What is the current standard for determining whether a defendant is mentally incompetent?

For criminal defendants, section 1367 is intended to codify the constitutional standard. (See People v. Welch (1999) 20 Cal.4th 701, 777.) It provides the following definition of incompetence: "A defendant is mentally incompetent ... if, as a result of mental disorder or developmental disability, the defendant is unable to understand the nature of the criminal proceedings or to assist counsel in the conduct of a defense in a rational manner." ( 1367, subd. (a), italics added.)