ALTHOUGH UNDERSTANDING OF RETARDATION HAS BECOME MORE SOPHISTICATED SINCE 1900, MYTHS SURVIVE IN COMMUNITIES WHERE LITTLE EFFORT HAS BEEN MADE TO EDUCATE CITIZENS. ACCORDING TO THE AMERICAN ASSOCIATION FOR MENTAL DEFICIENCY, THE CONDITION IS CHARACTERIZED AS 'SIGNIFICANTLY SUBAVERAGE INTELLECTUAL FUNCTIONING EXISTING CONCURRENTLY WITH DEFICITS IN ADAPTIVE BEHAVIOR AND MANIFESTED DURING THE DEVELOPMENTAL PERIOD.' BECAUSE MENTAL RETARDATION HAS MANY DIVERSE MANIFESTATIONS, IT IS SELDOM APPARENT THAT THE DEFENDANT IN A CRIMINAL CASE IS RETARDED. IT IS THEREFORE ESSENTIAL THAT WHENEVER THE POSSIBILITY OF MENTAL RETARDATION EXISTS THAT ADEQUATE ASSESSMENTS BE MADE. THE PECULIAR CHARACTERISTICS OF A MENTALLY RETARDED PERSON GREATLY INCREASE THE LIKELIHOOD THAT ANY DETENTION AT THE INITATION OF A POLICE OFFICER WILL RESULT IN ARREST. IN ADDITION, WHILE A MENTALLY RETARDED PERSON IS NOT ISPO FACTO PRECLUDED FROM MAKING AN EFFECTIVE WAIVER OF HIS RIGHTS AND RENDERING A VOLUNTARY, ADMISSIBLE CONFESSION, THE ULTIMATE TEST USED BY THE COURTS IS THE 'TOTALITY OF THE CIRCUMSTANCES,' WHICH INCLUDES BOTH THE CHARACTERISTICS OF THE PERSON INTERROGATED AND THE DETAILS OF THE INTERROGATION. THE QUESTION OF WHETHER A CRIMINAL DEFENDANT IS COMPETENT TO STAND TRIAL IS TO BE DISTINGUISHED FROM THE QUESTION OF WHETHER, DUE TO MENTAL DISEASE OR DEFECT, HE IS NOT LEGALLY RESPONSIBLE FOR THE CRIME. IN JACKSON V. INDIANA (1972), THE UNITED STATES SUPREME COURT HELD THAT DUE PROCESS IS VIOLATED BY A STATE PROCEDURE WHICH CALLS FOR THE INDEFINITE COMMITMENT OF A CRIMINAL DEFENDANT SOLELY ON ACCOUNT OF HIS INCOMPETENCY TO STAND TRIAL. THE TEST OF WHETHER A DEFENDANT IS COMPETENT TO PLEAD GUILTY IS THE SAME AS THAT USED TO DETERMINE IF HE IS COMPETENT TO STAND TRIAL. THIS TEST IS USED IN MANY STATES, INCLUDING MASSACHUSETTS. THE CRIMINAL RESPONSIBILITY DEFENSE MAY BE AVAILABLE TO THE MENTALLY RETARDED DEFENDANT BUT SHOULD BE CAREFULLY CONSIDERED BY DEFENSE COUNSEL BEFORE IT IS RAISED. FOOTNOTES ARE INCLUDED.