Locard's exchange principle

In forensic science, Locard's exchange principle (sometimes simply Locard's principle) holds that the perpetrator of a crime will bring something into the crime scene and leave with something from it, and that both can be used as forensic evidence. Dr. Edmond Locard (13 December 1877 – 4 May 1966) was a pioneer in forensic science who became known as the Sherlock Holmes of France. He formulated the basic principle of forensic science as: "Every contact leaves a trace". Paul L. Kirk^[1] expressed the principle as follows:

"Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value."

Fragmentary or trace evidence is any type of material left at (or taken from) a crime scene, or the result of contact between two surfaces, such as shoes and the floor covering or soil, or fibers from where someone sat on an upholstered chair.

When a crime is committed, fragmentary (or trace) evidence needs to be collected from the scene. A team of specialized police technicians go to the scene of the crime and seal it off. They both record video and take photographs of the crime scene, victim (if there is one) and items of evidence. If necessary, they undertake a firearms and ballistics examination. They check for shoe and tire mark impressions, examine any vehicles and check for fingerprints.

Famous cases

The case studies below show how prevalent Locard's Exchange Principle is in each and every crime and even everyday circumstances. The examples using Locard's Principle show not only how the transfer of trace evidence can tell the tale of what happened, but also how much care is required when collecting and evaluating trace evidence.

The Weimar Children Murders

Karola and Melanie Weimar, aged 5 and 7, lived with their parents, Reinhard and Monika, in Germany.^[2][3] They were reported missing on 4 August 1986. Their bodies were found on 7 August. They had been murdered.

Monika first said the children had breakfast, then went to a playground. Three weeks later she said they were already dead when she returned home the previous night: Reinhard was sitting on the edge of Karola's bed, weeping and confused; he then disposed of the bodies.

Both parents were suspected, but Monika was having an affair, and was seen where Melanie's body was later found. She was convicted, but after serving her sentence, was released in 2006.^[4]

Investigators determined what clothes Monika was wearing on 3 and 4 August, but not Reinhard's clothes, so only fibers from her clothing were identified on the children's bodies, yet they were also constantly in contact with him.

The bedding contained 14 fibers from Karola's T-shirt. Frictionless tests, simulating a dead child, matched that figure better than the friction tests, simulating a live child, so Karola could have lain lifelessly in bed wearing her T-shirt, as stated by her mother.

35 fibers from Monika's blouse were found on the back of Melanie's T-shirt, but only one on her bed sheet. In tests, between 6 and 10 fibers remained on the sheet. These higher numbers were thought to disprove Monika's claim that she gave her child a goodbye hug the previous day. However, there are several likely explanations. For example, the bedding was put in one bag, so fibers from the sheet could have been transferred to the cover and pillow. Only the central area of the top of the sheet was taped: it might have originally contained more than one blouse fiber, the others could have been transferred to the back or sides while in the bag.

The blouse fibers on Melanie's clothing were distributed evenly, not the clusters expected from carrying the body.

265 fibers from the family car’s rear seat covers were found on Melanie's panties and the inside of her trousers, but only a small number of fibers from the front seats was found on the children. This helped disprove the theory that they were killed on the front seats.

Melanie's clothes and hair were covered in 375 clinging fruits of goosegrass. As some of these itchy things were on the inside of her trousers and on her panties, the trousers must have been put on her after death.

No sand was found on the bodies or clothing (including socks and sandals) of either child, making the morning playground story unlikely.

The Westerfield-van Dam case

Danielle van Dam, aged 7, lived with her parents and brothers in San Diego, California. She was reported missing on 2 February 2002; her body was discovered on 27 February. Neighbor David Westerfield was almost immediately suspected, as he had gone camping in his RV, and he was convicted of her kidnapping and murder.

Hairs consistent with the van Dams’ dog were found in his RV, also carpet fibers consistent with Danielle's bedroom carpet. Danielle's nightly ritual was to wrestle with the dog after getting into her pajamas.^[5] The prosecution argued that those hairs and fibers got onto her pajamas through that contact, and were then carried on the pajamas to first Westerfield's house and then to his RV, when he kidnapped her from her bed. The alternative scenario is that they got onto her daytime clothes, and those of her mother and younger brother, and were carried to his house when they visited him earlier that week selling cookies.^[6] He said his laundry was out during that visit, so trace evidence from them could have got on it, and then been transferred to his bedroom and his RV (secondary Locard transfer).^[7] Also, his RV was often parked, sometimes unlocked, in the neighborhood streets, so Danielle could have sneaked inside, leaving behind that evidence.^[8]

No trace of Westerfield was found in the van Dam house.^[9]

14 hairs consistent with Danielle's were found in his environment. All but one were compared on only mitochondrial DNA, so they might have come from her mother or a sibling.^[10] Most (21) of the hairs were in a dryer lint ball in his trash can, so they might have got in his laundry before the kidnapping.^[11]

There were 5 carpet fibers in his RV, but none in his house, suggesting those were deposited by someone going directly from her house to his RV, or they may have come from another house in that development.^[12]

No Danielle pajama or bedding fibers were reported in his environment. There was no trace evidence in his SUV (which casts doubt on the belief that she was transported from his house to his RV in his SUV).^[13] He vacuumed his RV after the kidnapping, but no trace evidence was in the vacuum cleaner.^[14]

One orange fiber with her body was consistent with about 200 in his house and 20 in his SUV (none in his RV), while 21 blue fibers with her body were consistent with 10 in his house and 46 in his RV (none in his SUV). Contrary to media reports, only a few items from her house were tested so that can’t be excluded as the source.^[15] In particular, the clothes of Danielle and her family during the cookie sale were not determined and eliminated. There were apparently two different types of the orange fibers, dull and very bright (so the number which matched might have been much less than 200).^[16] There were red fibers with her fingernails, and many other fibers with her body, which could not be matched to his environment.^[17] The only non-Danielle hair found with her body wasn’t his, nor was any desert sand reported with the body, and no soil or vegetation from the dump site was reported on his shoes, laundry, shovel or RV.

To explain why so much expected evidence was missing, the prosecution argued that he went on a cleaning frenzy, and tossed out evidence.^[18]

In popular culture

• Locard's principle is mentioned in the sixteenth episode of CSI: Crime Scene Investigation, "Too Tough to Die", aired on 1 March 2001.^[19]
• "Locard's Exchange" is the title of episode #75 of the television medical drama Crossing Jordan, aired on 10 April 2005.
• Locard's principle is expounded in every one of the Lincoln Rhyme series of forensic detective stories by Jeffery Deaver.
• August 2009 Thomas Pynchon novel Inherent Vice.
• 2013 UK television movie Murder on the Home Front. Pathologist Lennox Collins insists evidence will be left behind at the crime scene of the murders.^[20]
• Mentioned by a detective in episode 15 of the third series of Father Brown, aired 23 January 2015.
• "Every Contact Leaves a Trace" is a novel by Elanor Dymott published by Jonathan Cape in 2012.
• Locard's exchange principle is consistently mentioned in TVB's Forensic Heroes II

References

External links

• Evidence Dynamics: Locard's Exchange Principle & Crime Reconstruction, W. J. Chisum, B. E. Turvey
• Fuller, John. (17 June 2008) "How Locard's Exchange Principle Works", HowStuffWorks.com.