'Warm glow' has been proposed as an explanation for public good contributions that exceed traditional theoretical predictions, yet little is known about why and when people exhibit warm glow in some voluntary settings. To investigate these issues, this research develops a model for the ``helping hand" hypothesis as an extension of warm glow. The hypothesis asserts that when an external environment faced by the subject seems not to provide a socially optimal level of the public good (non-incentive compatible), the subject, to some degree, gains utility by undertaking socially responsible behavior (offering a helping hand), and thus she over-contributes. Once the mechanism is established to be incentive compatible, the individual no longer offers a helping hand, but instead concentrates on maximizing her personal payoffs as predicted by the Nash equilibrium. Experimental results support the helping hand hypothesis, and show that contributions depend on the efficiency of the mechanism and not whether it is voluntary. We also find that contributions are positively correlated with an induced value of the public good even when free-riding is a dominant strategy in an non-incentive compatible mechanism. This would suggest that people's social preferences depend on an induced value of the public good and possess an efficiency concern.