Genetic variation at microsatellite loci is supposed to be constrained within some range in allele size. In this case, the average-square distance (delta mu)2 between two diverged populations moves asymptotically around and underestimates the time since the populations had split. A distance based on the between-locus correlation in the mean repeat scores, DR, is introduced. Numerical simulations show that DR is a linear function of time if the constraints are approximated by a linear centripetal force, which might be due to mutation bias toward a definite range or be caused both by directional mutation bias toward larger allele size and by selection against the greater number of repeats.