Diminished capacity is a defense people may use in criminal cases, arguing that they lacked the full capacity to understand what they were doing in a situation. It is not an attempt to be absolved of guilt in the case, but rather a technique for pleading down to a lower charge, thus reducing the sentence. The acceptance of diminished capacity cases varies worldwide and it should not be confused with an insanity plea, where people argue that they are not guilty by reason of not being able to understand their actions and should be allowed to go free or released into a mental health facility for treatment.
Generally, in criminal cases, the prosecution needs to prove that the defendant committed the crime while acting with the intent to commit harm. In a diminished capacity plea, the defense admits commission of the crime, but argues that the defendant's state of mind was such that intent cannot be proved. This plea may be used when people were intoxicated at the time of a crime or when drugs were involved, including prescription medications known to impair judgment or cause an altered mental state.
Diminished capacity pleas can backfire and must be undertaken with care. Someone who has been drinking and driving, for example, will not receive very much sympathy from the court because this is deemed a grave offense. A person arguing that alcohol or drug intoxication made comprehension of a situation difficult may be viewed prejudicially as a result of attitudes about intoxication. The defense will need to be able to show how the client failed to meet the intent standard for a harsher conviction.
Experts can be called upon to support a diminished capacity plea. These can include neurologists, psychologists, and other people familiar with altered mental states and capacity. The opposing side may call in its own expert witnesses to discredit testimony, providing evidence that the defendant in fact had mental capacity and could have chosen more wisely in the situation under discussion.
If a diminished capacity plea is successful, people will be convicted of a lesser charge than the original one and will face a less severe sentence. People can still be held liable in civil suits, though, and the victim or the victim's family may opt to pursue the matter in civil courts to recover damages. In these settings, the diminished capacity plea can be used against the defendant to prove negligence and liability.