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Unintended consequences refer to situations where an action results in an outcome that is not (or not only) what the actor intended. The unintended results may be foreseen or unforeseen, but they should be the logical or likely results of the action. One may class unintended consequences into roughly three types:

  • a positive unexpected benefit, usually referred to as serendipity or a windfall
  • a potential source of problems, according to Murphy's Law used in Systems Engineering
  • a negative or a perverse effect, which may be the opposite result of what is intended

Discussions of unintended consequences usually refer to the third situation of perverse results. This situation often arises because a policy has a perverse incentive and causes actions contrary to what is desired.

The Law of Unintended Consequences

The Law of Unintended Consequences has a feeble linguistic claim on the term "law". It is hardly a scientific law; even Murphy's law and natural law claim specific outcomes with some certainty. But this term persists as a solemn warning against certain disorder, that almost all human actions have at least one unintended consequence: "There shall be some unexpected result." In other words, each cause has more than one effect, and will include unforeseen effects. Less of a law or rule itself, it is more a call to decision makers to beware.


The idea dates to the Scottish Enlightenment and consequentialism, or judging by results. In the twentieth century, sociologist Robert K. Merton once again popularized the concept, sometimes referred to as the Law of Unforeseen Consequences. Merton (1936) spoke of the "unanticipated consequences" of "purposive social action", emphasizing 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" (p.895).


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 or other cognitive or emotional biases.

Robert K. Merton listed five causes of unanticipated consequences[1]:

  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)

Merton is also said to have stated that "no blanket statement categorically affirming or denying the practical feasibility of all social planning is warranted."


Of course, unintended consequences are common in everyday life, but many impact the greater society.

Examples of Unexpected Benefits:

  • Controversial research carried out by John J. Donohue and Steven Levitt and published in the Quarterly Journal of Economics suggests that legalized abortion in the United States can account for about 50% of the drop in national crime rates that occurred in the 1990s. As evidence, Donohue and Levitt cite the fact 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. Most convincingly, they found that "in high abortion states, only arrests of those born after abortion legalization fall relative to low abortion states."
  • In medicine, most drugs have unintended consequences associated with their use, which are known as 'side effects'. Many are harmful and are more precisely called 'adverse effects'. However, some are beneficial—for instance, aspirin, a pain reliever, can also thin the blood and help to prevent heart attacks. The existence of beneficial side effects also leads to off label use—prescription or use of a drug for a non-intended purpose.

Examples of Perverse Results:

  • The Streisand Effect occurs when an attempt to censor or remove a certain piece of information (such as photograph, file or website) instead causes the information in question to become widely known and distributed in a very short time. The fact that a piece of information is being restricted assigns to it a previously nonexistent value in the eyes of the public.
  • Standard economic theory implies that minimum wage laws increase unemployment among low wage workers (the workers whose wages the minimum wage law will affect). A survey of American Economic Association economists found that 45.6% fully agreed with the statement "a minimum wage increases unemployment among young and unskilled workers", 27.9% partially agreed, and 26.5% disagreed.[2]
  • 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 (Later, legislators stiffened penalties for leaving the scene of an accident when driving while intoxicated as well).
  • In 1990, driven by concern for the increasing number of cyclists' head injuries, the State of Victoria (Australia) made safety helmets mandatory for all bicycle riders. The expected significant reduction in the absolute number of head injuries occurred, but there was also a concomitant, entirely unexpected reduction in the number of juvenile cyclists. Research by Vulcan et al. found that the reduction in the number of juvenile cyclists was entirely because the youths considered wearing a bicycle helmet unfashionable.[How to reference and link to summary or text]
  • "Prohibition", in the 1920s U.S., 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. By the time the U.S. repealed Prohibition, the brewing industry had concentrated in a few major brewers, which had been able to ride it out. Sixty years later, the "War on Drugs," intended to suppress the illegal drug trade, likewise drove many small-time drug dealers out of business and consolidated the hold of organized drug cartels over the illegal drug industry. Additionally, it has led to the existence of street drugs of unknown strength and contamination; at least some drug-related (and particularly opiate-related) deaths are the result of accidental overdosing on drugs that a dealer neglected to dilute to the usual extent.

Failure mode and effects analysis

Failure mode and effects analysis (FMEA) is a fault tree method (first developed for systems engineering) that examines potential failures in products or processes. It may be used to evaluate risk management priorities for mitigating known threat-vulnerabilities.

FMEA helps select remedial actions that reduce cumulative impacts of life-cycle consequences (risks) from a systems failure (fault).[3]

Purposeful gaming to achieve unintended consequences

Another more restrictive use of the concept of unintended consequence is that it occurs when a mechanism that has been installed in the world with the intention of producing one result is used to produce a different (and often conflicting) result. "Gaming the system" illustrates this type of unintended consequence. One "games a system" (for example, the tax code) when one acts in such a way that one gains tax advantages by exploiting a tax rule that its drafters intended for some other purpose. Similarly, computer viruses, worms, and other such plagues are unintended consequences of the way certain computer systems are designed. Spam is an unintended consequence of the way the email system works. The preceding computer examples illustrate this sense of unintended consequence in that spammers hijack a mechanism, e.g., email, intended for interpersonal communication, for advertising.

This sense of unintended consequence excludes, for example, the proliferation of rabbits in Australia as an unintended consequence of their introduction. The proliferation of rabbits was indeed an unexpected (and unintended) consequence of their introduction, but it didn't result from the exploitation of a mechanism for some other purpose. It was just a consequence of a historical event. Many historical events have such unexpected (and unintended) consequences, but are not considered to exemplify this use of the term. The intent to "game the system" distinguishes this interpretation of unintended consequence from such a broader interpretation of unintended consequence as a result of simple historical contingency. See the Museum of Unintended Consequences for more examples.

Solution or mitigating actions

The concepts of interlock research and Information Routing Group were proposed to overcome the effects of this Law by ensuring that a spontaneously forming network of experts would feedback to the initiators of any proposed action or policy those consequences that the experts could foresee.

An associated concept was that of the Relevance Paradox, where an actor would seek out information that was obviously relevant, but that they did not see as relevant although it might well be.

See also


  1. Merton, Robert K. On Social Structure and Science. The University of Chicago Press, 1996.
  2. Fuller, Dan and Doris Geide-Stevenson (2003): Consensus Among Economists: Revisited, in: Journal of Economic Review, Vol. 34, No. 4, Seite 369-387 (PDF)
  3. Urban-wetland example showing unintended consequences (secondary and subsequent) of land-use zoning and flooding: Hazard Tree Analysis

External links

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