FIVE EXPERIMENTS WERE CONDUCTED IN AN ATTEMPT TO ISOLATE SPECIFIC LEARNING PATTERNS IN CHILDREN OF VARYING DEGREES OF MENTAL RETARDATION AND TO CONTRIBUTE TO THE KNOWLEDGE OF THE PHYLOGENETIC DEVELOPMENT OF INTELLIGENCE. FACTORS STUDIED WERE THOSE KNOWN TO INFLUENCE LEARNING IN NORMAL CHILDREN AND ADULTS--KIND OF REINFORCEMENT (VERBAL OR PHYSICAL), DELAY OR REWARD, SHIFT OF REINFORCEMENT, INTERTRIAL INTERVAL, AND REMINISCENCE. THE SUBJECTS WERE MALE AND FEMALE INSTITUTIONALIZED RETARDED CHILDREN, AGED 8 TO 18, AND CLASSIFIED AS MILDLY, MODERATELY, OR SEVERELY RETARDED. GROUPS VARIED IN SIZE, AND WERE EVALUATED IN THE COMPLETION OF MOTOR OR VERBAL TASKS (PURSUIT MOTOR, PEGBOARD, MEMORY DRUM TASKS). CONCLUSIONS DRAWN FROM THE SERIES OF MOTOR TASK EXPERIMENTS WERE AS FOLLOW--(1) MALES GENERALLY ACHIEVE A PERFORMANCE LEVEL SIGNIFICANTLY SUPERIOR TO FEMALES, (2) DIFFERENT REWARDS DO LEAD TO SIGNIFICANT DIFFERENTIATION IN PERFORMANCE, (3) DELAY OF REWARD DOES NOT HAVE A SIGNIFICANT EFFECT ON THE PERFORMANCE OF RETARDATES WHEN A SENSORY REWARD (SUCH AS LIGHT) IS USED, (4) MILD RETARDATES PERFORM AT A LEVEL SIGNIFICANTLY SUPERIOR TO MODERATE RETARDATES, AND BOTH GROUPS ARE SUPERIOR TO SEVERE RETARDATES, (5) THE EXPECTED ELATION AND DEPRESSION AS A RESULT OF A SHIFT IN REINFORCEMENT DO NOT APPEAR TO OCCUR IN RETARDATES, ALTHOUGH THESE SUBJECTS DO SHOW AN INTEREST EFFECT OR IMPROVEMENT IN PERFORMANCE FOLLOWING ANY SHIFT IN REINFORCEMENT, (6) THE LENGTH OF INTERTRIAL INTERVAL DOES NOT SIGNIFICANTLY AFFECT THE PERFORMANCE OF RETARDATES WHEN INTERVALS OF 10 SECONDS OR LESS ARE USED, (7) RETARDATES DO NOT SHOW A REMINISCENCE EFFECT IN VERBAL LEARNING, BUT THERE IS SOME INDICATION THAT THE REMINISCENCE DOES OCCUR IN MOTOR LEARNING, AND (8) AGE, IQ, AND MENTAL AGE CANNOT BE USED AS ACCURATE PREDICTORS OF PERFORMANCE ON EITHER VERBAL OR MOTOR TASKS. DATA ARE PRESENTED IN 17 TABLES AND 12 FIGURES. A BIBLIOGRAPHY LISTS 103 ITEMS. (DF)