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August 17, 2009 | by  | in Features | [ssba]

Forensic Science

CSI is awesome. It makes scientists seem totally bad‑ass. Using high‑tech gadgets to build a water‑tight case of incontrovertible forensic evidence, and presenting said incontrovertible evidence to baddies who crumble and confess in the face of such incontrovertible‑ness is all in a day’s work for the white‑coated CSI crime‑fighters.

One of the cases in the earliest written account of forensic science is remarkably similar to the narrative structure of procedural crime shows like CSI. In the 1248 book Xi Yuan Ji Lu (“Collected Cases of Injustice Rectified”) Song Ci relates the case of an investigator who solved the murder of a man killed with a sickle by gathering all the suspects (all the villagers who owned sickles, natch) with their sickles in the warm afternoon sun. Flies, attracted by the tiny fragments of blood, bone, and hair still on the murder weapon, congregated on the murderer’s sickle. In the face of the evidence before him, the murderer confessed. Who are you … who‑o, who‑o indeed.

This early example of forensic entomology (the study of insects on corpses and crime‑scenes), is one of the many subcategories of forensic science today, that range from fields like forensic anthropology (the study of skeletonised remains) to those like toxicology (the study of drugs and toxins).

Of course, forensic science is not all flies on sickles and baddies crumbling and confessing. Even if you haven’t suffered through the boredom of one of those chem or bio labs where you do a whole lot of squinting and pain‑staking measuring and timing for four hours and nothing very interesting happens, you probably at least suspect that that the writers of shows like CSI take more than their fair share of dramatic licence in their portrayal of forensic science. But real forensic science is arguably just as fascinating as its dramatised counterpart.

Developments in the science of DNA have been a boon to forensic science. A notable example is the development of the Polymerase Chain Reaction (PCR) technique, used to amplify a DNA sample so that millions of copies of a DNA sequence can be generated from just a few pieces, or even a single piece, of DNA. PCR was developed in 1984 by chemist Kary Mullis, who later won the Nobel Prize in Chemistry for his work.

PCR exploits the copying mechanisms of DNA to replicate a DNA sequence of interest. DNA does not normally exist as a single molecule (a single long strand of DNA), but as a bonded pair of molecules: the double helix. The backbones of the helix are made of five‑carbon sugars (deoxyribose—the D in DNA) joined together by phosphate groups. Between the backbones of the double helix, holding the molecules together, are hydrogen bonds between pairs of nucleotides (the nucleic N in DNA). There are only four bases in DNA, used over and over again: adenine, cytosine, guanine, and thymine (usually shortened to A, C, G, and T). The four bases bond in pairs: A with T, G with C.

In PCR, double stranded DNA containing the target sequence is heated so that it denatures, which means the strands separate. A heat resistant type of an enzyme called DNA polymerase, which catalyses the copying of DNA, is mixed in with the denatured DNA, along with lots of the DNA bases. As the polymerase moves down each of the strands of the denatured DNA, complementary bases bond to the strand, creating two copies of the original DNA sample: as the polymerase moves down each strand of DNA, the nucleotides follow, A joining to T, T to A, C to G, and G to C, so that you end up with two copies of the original sample. The process is repeated many times, amplifying the original sample of DNA until there are millions of copies.

The amplified sample can then be used to match a profile taken from a suspect.

Much of the forensic science work in New Zealand is performed by the Institute of Environmental Science and Research (ESR), a Crown Research Institute. In addition to environmental health programmes in areas like food safety and public health, ESR has a major focus on forensic science: DNA and forensic biology, physical evidence, crime scene investigation, drug analysis, fire investigation, and criminal and coronial toxicology.

ESR is the sole forensic science provider to the New Zealand police, and manages New Zealand’s National DNA Databank—a database of DNA profiles taken from convicted offenders and volunteers. The profiles in the database are used to try to link people to the scene of unsolved crimes. According to ESR’s 2005 Annual Report, “the overall success rate in DNA matching in New Zealand is world leading, with more than 55 percent of all unsolved cases loaded to the crime sample databases linked to individuals and more than 33 percent linked to another crime”.


