Law of Unintended Consequence

In the social sciences, unintended consequences are outcomes that are not the outcomes intended by a particular action. The unintended outcomes may be positive or negative. The concept has long existed but was named and popularized in the 20th century by the American sociologist, Robert K. Merton.

The law of unintended consequences is an adage or idiomatic warning that an intervention in a complex system always creates unanticipated and often undesirable outcomes. Akin to Murphy's law, it is commonly used as a wry or humorous warning against the hubristic belief that humans can fully control the world around them. Many fields of study in the sciences and humanities embrace this concept, including economics, history, philosophy, political science, and sociology.

Unintended consequences can be roughly grouped into three types:
- A positive, unexpected benefit (usually referred to as serendipity or a windfall).
- A negative, 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).
- A perverse effect contrary to what was originally intended (when an intended solution makes a problem worse), such as when a policy has a perverse incentive that causes actions opposite to what was intended.