AS A SCIENTIST, THE FORENSIC PSYCHOLOGIST HAS ACCESS TO THE VAST STORE OF DATA DERIVED FROM A CENTURY OF EXPERIMENTATION AND THE WIDE VARIETY OF RESEARCH METHODS AND TECHNIQUES APPLICABLE TO EVIDENTIAL PROBLEMS. FOR INSTANCE, THE FORENSIC PSYCHOLOGIST HAS ACCESS TO CUMULATIVE DATA OBTAINED FROM THE OBSERVATION OF REAL BEHAVIORS AND FROM EXPERIMENTATION WHICH MAY BE PRESENTED IN COURT AS CRITERIA OF NORMALITY OR REASONABLENESS. AS AN EXAMPLE, SOCIAL ATTITUDINAL DATA MAY BE RELEVANT TO DETERMINING COMMUNITY NORMS IN OBSCENTITY TRIALS, OR OCCUPATIONAL AND REHABILITATION RESEARCH DATA MAY AID IN ASSESSING LOSS IN COMPENSATION CASES. IN SOME CASES, RELEVANT DATA MAY NOT BE AVAILABLE; IN SUCH CASES SPECIFIC EXPERIMENTATION MAY BE UNDERTAKEN. IN SCANE VERSUS AINGER, RESULTS OF A STUDY OF 50 NORMAL CONTROLS AND 50 AGE-AND INTELLIGENCE QUOTIENT-MATCHED EXPERIMENTAL SUBJECTS WERE INCORPORATED INTO THE EVIDENCE OF A CASE INVOLVING MENTAL SUBNORMALITY IN A JUVENILE OFFENDER. THE TASK OF THE FORENSIC PSYCHOLOGIST MAY ALSO INCLUDE SHOWING THE PROBABLE TRUTH OR FALSITY OF WITNESS TESTIMONY. THERE ARE THREE GENERAL PURPOSES FOR THE EXPERT EVIDENCE OF A FORENSIC PSYCHOLOGIST: PROOF OR DISPROOF OF FACTS RELATING TO THE ACTUS REUS, PROOF OR DISPROOF OF THE CONDITION OF MENS REA, AND PROVISION OF FACTS IN MITIGATION. SETTING STANDARDS OF OBSCENITY OR SHOWING SUBNORMALITY OR INSANITY ARE EXAMPLES OF THE FIRST TWO PURPOSES OF EXPERT EVIDENCE. PROOF OF MITIGATING CIRCUMSTANCES MIGHT INVOLVE PSYCHOMETRIC TESTING AIMED AT ESTABLISHING MOTIVATION OF A SUSPECT OR IN DETERMINING DISPOSITION FOR TREATMENT FOLLOWING CONVICTION. QUANTITATIVE EVIDENCE USED FOR FORENSIC PURPOSES POSES TWO PROBLEMS. FIRST, SCIENTIFIC TERMINOLOGY IS OFTEN UNACCEPTABLE IN A COURT OF LAW, MAY BE SUBJECT TO INTENTIONAL OR UNINTENTIONAL MISINTERPRETATION OR DISTORTION, AND IS OFTEN REJECTED IN FAVOR OF 'COMMON SENSE' APPROACHES. SECOND, THE NATURE OF EMPIRICAL RESEARCH, USING INFERENCE AND THE HYPOTHETICAL-DEDUCTIVE METHOD, CAN NEVER PROVIDE POSITIVE PROOF; SCIENTIFIC EVIDENCE CAN THUS ONLY BE USED TO DISPROVE AN HYPOTHESIS. UNTIL THE BASIC PRINCIPLES OF EMPIRICAL INVESTIGATION BECOME MORE FAMILIAR TO JUDGES AND JURIES, EXPERIMENTAL EVIDENCE WILL STILL HAVE TO COMPETE WITH COMMON SENSE AND WILL REMAIN LIMITED TO THE DEGREE OPPOSING COUNSEL IS PREPARED TO ACCEPT IT. REFERENCES ARE INCLUDED. (JAP)