The outcome of many criminal law cases will depend upon the strength and admissibility of evidence -- including physical proof, scientific evidence, and witness testimony. Criminal evidence law can be complex, but this section will help make sense of the different rules and concepts surrounding evidence. Below you will find information on the concept of admissibility, witness testimony, the use of scientific evidence in court, and more.
In order to be admitted at court evidence must be relevant, material, and competent. To be relevant evidence must reasonably help prove or disprove some fact. The degree to which this evidence increases or decreases the likelihood of the fact for which it was introduced will influence the weight it is given by the judge or jury. Evidence is material if it is offered to prove a fact in dispute and it is competent if it falls within certain standards of reliability. Learn more about the admissibility of evidence and how these rules are interpreted and applied.
Evidence that might otherwise be admitted in a criminal case can be suppressed when it has been illegally obtained. Evidence produced as a result of an unlawful search and seizure, the failure to read Miranda rights, or evidence for which the chain of custody is broken may all be suppressed. Any evidence produced as the result of these flawed circumstances may also be suppressed as "fruit of the poisonous tree." Evidence that would normally be suppress-able may still be admitted where it would have inevitably been discovered, the officer was acting on good faith, or when an independent source would have produced the same evidence.
Hearsay describes when a witness repeats a third party statement in court to prove the truth of the statement itself. For Example: "My neighbor told me he saw her attack the victim." To say that there are exceptions to the hearsay rule is an understatement. The Federal Rules of Evidence list 24 exceptions, including a "catchall" rule that states that hearsay evidence that doesn't otherwise fit into the exceptions can still be admitted if it has sound guarantees of trustworthiness, helps prove a material fact, is more probative than other similar reasonably obtainable evidence, would forward the cause of justice, and all parties have been notified that it will be offered into evidence. Learn more about hearsay and the other 23 exceptions to the rule.
Character evidence is normally not permitted in criminal court to show a person is likely to be guilty. For example, a person's reputation for exaggeration or lying cannot be introduced as evidence that the person committed fraud. However, there are some circumstances when character evidence is permitted. At the sentencing stage character evidence is often introduced, but here it is not to prove guilt, but rather to argue for leniency or strictness in punishment. Habit rather than character can be used as evidence when it is specific, regular and consistently repeated. Civil suits have similar rules, but exceptions exist; especially in defamation suits and others where character is potentially central to the suit. Learn more about character evidence, admissibility and other issues.
Scientific and Forensic Evidence
There are many kinds of scientific evidence admitted to criminal courts including fingerprints, fiber analysis, DNA and other evidence. Any scientific evidence produced at trail must first be shown to be established within the scientific community and generally accepted as true before it can be asserted at trial. Fingerprint and DNA matching are reasonably well-understood, but there are times when less established kinds of scientific evidence are introduced. When necessary a hearing on the validity of a scientific theory takes place prior to the trial on the merits of the principal case. Learn more about kinds of forensic evidence and their current reception by courts.