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In history and the social sciences, unintended consequences are outcomes that are not (or not limited to) the results originally intended by a particular action. The unintended results may be foreseen or unforeseen, but they should be the logical or likely results of the action. For example, some historians have speculated that if the Treaty of Versailles had not imposed such humiliating conditions on Germany, World War II would not have occurred. From this perspective, one might consider the war an unintended consequence of the treaty. The concept has long existed but was popularised in the 20th century by the American sociologist, Robert K. Merton.

Unintended consequences can be grouped into roughly three types:
  • a positive unexpected benefit, usually referred to as serendipity or a windfall
  • perverse effect, that may be contrary to what was originally intended,. This situation can arise when a policy has a perverse incentive and causes actions contrary to what is desired.
  • a negative effect, where although the original policy had the desired effect - eg irrigation schemes do provide people with water for agriculture, they often increase water born disease which can a have a devastating negative health effect, such as Schistosomiasis.

Sometimes unintended consequences can far outweigh the intended effect; cane toads introduced into Australia to control insect pests failed to control the pests, while the toads became a major pest in their own right. Joseph Bazalgette introduced sewers into London, England in the nineteenth century to eliminate The Great Stink, which was thought to cause disease; in fact the sewers stopped contamination of the water supply and eliminated cholera—the stench, while relieved, was irrelevant.

The "law" of unintended consequences

The so called "law of unintended consequences" (or "law of unforeseen consequences") is not a true Scientific law such as Ohm's Law, but a humorous expression in common use according to which any purposeful action will produce some unintended, unanticipated, and usually unwanted consequences. Stated in other words, each cause has more than one effect, and these effects will invariably include at least one unforeseen side-effect. The unintended side-effect can be more significant than the intended effect.

Like Murphy's law, again a humorous expression rather than an actual law of nature, this law is a warning against the hubristic belief that humans can fully control the world around them.


The idea of unintended consequences dates back at least to Adam Smith, the Scottish Enlightenment, and consequentialism (judging by results). However, it was the sociologist Robert K. Merton who popularized this concept in the twentieth century.

In his 1936 paper, "The Unanticipated Consequences of Purposive Social Action", Merton tried to apply a systematic analysis to the problem of "unanticipated consequences" of "purposive social action". He emphasized that his term "purposive action… [is exclusively] concerned with 'conduct' as distinct from 'behavior.' That is, with action that involves motives and consequently a choice between various alternatives". Merton also stated that "no blanket statement categorically affirming or denying the practical feasibility of all social planning is warranted."


Possible causes of unintended consequences include the world's inherent complexity (parts of a system responding to changes in the environment), perverse incentives, human stupidity, self-deception, failure to account for human nature or other cognitive or emotional biases. As a sub-component of complexity (in the scientific sense), the chaotic nature of the universe – and especially its quality of having small, apparently insignificant changes with far-reaching effects (e.g., the Butterfly effect) – applies.

Robert K. Merton listed five possible causes of unanticipated consequences:

  1. Ignorance (It is impossible to anticipate everything, thereby leading to incomplete analysis)
  2. Error (Incorrect analysis of the problem or following habits that worked in the past but may not apply to the current situation)
  3. Immediate interest, which may override long-term interests
  4. Basic values may require or prohibit certain actions even if the long-term result might be unfavorable (these long-term consequences may eventually cause changes in basic values)
  5. Self-defeating prophecy (Fear of some consequence drives people to find solutions before the problem occurs, thus the non-occurrence of the problem is unanticipated)

The Relevance paradox where decision makers think they know the areas of ignorance about an issue, and go and obtain the necessary information to fill the ignorance, but neglect certain other areas of ignorance, because, due to not having the information, its relevance is not obvious, is also cited as a cause.


Unexpected benefits

  • The sinking of ships in shallow waters during wartime has created many artificial coral reefs: beautiful, scientifically valuable and an attraction for recreational divers.

  • Controversial research by John J. Donohue and Steven Levitt suggests that legalized abortion in the United States can account for much of the drop in crime rates that occurred in the 1990s. Their paper, The Impact of Legalized Abortion on Crime, presents evidence that states that legalized abortion before Roe v. Wade saw correspondingly earlier drops in crime; and states where abortion is common saw greater drops in crime than states where abortion is rare.

  • In medicine, most drugs have unintended consequences associated with their use, which are known as 'side effects'. However, some are beneficial—for instance, aspirin, a pain reliever, is also an anticoagulant that can help to prevent heart attacks as well as reduce the severity and damage from thrombotic strokes., The existence of beneficial side effects also leads to off-label use — prescription or use of a drug for a non-intended purpose.

Perverse results

  • The term Streisand Effect is applied to the internet phenomenon that occurs when an attempt to censor or remove a certain piece of information (such as a photograph, document, etc.) instead causes the information to become widely known and distributed. The fact that a piece of information is being restricted assigns to it a previously nonexistent value in the eyes of the public.

