We provide an elementary proof showing how in economies with an arbitrary number of agents an arbitrary number of public goods and quasi-linear utility functions, any efficient and individuaHy rational mechanism is not strategy-proof for any economy satisfyingWe provide an elementary proof showing how in economies with an arbitrary number of agents an arbitrary number of public goods and quasi-linear utility functions, any efficient and individuaHy rational mechanism is not strategy-proof for any economy satisfying a mild regularity requirement.[+][-]