Definition of Capacity Defense
Capacity defense is a defense which asserts the defendant does not have the ability to be held accountable for their negligence due to their age or mental capacity. The decreased mental capacity can be due to trauma, intoxication or disease, although it does not rise to the level of "mental insanity." This charge differs from "not guilty by reason of insanity" because it merely results in the defendant being convicted of a lesser offense. The diminished capacity defense argues that certain people, whether due to their mental state or a mental disease, do not have the mental faculties to commit a particular crime.
For example, if the courts are deciding whether a defendant is guilty of murder vs. manslaughter and they find the defendant has a diminished capacity this defense would argue that a certain defendant is incapable of intending to cause a death, and the court may find the defendant should be charged with manslaughter instead of murder. State laws vary so if you have been charged with a crime it is important to talk to a criminal defense lawyer.