Hair is one of the most common types of trace evidence. It varies extremely among both individuals and racial populations. Hair may in some cases rule out certain populations or help identify an unknown victim. Transfer of hair from a victim to a suspect or vice versa may substantially raise the probability that the victim and perpetrator were in contact.
The variability of hair characteristics can be seen through the use of comparison microscopy. Hair microscopy is a two-step process. Firstly examiners identify the questioned hairs and once this has been concluded, they compare the questioned and known hair, i.e hair found at the crime scene.
The purpose for this examination is to determine whether two or more individuals could have come into contact. This associative evidence is particularly helpful in crimes of violence, like homicide and sexual assault where physical contact may have occurred. Crimes like burglary typically involve the recovery of debris and articles of clothing which may also contain hairs useful for identifying suspects.
Hair Examination Process
The hair examination process involves many different steps:
- The examiner determines whether the hair in question originated from an animal or a human being. If the hair originated from an animal, it is possible to further identify it to a particular type of animal. Although certain hairs can be attributed to species, it is not possible to identify hairs to a specific animal to the exclusion of other similar animals.
- The examiner conducts a Racial Determination- A human hair can be associated with a particular racial group based on established models for each group. Forensic examiners differentiate between hairs of Caucasoid (European ancestry), Mongoloid (Asian ancestry), and Negroid (African ancestry) origin. Head hairs are generally considered best for determining race. Racial determination from the microscopic examination of head hairs from infants, however, can be difficult, and hairs from individuals of mixed racial ancestry may possess microscopic characteristics attributed to more than one racial group). Lastly,
- The examiner may be able to determine the age and sex of the individual- The age of an individual cannot be determined definitively, however, the microscopic appearance of certain human hairs, such as those of infants and elderly individuals, may provide a general indication of age. The hairs of infants, for example, are generally finer and less distinctive in microscopic appearance. As individuals age, hair can undergo pigment loss and changes in the configuration of the hair shaft to become much finer and more variable in diameter. Although the sex of an individual is difficult to determine from microscopic examination, longer, treated hairs are more frequently encountered in female individuals. Sex can be determined from a forcibly removed hair (with tissue), but this is not routinely done.
Possible conclusions after hair analysis
Hair examiners can come to the following conclusions after a hair analysis:
- The questioned hair exhibits the same microscopic characteristics as the hairs in the known hair sample
- The questioned hair is microscopically dissimilar to the hairs found in the known hair sample and, accordingly, cannot be associated to the source of the known hairs.
- Similarities and slight differences were observed between the questioned hair and hairs in the known hair sample. Accordingly, no conclusion could be reached as to whether the questioned hair originated from the same source as the known hairs.