throat was crucial to his first conviction in 1992 – after which he was labeled “The Snaggletooth Killer.” In 1996, his conviction was reversed on appeal, on the basis of the prosecution’s concealment of evidence from Krone’s lawyers. He was re-convicted in 1996, again on bite-mark testimony, despite DNA tests which indicated that blood found on the victim was not his – or hers.
Krone continued to proclaim his innocence. Finally, in April 2002, DNA tests of blood and saliva on his supposed victim implicated a man by then in prison for sexually assaulting and choking a 7-year-old girl. Four days later, Krone was set free.
In 1992, when a 3-year-old girl was found raped and strangled near Brooksville, Mississippi, police arrested Kennedy Brewer, 21. As the victim’s mother’s boyfriend, he was automatically a person of interest. He was also home alone with the victim and his own two children on the night the little girl vanished. The traces of semen found in the child’s body were too small for 1992 DNA techniques to use. But a confident forensic dentist testified that the 19 mysterious marks on the child’s body were bite marks, five of which matched Brewer’s teeth “with reasonable medical certainty.” (A defense expert said they were insect bites, sustained during the two days the child’s body lay undiscovered in the open.)
Brewer was convicted and sent to death row. In May 2002, new DNA tests indicated that two unknown men raped the tiny victim, and a new trial was ordered. Brewer’s conviction was “vacated” and he was moved from death row to pre-trial detention, where he remained until 2007, when he was released, pending a new trial. In the meantime, investigations by the Innocence Project led to the identification of Justin Albert Johnson, one of the original suspects in the crime, as the perpetrator. Johnson subsequently confessed. Finally, in February 2008, charges against Kennedy Brewer were dropped and he was exonerated.
Yet, some skin bite-mark opinions can be delivered with a degree of confidence. Forensic odontologists can certainly give opinions on whether or not a bite mark is self-inflicted. Their opinion in such cases is based on anatomical as well as dental issues. A person cannot, as Melbourne forensic odontologist Dr. Tony Hill points out, bite themselves on the back, or on the genitals. Hill has testified in several rape cases where he has been able to testify that a bite mark is human, not animal, and made by an adult, rather than a child. He has also been able to state that someone else, not the victim, inflicted the bite. But was that person the accused? He has not been able to say.
Forensic odontologists can also use bite marks to exclude certain suspects – as in a case where an alleged biter has a missing front tooth or unusually crowded teeth and the bite mark doesn’t show the same peculiarity. A biter with unusual teeth might also be able to be identified in a “closed” incident where the culprit has to be one of several suspects (and no one else has had access to the victim). In one South Australian case involving allegations of abuse of a baby girl of approximately 20 months of age, experts from Adelaide University’s Forensic Odontology Unit examined the teeth of five suspects and were able to eliminate the chief suspect and three of the others. The fifth one was the culprit, who was convicted.
