All psychological and psychophysiological traits vary, often widely, about their stable set-point values, due to transitory environmental influences. Because it is this stable set-point that embodies the genetically determined component of the trait, twin and family data based on one-time trait measurements must underestimate true trait heritability. The means of multiple measurements, taken months or years apart, then correlated within pairs of monozygotic twins, would yield an accurate estimate of the broad heritability of the set-point value, but such data are rare and expensive. Given just two measurements sufficiently far apart, the cross-twin cross-time correlation (RCT), divided by the retest or within-twin cross-time correlation (RWT), provides a valid estimate of set-point heritability. This article examines data from young and middle-aged twins who were tested twice, 3 or more years apart, on heart rate and blood pressure, personality traits, self-rating items, occupational and recreational interests, as well as on Wechsler Intelligence Scales. In every case, the disattenuated RCT revealed substantially higher heritability than indicated by correlations based on single measurements.