In the social sciences, unintended consequences (sometimes unanticipated consequences or unforeseen consequences, more colloquially called knock-on effects) are outcomes of a purposeful action that are not intended or foreseen. The term was popularized in the 20th century by American sociologist Robert K. Merton.

Unintended consequences can be grouped into three types:

The idea of unintended consequences dates back at least to John Locke who discussed the unintended consequences of interest rate regulation in his letter to Sir John Somers, Member of Parliament.

The idea was also discussed by Adam Smith, the Scottish Enlightenment, and consequentialism (judging by results).

The invisible hand theorem is an example of the unintended consequences of agents acting in their self-interest. As Andrew S. Skinner puts it:

"The individual undertaker (entrepreneur), seeking the most efficient allocation of resources, contributes to overall economic efficiency; the merchant's reaction to price signals helps to ensure that the allocation of resources accurately reflects the structure of consumer preferences; and the drive to better our condition contributes to economic growth."

Influenced by 19th century positivism and Charles Darwin's evolution, for both Friedrich Engels and Karl Marx, the idea of uncertainty and chance in social dynamics (and thus unintended consequences beyond results of perfectly defined laws) was only apparent, (if not rejected) since social actions were directed and produced by deliberate human intention.

While discerning between the forces that generate changes in nature and those that generate changes in history in his discussion of Ludwig Feuerbach, Friedrich Engels touched on the idea of (apparent) unintended consequences: