The incidence of bilateral nonmetric skeletal traits: a reanalysis of sampling procedures.

For reporting the incidence of bilateral skeletal traits, the choice between sampling statistics depends upon more than their relative efficiencies. Of overreaching importance are the fundamentally different assumptions about the genetic significance of bilateral asymmetry represented by the two principal sampling approaches. Sampling by side is consistent with the premise that trait expression on each side reflects an additive component of genetic variation. Implicit in sampling by individual, by contrast, is the proposition that asymmetries of expression result chiefly from developmental noise. The pattern of age-regression indicated for many of these traits, suggesting a transient developmental role for unilateral expression, supports both this latter view and the thesis of stress asymmetry. Given this pattern, furthermore, incorporating unadjusted trait frequencies into divergence statistics would seem injudicious.