Conventional economics assumes workers provide the same effort under penalty contracts and economically equivalent bonus contracts. However, prior research finds that although workers prefer bonus contracts, they provide more effort under penalty contracts. Given these findings, the prevalence of bonus contracts in practice is puzzling. If penalty contracts yield more worker effort, why would employers not use them more often? We conduct experimental labor markets to test whether the prior finding of more effort under penalty contracts than bonus contracts (i.e., the contract frame effect) persists when workers can choose their contract and know that their employer intentionally offered the contract they choose. As predicted, these features of labor markets eliminate the difference in effort between penalty and bonus contracts reported in prior studies. This finding suggests employers may use bonus contracts more often than penalty contracts because they can offer the contract most workers prefer without sacrificing worker effort.