Focused power: Experiments, the Shapley-Shubik power index, and focal points

Experiments evaluate the fit of human behaviour to the Shapley-Shubik power index (SSPI), a formula of voter power. Groups of six subjects with differing votes divide a fixed purse by majority rule in online chat rooms. Earnings proxy for measured power. Chat rooms and processes for selecting subjects reduce or eliminate extraneous forces. Logrolling remains as the primary political force. Subjects' initial proposals for division of the purse allow measurement of effects from focal points and transaction costs. Divisions of purses, net of those effects, closely fit the SSPI, averaging 1.033 of their SSPI values. The SSPI can serve as a control for power imbedded in voting blocs, permitting fuller analysis of other factors that affect political outcomes.