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Murphy's law (also known as Finagle's law or Sod's law) is a popular adage in Western culture, which broadly states that things will go wrong in any given situation. It is most commonly formulated as "Anything that can go wrong will go wrong." The law was named after Edward A. Murphy, Jr., a development engineer working for a brief time on rocket sled experiments done by the United States Air Force in 1949.
The letter of the law
Accounts differ as to the precise origin of Murphy's law and the details about how it was initially formulated. From 1947 to 1949, a project known as MX981 took place on Muroc Field (later renamed Edwards Air Force Base) for the purpose of testing the human tolerance for g-forces during rapid deceleration. The tests used a rocket sled mounted on a railroad track with a series of hydraulic brakes at the end.
Initial tests used a humanoid crash test dummy strapped to a seat on the sled, but subsequent tests were performed by John Paul Stapp, then a Captain. During the tests, questions were raised about the accuracy of the instrumentation used to measure the g-forces Captain Stapp was experiencing. Edward Murphy proposed using electronic strain gauges attached to the restraining clamps of Stapp's harness to measure the force exerted on them by his rapid deceleration. Murphy's assistant wired the harness, and a trial was run using a chimpanzee.
The sensors provided a zero reading, however; it became apparent that they had been installed incorrectly, with each sensor wired backwards. It was at this point that Murphy made his pronouncement. According to George Nichols , another engineer who was present, Murphy, in frustration, blamed the failure on his assistant, saying "If that guy has any way of making a mistake, he will". Nichols' account is that "Murphy's law" came about through conversation among the other members of the team; it was condensed to "If it can happen, it will happen", and named for Murphy in mockery of what Nichols perceived as arrogance on Murphy's part. Others, including Edward Murphy's surviving son Robert Murphy, deny Nichols' account, and claim that the phrase did originate with Edward Murphy. According to Robert Murphy's account, his father's statement was along the lines of "If there's more than one way to do a job, and one of those ways will result in disaster, then somebody will do it that way".
In any case, the phrase first received public attention during a press conference in which Stapp was asked how it was that nobody had been severely injured during the rocket sled tests. Stapp replied that it was because they took Murphy's Law under consideration; he then summarized the law and said that in general, it meant that it was important to consider all the possibilities before doing a test.
Murphy's law has taken on many different formulations. In 1952, the proverb was phrased "Anything That Can Possibly Go Wrong, Does" in the epigraph of John Sack's The Butcher: The Ascent of Yerupaja. Possibly the earliest printed use of Murphy's name in connection with the law is in Lloyd Mallan's 1955 book, Men, Rockets and Space Rats: "Colonel Stapp's favorite takeoff on sober scientific laws—Murphy's Law, Stapp calls it—'Everything that can possibly go wrong will go wrong'".
Yet another formulation can be found in the Jargon File, which states that the "correct, original" law is "If there are two or more ways to do something, and one of those ways can result in a catastrophe, then someone will do it." 
The spirit of the law
Regardless of the exact composition and origin of the phrase, its spirit embodies the principle of defensive design — anticipating the mistakes the end-user is likely to make. Murphy's g-force sensors failed because there existed two different ways to connect them; one way would result in correct readings, while the other would result in no readings at all. The end-user — Murphy's assistant, in the historical account — had a choice to make when connecting the wires. When the wrong choice was made, the sensors did not do their job properly.
In most well-designed technology intended for use by the average consumer, incorrect connections are made difficult. For example, the 3.5-inch floppy disk used in many personal computers will not easily fit into the drive unless it is oriented correctly. In contrast, the older 5.25-inch floppy disk could be inserted in a variety of orientations that might damage the disk or drive. The newer CD-ROM technology permits one incorrect orientation — the disc may be inserted upside-down. A defensive designer knows that if it is possible for the disc to be inserted the wrong way, someone will eventually try it.
From its initial public announcement, Murphy's law quickly spread to various technical cultures connected to aerospace engineering. Before long, variants had passed into the popular imagination, changing as they went. Generally, the spirit of Murphy's law captures the common tendency to emphasize the negative things that occur in everyday life; in this sense, the law is typically formulated as some variant of "If anything can go wrong, it will", a variant often known as Finagle's law or Sod's law (chiefly British). An often-quoted example of this tendency to emphasize negatives is that whenever a buttered slice of bread falls on the floor, people tend to remember more vividly the times that it fell buttered-side-down, since a buttered-side-up landing is of lesser consequence. Hence, one gets the impression that the bread always falls buttered-side-down, regardless of the actual probability of either happening. Laws such as Murphy's are a direct expression of such seeming perversities in the order of the universe.
Additional mutations of the law and its corollaries have developed, many of them meta-laws in some way, either through some form of self-reference or referral to other laws or analogies. For instance, the buttered-bread analogy could be further extended: "The chance of a dropped slice of bread landing buttered-side down on a new carpet is proportional to the price of the carpet". (If the buttered side falls facing up, then obviously the wrong side is buttered.) A further example is Murphy's Ultimate Corollary: "If it could have gone wrong earlier and it did not, it ultimately would have been beneficial for it to have". John Gall's systemantics offers further expansion of Murphy's law.
"Laws" can occasionally be found to lead to a paradox, or which have positive outcomes; for example: when a cat is dropped from above a certain height, it will always land on its feet. In almost a canonical example of the hackish love for wordplay and cultural in-jokes, it has been noted that, therefore, if you strap a piece of buttered toast to the back of a cat, butter side up, and drop the cat out a window, it will fall to approximately a foot above the street, and hover there, spinning.
Some state that Murphy's law cannot operate as a subset of something useful; for example: "It will start raining as soon as I start washing my car, except when I wash the car for the purpose of causing rain". O'Toole's commentary on Murphy's law is: "Murphy was an optimist!" These mutant versions demonstrate Murphy's law acting on itself, or perhaps Finagle's law acting on Murphy's law.
Author Arthur Bloch has compiled a number of books full of corollaries to Murphy's law and variations thereof. These include the original Murphy's Law, which is very general in scope, and the domain-specific volumes, Murphy's Law: Doctors: Malpractice Makes Perfect and Murphy's Law: Lawyers: Wronging the Rights in the Legal Profession!.
- Finagle's law
- Hanlon's razor
- Sturgeon's law
- Laws of infernal dynamics
- unintended consequence
- Peter principle (Laurence J. Peter is said to have interviewed Edward A. Murphy around the time of writing the books in which he proposed the Peter Principles)
- Adages named after people
- Jargon File
- The Annals of Improbable Research tracks down the origins of Murphy's law
- The Ultimate Collection of Murphy's Laws - a website on Murphy's Law
- The House of Murphy - a website on Murphy's Law
- Mathematical formula for Murphy's Law
- Examples of the mathematical formula for Murphy's Law
- 1952 proverb citation
- 1955 term citation of phrase "Murphy's Law"
- Reference to 1941 citation of the proverb
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