Hair evidence is one of the most common types of evidence encountered in criminal investigations. During the course of the normal hair-growth cycle, hairs are readily lost from individuals, and these hairs may be transferred during the course of a criminal activity.
Edmond Locard was the first forensic scientist to formally articulate the foundation for the transfer event (Locard 1930). Now known colloquially as the Locard Exchange Principle, it states that any time there is contact between two surfaces, an exchange of materials will occur. One of the materials that can be readily collected, identified, and compared is hair evidence.
The forensic analysis of hair evidence can be extremely valuable in the examination of physical evidence by (1) demonstrating that there may have been an association between a suspect and a crime scene or a suspect and a victim or (2) demonstrating that no evidence exists for an association between a suspect and a crime scene or a suspect and a victim. Although the science of microscopic hair examination can never result in an identification, that is, conclude that a hair came from one individual to the exclusion of all others, the vast amount of both macroscopic and microscopic information available from hair analysis can provide a strong basis for an association and certainly provides strong exculpatory evidence. The final aim of any forensic examination must be to provide statements based on objective scientific observation that will be of value in a court of law or to any interested party involved in an investigation.
The purpose of this document is to review the bases for microscopic hair analysis and comparison. Hair examinations involve the analysis and comparison of the morphological characteristics present in hair. Based on these morphological characteristics, the first determination that can be made is whether the hair came from a human or an animal (for the purposes of this document, any reference made to animal means nonhuman animal). Within each of these two groups, additional information regarding the potential donor can be obtained using these same microscopic characteristics. Finally, a comparison can be conducted between a hair of unknown origin and a known sample of hairs from an appropriate known sample
Scientific Basis for Microscopic Hair Examinations
All organisms differ widely in many dimensions, including morphological appearance, physiology, and genetic makeup. Some groups of organisms clearly are more similar to some groups than to others. For instance, a monarch butterfly is more similar to a tiger swallowtail butterfly than either is to a ladybird beetle. Biologists seek to identify these differences and use them to organize and classify the world around them. They use these differences to generate classification schemes that can be used for many purposes, from examining how traits evolve to solving crimes.
These classification schemes have their roots in the field of taxonomy. Taxonomy is the practice of classifying biodiversity, and it has a long and venerable history. In 1758, Carl Linnaeus proposed a system that has dominated classification for centuries. He proposed...
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