We investigate the effect of a donation incentive tied to contributions to a public good when group members can decide on the size of the donation to be made. An up to 20 % donation of the public good was implemented either exogenously or endogenously by group members. In the Vote treatment, groups could either decide in favor of or against a donation of 20 % of the public good; in the Vote Share treatment, subjects could decide on a donation share of between 0 % and 20 %. Results show that a large percentage of the participants vote in favor of implementing a donation share in both treatments. Voting in favor of a 20 % donation share or endogenously implementing a high donation share in the Vote Share treatment has positive effects on contributions to the public good compared to an exogenously implemented donation share.