In the standard Voluntary Contribution Mechanism (Hichri 2004), a linear public goods game, the dominant strategy equilibrium is to donate zero; therefore, any error or noise will appear to be altruism. I avoid this ``corner critique'' (Laury and Holt 2008) by using a nonlinear tax system to get an interior Nash equilibrium. It is also possible to do this by means of a threshold for the good to be provided; however, this induces a multiplicity of equilibria. My method induces a unique, sharp dominant strategy equilibrium, in which a player's best response does not depend on any other players' choices. Furthermore, by using a differentiable payoff function, I can measure the marginal cost of deviation from the dominant strategy equilibrium, inferring a willingness-to-pay for other-regarding behavior. Using software that provides real-time feedback about payoffs, I will test whether participants contribute more than the dominant strategy equilibrium, under treatments that vary the payoffs and the inequality of endowments such that both the equilibrium and the Pareto optimum are different across each treatment condition.