Crime Library: Criminal Minds and Methods

Trace Evidence

Hair Evidence Analysis

Like fibers, hair specimens are also understood in forensic research as "class characteristic."  At best, a hair may have enough similar properties compared with a known sample to be "consistent with" the sample; it can't be said definitively to be a perfect match.  While hair samples can be used to exclude a suspect, as with a negroid hair excluding white perpetrators, they can only be considered as contributing evidence.

In homicide cases, hair is picked up at the scene and is generally collected from several different parts of the body, including several areas of the scalp.  Because different hairs on the same person can show many variations, the larger the sample for analysis, the better.  An average sample ranges from 24 to 50 pieces, although those samples on which DNA can be done can be much smaller.

Human Cells (AP)
Human Cells (AP)

Hair analysis can indicate whether the source is human or animal, and also whether the source is a member of a particular race.  It can determine if the hair has been dyed, cut in a certain way or pulled out, and where on the body it was located.  In some cases, evidence of poisoning shows up in the hair.  The hair shaft with a follicle can also offer genetic determinations, such as blood type or DNA, and since the external layer of the shaft resists decomposition, it's the kind of evidence that has real staying power.

Vernon J. Gerberth, in Practical Homicide Investigation, points out that hair (and fiber) evidence is useful in

  1. Helping to establish the scope of the crime scene
  2. Placing a perpetrator at a scene
  3. Connecting a suspect with a weapon
  4. Supporting witness statements
  5. Connecting crime scene areas (abduction, vehicle used, dump site)

Forensic analysis of hair centers on color and structure, determined through microscopic magnification. If the hair has been pulled out, it should include the follicle, and that helps to see the hair's full length.  The shaft has three forensically relevant layers: the cuticle, cortex, and medulla.  The cuticle has overlapping external scales, which helps in species identification: Animals are different from humans.  Within the cuticle is the cortex, made up of spindle-shaped cells that contain the color pigment, and the way the pigment is distributed helps to identify hairs from particular individuals.  The center of the shaft is the medulla, which is also valuable for species differentiation: An animal's medullary index (diameter relative to the shaft's diameter) is larger than a human's.  However, the medulla is often fragmented or interrupted, and may differ from one hair to another on the same person.

Negroid hairs are kinky, with dense pigments, while Caucasian hairs are generally straight or wavy, with finer pigmentation.  Pigment distribution is also different between the two races. The hair of an infant or young child tends to be finer than adult hair, but it is difficult to establish gender from hair samples.  Hair that has follicle tissue was probably pulled out, and that tissue offers the possibility of DNA analysis through the PCR method (which recreates the DNA molecules).    

In the 1950s, a technique called neutron activation analysis became a valuable forensic tool.  A sample such as hair is bombarded with neutrons while inside the core of a nuclear reactor. The neutrons collide with the components of the trace elements and make them emit gamma radiation of a characteristic energy level.  That way, the scientist can measure every constituent part of the sample, no matter how small.  In a single hair, for example, fourteen different elements can be identified. 

The first case to utilize neutron activation analysis was the 1958 murder of 16-year-old Gaetane Bouchard in Canada.  Her former boyfriend, John Vollman, lived across the border in Maine and he was seen with her just before she was discovered dead.  Flakes of paint from the place where they had been together were matched to his car.  Also, the victim's color of lipstick was found on candy in his glove compartment.  However, it was the strands of hair found clasped in the victim's hand that ultimately convinced the jury.  These were matched to Vollman via a ratio of sulpher radiation to phosporus, which was closer to his ratio than hers.

As forensic science advances with computers and increasingly more accurate means of detecting the component parts of small samples, trace evidence may soon play even more significant roles.  As it is, the more trace evidence an investigator can collect at a crime scene, the better the chances of making a case.

Although hair and fiber are currently the most frequently analyzed trace evidence, there are other types that ought to be noted.  A few have offered important clues for solving a crime.

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