Holzinger's Hc revised.

The formula H'C = 1 - CDZ/CMZ is suggested as a better summary of twin concordance data than the familiar Holzinger concordance formula, HC = (CMZ - CDZ)/(1 - CDZ). The new formula better estimates degree of genetic determination, G, as calculated from a threshold model, but never exceeds unity, as G sometimes does. For high concordance rates, if CMZ + CDZ greater than 1, HC may be more useful than either G or H'C.