The instrumental variable method relies on a strong no-deers condition, which requires that the instrument aect every subject's treatment decision in the same direction. This paper shows that no-deers can be replaced by a weaker compliersdeers condition, which requires that a subgroup of compliers have the same size and the same distribution of potential outcomes as deers. This condition is necessary and sucient for IV to capture causal eects for the remaining part of compliers. In many applications, compliers-deers is a very weak condition. For instance, in Angrist & Evans (1998), 94% of DGPs compatible with the data satisfy compliers-deers, while 0% satisfy no-deers.