A more thorough explanation:

Definition: Diminished capacity is a legal defense theory that argues that a person, due to unique factors, could not meet the mental state required for a specific intent crime. It is different from the "not guilty by reason of insanity" defense because it only results in the defendant being convicted of a lesser offense, rather than being found not guilty and committed to a mental institution.

For example, if someone is charged with second-degree murder, which requires the intent to cause the death of a person, but they have a mental impairment or disease that makes them incapable of forming that intent, they could use a diminished capacity defense to argue that they should be convicted of a lesser offense, such as second-degree manslaughter, which only requires recklessness.

One famous example of a diminished capacity defense is the "Twinkie defense" in the 1979 case California v. White. Dan White, a former city supervisor, shot and killed the mayor of San Francisco and another city supervisor. His defense argued that a diet of only junk food had created a chemical imbalance in his brain, making him unable to premeditate murder. The jury convicted him of voluntary manslaughter instead of first-degree murder.

The federal sentencing guidelines allow for a plea of diminished capacity if a "significantly reduced mental capacity contributed substantially to the commission of the offense." This can result in a reduced sentence, but not a "not guilty" verdict.

Example: John is charged with second-degree murder for killing his wife. However, John has a history of severe mental illness and was in a psychotic state at the time of the killing. His defense team argues that he was incapable of forming the intent required for second-degree murder due to his mental illness, and instead should be convicted of a lesser offense, such as manslaughter. This is an example of a diminished capacity defense.

Explanation: John's mental illness is a unique factor that could make him incapable of forming the intent required for second-degree murder. By using a diminished capacity defense, his defense team is arguing that he should be convicted of a lesser offense that does not require that intent, such as manslaughter. If successful, this defense could result in a reduced sentence for John.