Scientific evidence is demonstrative evidence or evidence that is introduced to prove a fact by some mode other than oral testimony. Scientific evidence is obtained by using the scientific method, a process of scientifically investigating physical evidence. The information obtained is considered valid because it has been tested. Any scientific evidence that is admitted during a trial must be both relevant (it tends to prove or disprove the factual matter being considered by the court) and reliable or trustworthy. Handwriting analysis, fingerprint and footprint analysis, DNA analysis, and lie detector test results are all examples of scientific evidence. Generally, an expert witness is called to testify about the reliability of the scientific evidence sought to be introduced at trial.
Handwriting analysis is admissible in court. An expert compares samples of a person’s handwriting to the writing in a disputed document and gives an opinion about whether the person wrote or signed the document. Whether a witness qualifies as an expert is up to the trial judge. Handwriting analysis is used in criminal cases and in disputes involving contracts and other written documents.
Fingerprints and Footprints
Fingerprint and footprint identification is considered highly reliable. A person’s fingers contain a pattern of ridges and swirls that is unique to each individual. Similarly, a person’s bare feet contain what is known as friction ridge patterns that are unique to each person. Fingerprints and footprints that are taken at a crime scene can be used to help identify the person who committed the crime. Fingerprints and footprints can also be used to identify the victim of a crime. The validity of the scientific method used for fingerprinting and footprinting is accepted by the courts.
DNA (Deoxyribonucleic acid) Analysis
Each person’s genetic makeup, which contains DNA, is unique. DNA is obtained through blood, saliva, skin, semen, or hair samples. It is used as a way to identify a person. If a drop of blood or a strand of hair is found at a crime scene, it can be compared to a person’s known DNA to see if there is a match, thereby linking the person to the crime scene. An expert witness can give an opinion about the likelihood that the blood found at the crime scene came from the individual whose sample was compared. DNA analysis may also used to establish paternity.
Lie Detector or Polygraph Tests
Most courts refuse to admit the results of a polygraph test into evidence because they consider such tests to be unreliable. A polygraph measures a person’s unconscious physiological responses, such as breathing, heart rate, and galvanic skin response while the person is being questioned. The underlying theory is that stress occurs when a person lies and that this stress can be measured by changes in the person’s physiological responses. There is a concern that some individuals can conceal stress when lying. Polygraph tests also are considered unreliable because it is not possible to determine conclusively whether the stress measured during the test is caused by the test itself or by a lie.
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