“Which test provides the better measurement of intelligence, the Wechsler Adult Intelligence Scale-Third Edition (WAIS-III) or the Wechsler Adult Intelligence Scale-Fourth Edition (WAIS-IV)?” is an important question to professional psychologists; however, it has become a critical issue in Atkins cases wherein courts are often presented with divergent Full-Scale IQ (FSIQ) scores on the WAIS-III and WAIS-IV. In these instances, courts are required to render a decision stating which test provided the better measure of an inmate’s intellectual functioning. This study employed structural equation modeling to empirically determine which instrument; the WAIS-III or the WAIS-IV, provides the better measure of intelligence via the FSIQ score. Consistent with the publisher’s representation of intellectual functioning, the results from this study indicate the WAIS-IV provides superior measurement, scoring, and structural models to measure FSIQ when compared to the WAIS-III.