Famous Laws and Principles, Explained

“What’s the name of the rule where work expands to fill the time?” “What’s the law about the simplest explanation?” Many useful rules of thumb are named after the person who stated them. Here are the best-known ones, with their usual wording, where they come from and a link to a full explanation where we have one.

A note on the word “law”: almost none of these are scientific laws. They are observations, jokes with a serious point, or heuristics that hold often enough to be worth remembering.

Jump to: Planning & projects · Organizations & metrics · Thinking & reasoning · Technology · Economics & behavior

Planning and projects

Murphy’s Law

“Anything that can go wrong will go wrong.” Named after US Air Force engineer Edward A. Murphy Jr. and rocket-sled tests at Edwards Air Force Base around 1949; popularized by the project’s physician John Paul Stapp. Engineers use it as a design principle: make mistakes impossible rather than hoping no one makes them.

Parkinson’s Law

“Work expands so as to fill the time available for its completion.” The opening line of a satirical 1955 essay in The Economist by naval historian C. Northcote Parkinson. Practical lesson: set deliberate, realistic deadlines.

Hofstadter’s Law

“It always takes longer than you expect, even when you take into account Hofstadter’s Law.” From Douglas Hofstadter’s Gödel, Escher, Bach (1979). Psychologists call the underlying bias the planning fallacy.

Brooks’s Law

“Adding manpower to a late software project makes it later.” Fred Brooks, The Mythical Man-Month (1975). New people need training and add communication overhead.

Gall’s Law

“A complex system that works is invariably found to have evolved from a simple system that worked.” John Gall, Systemantics (1975). Start simple, then grow – a cousin of KISS and make it work, make it right, make it fast.

Organizations, management and metrics

Goodhart’s Law

“When a measure becomes a target, it ceases to be a good measure.” Economist Charles Goodhart described the effect in 1975; this popular wording is anthropologist Marilyn Strathern’s. The counterweight to “if you can’t measure it, you can’t improve it”.

Campbell’s Law

The more a quantitative indicator is used for social decision-making, the more it will be corrupted and the more it will distort the process it is meant to monitor. Social scientist Donald T. Campbell, 1976 – a close relative of Goodhart’s Law, often cited about standardized testing.

Conway’s Law

Organizations design systems that mirror their own communication structure. Melvin Conway, “How Do Committees Invent?” (1968). As the hacker Jargon File later summarized it: if four groups work on a compiler, you get a four-pass compiler.

The Peter Principle

“In a hierarchy every employee tends to rise to his level of incompetence.” Laurence J. Peter and Raymond Hull, The Peter Principle (1969). People are promoted for doing their current job well until they reach a job they do not do well – and stay there.

The Pareto Principle (80/20 rule)

A large share of effects often comes from a small share of causes. Named by quality expert Joseph Juran after economist Vilfredo Pareto, who observed that about 80% of land in Italy was owned by about 20% of the population. The exact ratio varies.

Thinking and reasoning

Occam’s Razor

Among explanations that fit the evidence equally well, prefer the one with the fewest assumptions. Named after the 14th-century philosopher William of Ockham. Often simplified to “the simplest explanation is usually correct”, which is a rule of thumb, not a guarantee.

Hanlon’s Razor

“Never attribute to malice that which is adequately explained by stupidity.” Credited to Robert J. Hanlon in Murphy’s Law Book Two (1980). A kinder reading: assume mistakes before bad intent.

Sturgeon’s Law

“Ninety percent of everything is crap.” Science-fiction author Theodore Sturgeon, defending his genre in the late 1950s: most work in any field is mediocre, so judge a field by its best. See also quality over quantity.

Betteridge’s Law of Headlines

“Any headline that ends in a question mark can be answered by the word no.” Technology journalist Ian Betteridge, 2009. A reminder that question headlines often hide weak evidence.

The Dunning–Kruger Effect

People with low skill in an area tend to overestimate their ability. From a 1999 paper by psychologists David Dunning and Justin Kruger. Later research debates how much of the effect is a statistical artifact, so treat popular charts of it with caution.

Technology and software

Moore’s Law

The number of transistors on a chip doubles about every two years. Gordon Moore predicted yearly doubling in 1965 and revised it to every two years in 1975. It is an industry trend, not a law of physics, and has slowed in recent years.

Postel’s Law (the robustness principle)

“Be conservative in what you do, be liberal in what you accept from others.” Jon Postel, in the 1980 specifications for TCP. It helped early internet systems work together; modern security practice is more cautious about accepting malformed input.

Linus’s Law

“Given enough eyeballs, all bugs are shallow.” Coined by Eric S. Raymond in The Cathedral and the Bazaar (1997) and named after Linux creator Linus Torvalds. A close cousin of two heads are better than one.

Knuth’s optimization principle

“Premature optimization is the root of all evil.” Donald Knuth, 1974, in a sentence that begins “We should forget about small efficiencies, say about 97% of the time”. See the full explanation.

Hick’s Law

The time it takes to make a decision increases with the number of choices. From experiments by psychologists William Hick and Ray Hyman in the early 1950s. Designers use it to keep menus short – see don’t make me think.

The Lindy Effect

For non-perishable things like ideas, books and technologies, the longer something has survived, the longer it is likely to last. Named after Lindy’s delicatessen in New York; popularized by Nassim Nicholas Taleb.

Economics and behavior

Gresham’s Law

“Bad money drives out good.” When two forms of money have the same face value but different real value, people hoard the good one and spend the bad one. Named in 1858 by economist Henry Dunning Macleod after the Tudor financier Sir Thomas Gresham.

The Yerkes–Dodson Law

Performance improves with arousal or pressure – up to a point, after which it declines. From a 1908 study by psychologists Robert Yerkes and John Dodson. A little deadline pressure helps; too much hurts.

Cunningham’s Law

“The best way to get the right answer on the internet is not to ask a question; it’s to post the wrong answer.” Named after wiki inventor Ward Cunningham by a former colleague; Cunningham himself has said it misquotes him.

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