A REVIEW OF SEXUAL AROUSAL MEASUREMENTS AND SEXUAL PREFERENCE ASSESSMENTS THAT HAVE BEEN USED IN RESEARCH ON SEXUAL OFFENDERS ALSO IDENTIFIES THE FACTORS THAT ARE USED IN DESIGNING TREATMENT OR IN CONSIDERING SUITABILITY FOR DISCHARGE FROM DETENTION UNDER SECURE CONDITIONS. THESE FACTORS (GENDER IDENTITY AND ROLE BEHAVIOR, SEXUAL PREFERENCE AND ATTITUDES, SEXUAL GRATIFICATION, SEXUAL AROUSAL, AND SEXUAL GRATIFICATION, SEXUAL AROUSAL, AND SEXUAL ACTIVITY--FANTASIED OR REAL) ARE DISCUSSED WITH REGARD TO THE RESEARCH CONDUCTED TO MEASURE THEM, INCLUDING PHYSIOLOGICAL METHODS OF PHALLOPLETHYSMOGRAPHY OR PENCIL AND PAPER TESTS. ATTENTION IS GIVEN TO A 'SHUTTER' EXPERIMENT AIMED AT DIFFERENTIATING BETWEEN HOMOEROTIC OR HETEROEROTIC INTEREST IN A MALE SUBJECT, A 'SEXUAL INTEREST LATENCY' EXPERIMENT, AND ANIMAL EXPERIMENTS USING RESPONSE RATES. IN ONE OF THE PROTOTYPE EXPERIMENTS EXAMINING CONCURRENT PERFORMANCE, PIGEONS WERE EXPOSED TO A TWO-KEY CONCURRENT PROCEDURE. THE PIGEONS WERE PENALIZED FOR ALTERNATING IN RESPONSE BETWEEN THE TWO KEYS. THE RELATIVE FREQUENCY OF RESPONDING ON A GIVEN KEY CLOSELY MATCHED THE FREQUENCY OF REINFORCEMENT ON THAT KEY. THIS RELATIONSHIP HAS BEEN DESIGNATED THE 'MATCHING LAW.' THIS FORMULA IS DIAGRAMED AND EXPLAINED AS A METHOD FOR MEASURING SEXUAL ORIENTATION BY CALCULATING THE RELATIVE REINFORCEMENT MAGNITUDES OF QUALITATIVELY DIFFERENT STIMULI. FINALLY, SOLUTIONS TO THE PROBLEM OF FAKING ARE SUGGESTED, WITH EMPHASIS ON TECHNOLOGICAL DEVICES THAT CAN HELP TO RENDER THIS FORMULA A HIGHLY SOPHISTICATED MEASURE OF SEXUAL ORIENTATION. A LIST OF 33 REFERENCES IS PROVIDED.