Are discriminatory procurement policies motivated by protectionism?

When purchasing goods and services, governments often discriminate in favour of domestic suppliers. It is widely assumed that such behaviour is motivated by protectionism. Although this interpretation is sometimes valid, it is also puzzling. After reviewing some of the puzzles, the paper proposes an alternative explanation of preferential procurement based on the assumption that governmental buyers want to purchase goods and services at minimum cost, but must do this in a context in which, because of the presence of unverifiable services, contracts are necessarily incomplete. The paper argues that preferential purchasing can guarantee the efficient delivery of these unverifiable services.