  • The introduction of exotic animals and plants for food, for decorative purposes, or to control unwanted species often leads to more harm than good done by the introduced species.
    • The introduction of rabbits in Australia and New Zealandmarker for food was followed by an explosive growth in the rabbit population; rabbits have become a major feral pest in these countries.
    • Cane toads, introduced into Australia to control canefield pests failed to do that and have become a major pest in their own right.
    • Kudzu, introduced as an ornamental plant and later used to prevent erosion in earthworks, has become a major problem in the South Eastern United States. Kudzu has displaced native plants, and has effectively taken over significant portions of land.

  • The stiffening of penalties for driving while intoxicated in the United States in the 1980s led, at first, to an increase in hit and run accidents, most of which were believed to have been drunken drivers trying to escape the law. Legislators later stiffened penalties for leaving the scene of an accident.

  • In 1990, the Australian state of Victoriamarker made safety helmets mandatory for all bicycle riders. While there was a reduction in the number of head injuries, there was also an unintended reduction in the number of juvenile cyclists – fewer cyclists obviously leads to fewer injuries, all else being equal. Research by Vulcan et al. found that the reduction in juvenile cyclists was because the youths considered wearing a bicycle helmet unfashionable. A health benefit model developed at Macquarie University in Sydney suggests that helmets laws are counterproductive in terms of net health, with net health costs of 0.4 to 5 billions of US dollars per year in Australia, Britain, The Netherlands, and the USA .

  • Prohibition in the 1920s United States, originally enacted to suppress the alcohol trade, drove many small-time alcohol suppliers out of business and consolidated the hold of large-scale organized crime over the illegal alcohol industry. Since alcohol was still popular, criminal organizations producing alcohol were well funded and hence also increased their other activities. The War on Drugs, intended to suppress the illegal drug trade, has likewise consolidated the hold of organized drug cartels over the illegal drug industry.

  • Father Mathew's temperance campaign in 19th-century Irelandmarker – in which thousands of people vowed never to drink alcohol again – led to the consumption of ether, a much more dangerous intoxicant, by those unwilling to break their pledge.

  • Rent control in many major cities is intended to make housing more accessible for lower income tenants. Instead, it creates a housing shortage, making housing less accessible. Landlords often circumvent the rent control by extracting bribes in the form of key money.


A related idea is that of emergence, where complex patterns arise out of simple causes not intended to produce them; for example stock markets, where people buy and sell with the intention of making a profit, control the value of companies.

See also


  1. Robert K. Merton, Versatile Sociologist and Father of the Focus Group, Dies at 92, Michael T. Kaufman, New York Times.
  2. Library of Economics and Liberty:Unintended Consequences, Rob Norton
  3. Renowned Columbia Sociologist and National Medal of Science Winner Robert K. Merton Dies at 92 Columbia News
  4. Robert K. Merton Remembered Footnotes, American Sociological Association
  5. Robert K. Merton, Versatile Sociologist and Father of the Focus Group, Dies at 92, Michael T. Kaufman, New York Times.
  6. Merton, Robert K. On Social Structure and Science. The University of Chicago Press, 1996.
  12. BBC 15 February 2001, Aspirin heart warning
  13. Canton, David. "Today's Business Law: Attempt to suppress can backfire", London Free Press, November 5, 2005. Retrieved July 21, 2007. The "Streisand effect" is what happens when someone tries to suppress something and the opposite occurs. The act of suppressing it raises the profile, making it much more well known than it ever would have been".
  14. Evaluating the Health Benefit of Mandatory Bicycle Helmet Laws, Piet De Jong, Macquarie University - Actuarial Studies, 26 October 2009
  15. Principles of Economics, by Gregory Mankiw. 4th edition. Chapter 2, Page 31. Chart shows that 93% of economists agree with the statement: "a ceiling on rents reduces the quality and quantity of housing available"


  • The Unanticipated Consequences of Purposive Social Action by Robert K. Merton, American Sociological Review, Vol 1 Issue 6, Dec 1936, pp.894-904
  • Unintended Consequences entry in Concise Encyclopedia of Economics
  • Museum of Unintended Consequences
  • Edward Tenner, Why Things Bite Back: Technology and the Revenge of Unintended Consequences, Vantage Books, 1997.
  • Tomislav V. Kovandzic, John Sloan III, and Lynne M. Vieraitis. Unintended Consequences of Politically Popular Sentencing Policy: The Homicide-Promoting Effects of 'Three Strikes' in U.S. Cities (1980-1999). Criminology & Public Policy, Vol 1, Issue 3, July 2002.
  • Vulcan, A.P., Cameron, M.H. & Heiman, L., "Evaluation of mandatory bicycle helmet use in Victoria, Australia", 36th Annual Conference Proceedings, Association for the Advancement of Automotive Medicine, Oct 5-7, 1992.
  • Vulcan, A.P., Cameron, M.H. & Watson, W.L., "Mandatory Bicycle Helmet Use: Experience in Victoria, Australia", World Journal of Surgery, Vol.16, No.3, (May/June 1992), pp.389-397.

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