The strength of strict churches is neither a historical coicidence nor a statiscal artifact. Strictness makes organizations stronger an more attractive because it reduces free riding. It screens out members who lack commitment and and stimulates participation among those who remain. Rational choice theory thus explains the success of sect, cults, and conservative denominations without recourse to assumptions of irrationality, abnormality, or misinformation. The theory also predicts differences between strict and lenient groups, distinguishes between effective and counterproductive demands, and demonstrates the need to adapt strict demands in response to social change.