Conditions for the optimality of simple majority decisions in pairwise choice situations

Recently it has been proved in a number of studies, that, under a proper set of assumptions, the optimal group decision rule in pairwise choice situations is a weighted majority rule, with weights that are proportional to the logarithms of the decision makers' odds of choosing the correct alternative. The purpose of the present note is to specify the necessary and sufficient conditions for this rule to coincide with the simple majority rule, and with restricted simple majority rules (which are defined as rules of simple majority, based on some subset of the most competent group members). These conditions are formulated in terms of inequalities between the group members' weights, thereby permitting easy verification of the optimality of the above mentioned rules.