Unintended consequences

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 the American sociologistRobert K. Merton.

Unintended consequences can be grouped into three types:

• Unexpected benefit: an unexpected benefit (also known as serendipity).
• Unexpected drawback: an unexpected detrimental effect, occurring in addition to the desired one. For example, while irrigation schemes may provide farmers with water for agriculture, they may also lead to a rise in the prevalence of waterborne diseases such as schistosomiasis.
• Perverse result: an effect contrary to what was originally intended. For example, while irrigation schemes intended to increase yields may improve water availability and increase agricultural yields in the short term, they might also lead to soil salination and the depletion of freshwater resources, decreasing yields in the long term.