In the social sciences, unintended consequences (sometimes unanticipated consequences or unforeseen consequences) are outcomes of a purposeful action that are not intended or foreseen. The term was popularised in the twentieth century by American sociologist Robert K. Merton.

Unintended consequences can be grouped into three types:

• Unexpected benefit: A positive unexpected benefit (also referred to as luck, serendipity or a windfall).
• Unexpected drawback: An unexpected detriment occurring in addition to the desired effect of the policy (e.g., while irrigation schemes provide people with water for agriculture, they can increase waterborne diseases that have devastating health effects, such as schistosomiasis).
• Perverse result: A perverse effect contrary to what was originally intended (when an intended solution makes a problem worse).