Triple Differences

Changes in log wages (%) before and after a state mandate that raises costs of employing married women aged 20–40 (the affected group). A state-wide shock hits everyone in the treated state; a national trend hits the affected group everywhere. DiD for the affected group picks up the state shock; DDD subtracts it using other workers in the same state. Only a shock specific to the treated state and the affected group (the violation) biases DDD.