What are the pros and cons of hair fiber evidence as opposed to other forms of physical evidence?
Hair fiber evidence can be crucial in connecting a particular individual to a crime, such as rape, but it is also the most difficult to process and the most prone to misuse.
Strands of human hair located at the scene of a crime provides very strong indication that an individual was at that location, but it can also prove worthless if the fibers were found outside and, consequently subject to the elements, including wind, and if the full length of the strand is not present. Hair by itself is important, but the most important DNA evidence associated with hair fibers comes from the cellular material at the root of the hair strand. In other words, the microscopic particles of follicle provide the most reliable evidence. The protein keratin is the key substance that accompanies the follicle, and that provides the greatest identifying evidence.
If the hair is pulled out by the root, such as would occur by an individual defending him or herself against an attacker, then the hair fibers are very important. Hair that does not include follicular material is much less reliable. Hair fibers that do not include keratin are useful in identifying a category of individual into which a suspect might fit, but they are not definitive proof that the hair came from the suspect. For this reason, hair fibers are less valuable than fingerprints and DNA evidence derived from saliva or blood. Fingerprints are not DNA evidence, but they remain very reliable forms of evidence. DNA derived from bodily fluids, however, is the most valuable form of evidence available.
Hair and fiber evidence can be a powerful tool in forensics to trace the paths of suspects or other people at a crime scene, as people leave hair and fibers behind wherever they go. This type of evidence, which must be very carefully collected, can help place a person at a crime scene, and it can also help connect people to different crimes so that their activities and modus operandi can be traced. Human hair can be used to figure out the gender, race, and age of a person. It has an advantage over other forms of evidence because people do not realize that they are leaving fibers or hair behind at a crime scene.
However, evidence from hair and fiber is not considered absolutely reliable or conclusive, and it has this disadvantage over other forms of physical evidence (such as DNA or fingerprints). There is no conclusive quantitative analysis of hair that shows how likely it is that a hair came from a certain individual, though if the hair contains DNA, it can be used as more conclusive evidence. In addition, fiber evidence is not conclusive. There is no way to prove that a fiber came from a certain person's clothes or carpets, but the statistical probability that it did can be quantified.