It is now well established that people differ with respect to their tax compliance degree. In this laboratory experiment, we investigate whether individuals are willing to contribute, even in a small proportion, to a fund used to increase deterrence of evaders. This voluntary contribution scheme is composed of several experimental treatments that differ as to the degree of freedom left to contributors in their funding and in the levels of contributions in case they are predefined. We first find that a significant proportion of subjects contributes to this fund but less frequently when the cost of the contribution is increased. Secondly, contributors are characterized by a relatively higher compliance level, their reported income is above the average reported income of other group members. Thirdly, as expected the impact on the willingness to contribute is negative, respectively positive, for risk aversion and inequity aversion. Additionnally we have tested the impact of social norms as an additionnal potential explanation to contributions. We conclude that when: (i) the difference in declarations is large, (ii) the declaration made by the others is low, (iii) or when the situation of declarations is seen as inappropriate; the decision to contribute to the fund is seen as a very socially appropriate decision. Finally, looking at reported income after contribution decisions, estimation of difference-in-differences models highlights a strong causal effect: reported incomes are higher in groups in which the audit probability has increased thanks to individuals' contributions.