Forensic linguistics, also known as forensic stylistics, applies the study of written or spoken material to legal investigations, generally for the purpose of identifying the author or speaker. The field of work is often applied to various pieces of evidence. Suicide letters are sometimes studied to determine whether they were actually written by the deceased or whether they were created in attempts to cover up a homicide. Ransom notes and anonymous threatening letters can be examined in order to aid in the identification of the writer through various means. Recorded emergency service calls are sometimes analysed if there are suspicious circumstances surrounding a particular event. It has also been used in the analysis of cybercrime, identifying particular codes used by certain hackers, such as in the case of Onel de Guzman. Forensic linguistics may also be utilised in civil cases. For example, in cases of authorship dispute, pieces of writing may be analysed in attempts to confirm or refute a particular individual as the author of a text. However one of the most common uses of forensic linguistics is to aid in identifying an anonymous writer or speaker.
The forensic study of the written and spoken language can potentially provide a variety of information. With the appropriate knowledge and experience, it may be possible to make estimations regarding an individualís age, gender, race, religion, level of education, culture, socioeconomic background, and more.
The majority of individualsí speech patterns will include at least traces of local dialect from the area in which they grew up, along with hints of accent from other areas in which they have spent a significant period of time.
Written language is less informative than speech. However spelling, grammar and vocabulary can be indicative of level of education. Written communications may include slang terms or colloquialisms that can indicate age and geographic area. The ordering of nouns, verbs and subject words in a sentence can give some clue as to the native language of the speaker or writer. For example, whereas an English speaker will generally write in a subject-object-verb order, many non-English speakers take on a subject-object-verb order. So even if a non-English speaker is speaking or writing in English, they may still use the word ordering of their own language.
Age can often be established from spoken or written word. Grammar use, apparent educational level, and references to particular movies, music or television shows can indicate the age of the individual. Certain gender characteristics may also be displayed in language. Females tend to use more emotional and less self-confident language than males.
Numerous other factors can also potentially be established. Certain religious or philosophical beliefs may also be indicated by the use of biblical references and similar. The occupation of the writer or speaker could be suggested by the use of technical language or references to specific topics.
Not only can forensic linguistics help establish the identity of the subject, but it may also be useful in determining the likelihood of past or previous actions. For example by analysing a spoke or written threat, experts may be able to estimate how likely it is that the subject will carry out these threats. Similarly, the language used could also indicate the suspectís psychological stability or motivation for the incident.
In the 1960s a technique was developed known as voice spectrography. This technique can produce a visual representation of sound, allowing for what is essentially a voiceprint to be developed. The most common format consists of a graph with a horizontal axis representing time and a vertical axis displaying frequency. Amplitude may also be incorporated. Studying these graphs of sounds can allow for voices to be compared and analysed to establish if two voices are the same or if a particular voice is displaying signs of stress.
Linguistic investigations may involve the use of a polygraph test, more commonly (and inaccurately) known as a lie detector test. This test is essentially utilised in attempts to give indication as to whether or not the subject is lying.
Throughout the test the machine measures a number of physiological changes in the subjectís body, namely blood pressure, respiration rate, heart rate, and perspiration production. The theory behind the test is that when an individual tells a lie there will be an increase in certain physiological responses. The individualís rate of respiration is measured by the pneumograph portion of the machine, which places tubes around their chest and abdomen to measure air pressure changes as they breathe. The cardio-sphygmograph component involves an inflated cuff wrapped around the arm of the subject which measures heart rate. Finally, the galvanograph component of the polygraph will record the amount of sweat produced. Electrical sensors are attached to the subjectís fingertips. As sweat production changes, as does the resistance of the electrical current, allowing for perspiration amount to be recorded.
Throughout the test, the subject will be asked a series of questions. Beforehand the examiner may speak with the subject and ask control questions to obtain a basic reading of the individualís physiological responses to standard questions. These may be basic yes or no questions regarding simple facts about the subject. Questions relevant to the investigation may then be asked.
A baseline reading for the various physiological responses can be established through various forms of control questioning. Once this has been established, any future deviation from this baseline can indicate deceptiveness.
There are numerous types of polygraph test, the most common being the Control Question Test (CQT). In this, the control questions are designed so that the subject can answer them honestly, though they are composed in such a way that they may provoke an emotional response. This way, the examiner can gain an understanding of the subjectís physiological responses to provoking questions. "This test is often used to determine whether certain criminal suspects should be prosecuted or classified as uninvolved in the crime" (American Psychological Association).
Positive Control Tests (PCT) are a form of polygraph test which uses a relevant question as a control. The subject is first directed to answer the question truthfully and then falsely.
Another form of test is the Directed Lie Test (DLT), in which the examiner substitutes extremely broad questions as the controls and the subject is asked to respond to them with lies. This allows for an understanding of physiological responses linked with the subject telling a lie.
Finally, the Truth Control Test (TCT) uses a series of control questions which are designed to give the impression that the individual is being questioned regarding a fictional crime. This allows the examiner to study how the subject will respond to honest denial.
Early polygraph machines included strips of paper and pens which moved slowly according to physiological responses. However modern devices are digital, with results being displayed and analysed by computers. Following the test, the examiner will analyse the results and essentially score the subjectís physiological responses in order to establish whether it is likely that they were lying or not.
However there is much debate over the accuracy of polygraph tests. According to the American Polygraph Association, polygraph tests have an accuracy of between 85 and 95%. However research by the Congressional Office of Technology Assessment has shown false positives in polygraph tests can be as high as 75%. These inaccuracies may have numerous causes.
Changes in certain physiological responses, such as heart rate, blood pressure and perspiration, can be caused by a wide range of occurrences. Anger, nervousness and stress can all produce these changes. Such emotions are likely to be linked to situations in which an individual may be subjected to the polygraph test, essentially rendering results useless. Medical conditions such as colds and viruses can also lead to such physiological responses. Similarly, some individuals may have better control over such physiological responses even when they are lying. The uncertainty of polygraph accuracy has led to polygraph evidence being inadmissible in court in many countries and states.