An unintended consequence comes about 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. The notion of "gaming the system" illustrates the idea of an 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 was intended for some other purpose. Similarly, computer viruses, worms, and other such plagues are unintended consequences of the way certain computer systems are designed. And spam is an unintended consequence of the way the email system works.

It's important to distinguish between unintended consequences in this sense and simple historical contingencies. It would not be appropriate to characterize—as this page does below—a negative side effect of a drug as an unintended consequence. It certainly is true that negative side effects are consequences that were not intended. But much of what happens in the world is not (directly) intended. The term unintended consequence should be reserved for the exploitation of a deliberately designed and deployed mechanism to produce an effect that the mechanism is capable of producing but which it was not intended to produce. See the Museum of Unintended Consequences for more examples.

Much of the rest of this page takes a broader view of unintended consequence and would (inappropriately) apply the term to virtually any (unexpected) historical contingency.

Unintended consequences, or situations where the final outcome is unexpected, can be classed into roughly three groups:

• positive, usually referred to as serendipity or a windfall
• a source of problems, according to Murphy's law
• definitively negative or a perverse effect, which is the opposite result to the one intended

The Law of Unintended Consequences holds that almost all human actions have at least one unintended consequence. In other words, each cause has more than one effect, including unforeseen effects. The idea dates to the Scottish Enlightenment, which influenced people such as Thomas Jefferson.

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… exclusively concerned with "conduct" as distinct from "behavior". That is, with action which involves motives and consequently a choice between various alternatives" (p.895).

Causes

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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. Ignorance (It is impossible to anticipate everything)
2. Error (Incomplete 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 also stated that "no blanket statement categorically affirming or denying the practical feasibility of all social planning is warranted."

Examples

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Of course, unintended consequences are common in everybody's life, but some can impact the greater society. Here are some examples:

• In medicine, most drugs have unintended consequences associated with their use, which are known as "side effects," or more precisely, "adverse effects". As with other unintended consequences, these side effects are often negative, but 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.)

• The introduction of rabbits into Australia for sport led to an explosive growth in the rabbit population; rabbits became a major feral pest in Australia.

• Driven by concern for the increasing number of cyclist’s head injuries, the State of Victoria (Australia) legislated to make safety helmets mandatory for all bicycle riders in 1990. Whilst the expected significant reduction in the absolute number of head injuries was certainly observed, 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 due to the fact that wearing a bicycle helmet was not considered to be "cool".

• "Prohibition" in the 1920s U.S., originally intended 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. When Prohibition was repealed, the brewing industry was then concentrated in a few major brewers who were able to ride it out. It would be fair to say that quality of American beer has never fully recovered. Sixty years later, the "War on Drugs," intended to suppress the illegal drug trade, has likewise driven 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 caused by accidental overdosing on drugs which a dealer neglected to dilute to the usual extent.

• The medieval policy of setting up large hunting reserves for the nobility has preserved green space, often as parks, throughout England and other places in Europe.

• The wartime practice of sinking ships in shallow waters has created some artificial coral reefs.

• In CIA jargon, "blowback" describes the unintended, even undesirable consequences of covert operations. Examples include:
  • Operation Ajax, which contributed to the 1979 Iranian Revolution & the Iran hostage crisis
  • Covert funding of the Contras in Nicaragua, which lead to the Iran-Contra Affair

• Numerous attempts by governments to reduce rent by introducing rent controls has led to the unintended consequence of property shortages and reduction in quality housing, increased difficulty for less desirable renters to even get or retain housing and even to the creation of slums—areas where rental property is not properly maintained.

• 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 has accounted for as much as 50% of the drop in national crime rates. As evidence, Donohue and Levitt cite the fact that states which legalized abortion before Roe v. Wade saw correspondingly earlier drops in crime, and that 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."

• Also controversially, it has been suggested that legalized abortion has led to fewer so-called 'crack babies'—children born with a drug dependency due to their mothers' drug use while pregnant. The availability of abortion means that a drug-addicted mother has the option to abort a fetus that her drug use is likely to render unhealthy, even if this consequence is not a primary reason for her choice to abort. (Note: the theory that fetuses exposed to crack cocaine would become inferior human beings prone to violence, crime or addiction has since been proven false. Crack babies perform as well as their age cohorts and show no additional tendency to violence, crime or addiction. Babies with fetal alcohol syndrome, exposed to alcohol during the first trimester of life, do show significant mental delay, tendency to violence and crime, and a tendency toward drug and alcohol abuse.)

• Gun control, perhaps the most controversial of all, is frequently cited as the reason that places such as Washington DC and New Jersey have experienced unexpected increases in crime. One possible reason for the effect is that the enactment of gun control laws leave citizens vulnerable to criminals who do not respect the prohibitions on gun ownership, thereby making crime a much safer occupation. Conversely, places such as Switzerland and Vermont are cited as examples of places where weapons are common (and, importantly, equally distributed), yet the crime rates are extraordinarily low.

See also

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• Counter-intuitive
• Hutber's law
• Kludge
• Moral hazard
• Murphy's law
• Nocebo
• Perverse effects of vaccination
• Perverse incentive
• Placebo
• Placebo (origins of technical term)
• Regression testing
• Side effect
• Unforeseen effects of species introduction

References

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• http://cs.calstatela.edu/wiki/index.php/Courses/CS_461/Museum_of_unintended_consequences Museum of Unintended Consequences]
• The Unanticipated Consequences of Purposive Social Action by Robert K. Merton, American Sociological Review, Vol 1 Issue 6, Dec 1936, pp.894-904
• Atlantic magazine article: "Blowback"
• Observer article: Why 'blowback' is the hidden danger of war
• MSNBC article on Bin Laden and blowback
• 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.

Adages

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