RCW 9A.12.010: Insanity.

To establish the defense of insanity, it must be shown that:

(1) At the time of the commission of the offense, as a result of mental disease or defect, the mind of the actor was affected to such an extent that:

(a) He or she was unable to perceive the nature and quality of the act with which he or she is charged; or

(b) He or she was unable to tell right from wrong with reference to the particular act charged.

(2) The defense of insanity must be established by a preponderance of the evidence.