What is the test for determining whether a defendant is mentally unfit to stand trial?

Competency hearings are required only when a trial court has "reasonable cause to believe that the defendant may presently be suffering from a mental disease or defect rendering him mentally incompetent...." 18 U.S.C. 4241(a); see also Pate, 383 U.S. at 385-86; United States v. Quintieri, 306 F.3d 1217, 1232-33 (2d Cir. 2002). The "[d]etermination of whether there is 'reasonable cause' to believe a defendant may be incompetent rests in the discretion of the district court," United States v. Vamos, 797 F.2d 1146, 1150 (2d Cir. 1986), and we review such determinations only for abuse of discretion. Quintieri, 306 F.3d at 1232-33. "In deciding that an evidentiary hearing is unnecessary, a court may rely not only on psychiatrists' reports indicating competency but also on its own

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observations of the defendant." United States v. Nichols, 56 F.3d 403, 414 (2d Cir. 1995).