 Take the classic account of the efficacy of interdemic or group selection, the case that even G.C. Williams acknowledged was hierarchical selection. Lewontin and Dunn (Lewontin &amp; Dunn 1960 and Lewontin 1962), in investigating the house mouse, found first, that there was segregation distortion, in that over 80% of the sperm from mice heterozygous for the t-allele also carried the t-allele, whereas the expected rate would be 50%. Second, they also found that male homozygotes (those with two t-alleles) tended to be sterile. (In several populations the t-alleles were homozygous lethal, but in the populations in question, homozygous males were sterile.) Third, they also found a substantial effect of group extinction based on the fact that female mice would often find themselves in groups in which all males were sterile, and the group itself would therefore go extinct. This, then, is how a genuine and empirically robust hierarchical model was developed.