As impressive as shows like CSI might make forensic science seem, the use of such scientific techniques in criminal investigations is not without controversy.

Science is constantly developing, and techniques that were once considered state of the art have been shown to be less reliable than once thought—meaning the evidence they produce is given less weight in criminal investigations than it once was. Some techniques have been abandoned altogether.

Forensic dentistry, which involves matching bite marks found on victims’ bodies with the teeth of suspects, has been used to convict people of murder—some of whose convictions were later overturned on the basis of DNA evidence.

Starting with the investigation of the assassination of US President John F Kennedy in 1963, and continuing until 2005, the FBI used a technique called “comparative bullet‑lead analysis” in which investigators traced bullets back to their batch in an attempt to link bullets at a crime scene with those owned by a suspect. The theory behind the technique was that each batch of bullets had a distinct chemical composition. The FBI abandoned the technique when studies found the technique, used in criminal investigations for more than 40 years, was unreliable.

The CSI effect

You might find the fact that a forensic science technique can be used to help achieve convictions for four decades before being abandoned as unreliable to be a chilling thought. Some commentators have expressed concern at the apparent overestimation of the importance of forensic evidence by experts and lay people alike in the justice system.

Some researchers point to dramatisations of forensic science in shows like CSI distorting people’s expectations of the evidence produced by such techniques, dubbing it the “CSI Effect”. Research into this effect is relatively new however, and to date, whether or not such an effect exists and to what extent is not clear. Findings from studies into the “CSI Effect” have ranged from no evidence of an effect to jurors being more skeptical of forensic science evidence presented at trial, to jurors expecting such evidence to be presented at trial whether or not it is relevant to the case.

On its website, the Institute of Environmental Science and Research explains that it “is not possible to definitively state that an unknown DNA profile came from a given individual, only whether or not it is possible”. Forensic scientists make statistical assessments of the likelihood that a DNA sample has come from a certain individual.

Although it states that with the profiling system it currently uses, “the chance of a person being incorrectly linked to a crime because their DNA just happens to match DNA found at the crime scene is very small”, ESR is careful to point out that DNA evidence does not in itself prove a person’s guilt or innocence but only addresses “the question of whether or not an unknown DNA profile from a crime scene matches individual profiles”, and that “while this can link a person to a crime with a high degree of probability, it says nothing regarding the question of a person’s guilt or innocence”.

Freed by Forensic Science

On the other side of the coin, forensic science techniques, especially DNA analysis, have increasingly been used to overturn wrongful convictions.

In a story of the type becoming increasingly familiar, earlier this month a Texas man, Ernest Sonnier, was released from jail 23 years into a life sentence for rape. Sonnier was jailed on the basis of the victim’s testimony. The victim picked Sonnier out from a photo‑array and a line‑up.

18 months of DNA testing of evidence found on the victim’s clothing and at the scene of the crime cleared Sonnier—and implicated two other men.

In the United States alone, since 1989, DNA evidence has cleared 241 people convicted of crimes they did not commit.

Most New Zealanders will remember the case of David Dougherty, who was found guilty and jailed for a 1992 abduction, sexual violation and rape of an 11 year old girl.

Dougherty was jailed in 1993 largely on the basis of the victim’s testimony. At Dougherty’s trial, an ESR scientist told the court that DNA evidence was a match with another man, but that traces of DNA found at the scene could not exclude Dougherty.

Dougherty was released in 1996 upon appeal—the court heard evidence from other scientists whose interpretation of the evidence differed markedly from that of the scientist who gave evidence at the original trial.

In 2003, Nicholas Reekie was charged and convicted for the rape: DNA evidence indicated that semen found on the victim’s clothing was 700,000 million times more likely than any other unrelated man to belong to Reekie.

It is a case that starkly illustrates the importance of the interpretation and use of forensic science: Dougherty’s conviction and its subsequent overturning were both influenced by evidence gleaned by forensic science.

Rather than forensic science in and of itself being ‘bad’ or ‘good’, it is our use and interpretation of the science that can create the difference between a wrongful conviction, and a white‑coated crime‑fighter making baddies crumble at the flick of a test‑tube.


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