Murphy's Law: Why the Toast Lands Butter Side Down

Three mechanisms sit underneath the joke. The first is that some mishaps have a real cause that points in one direction. Toast lands butter side down because tables are the height they are and toast rotates at the rate it does, so what feels like malice is mechanics. Matthews's 1995 paper worked through the dynamics of a slice sliding off an edge and found that the toast cannot rotate far enough in a fall from table height to bring the buttered side back up; the butter is irrelevant, and an unbuttered slice with one face marked lands the same way.

The second mechanism is lopsided odds. In any situation with several options and one of them yours, most of the time some other option does better. In a supermarket with five similar lanes and no way to tell in advance which will move fastest, yours is the fastest about one trip in five, and the four losses register while the win does not. The same arithmetic covers the highway lane and the airport security queue. Nothing is going wrong; the count is against you by construction.

The third mechanism is memory. Baumeister and colleagues' 2001 review, Bad Is Stronger Than Good, gathered the evidence that negative events are processed more thoroughly and weighted more heavily than positive ones of the same size. Its memory evidence is more mixed, and the clearest advantage for bad events is a short-term one: whether a mishap stands out also depends on your mood, on how much it says about you and on how long ago it was. Tversky and Kahneman showed in 1973 that people judge how often something happens by how easily examples come to mind. Put the two together and, of the four combinations of rain and umbrella, the soaking is the one that comes back, so rain seems to wait for the umbrella to be left behind.

The three reinforce one another: a real cause supplies a few vivid mishaps, the odds a steady run of small ones, and memory holds more of those than of the uneventful days between, until the result is less a belief about probability than a mood.

The law has a documented origin and a documented physics, forty-six years apart. The name comes from 1949 and from Edwards Air Force Base in California, then Muroc, where Colonel John Paul Stapp was running rocket sled tests of how much deceleration a human body could survive. Captain Edward A. Murphy Jr., an engineer from Wright Field, had supplied strain gauges for the harness, and when they were installed they were wired so that every one read zero. Murphy's remark about the technician, in one version that if there was any way to do it wrong he would find it, was turned into a law by George Nichols, the project manager, and Stapp quoted it at a press conference as the reason the program had a good safety record: the team assumed anything that could go wrong would, and planned accordingly. Nick Spark, who interviewed Nichols, Murphy's son and others for a history published in 2006, found that they disagreed about the exact words and about who first called it Murphy's law.

The physics came in 1995, when Robert Matthews published "Tumbling toast, Murphy's Law and the fundamental constants" in the European Journal of Physics. He treated the slice as a rigid body leaving the edge of a table, worked out how fast it rotates once its center of mass passes the edge, and compared the rotation needed to come back butter side up with the time available before the floor. From table height the slice does not get far enough round, and lands butter side down. For it to rotate far enough to land butter side up it would need to fall from about three meters, which is why the law holds in kitchens. Matthews added a wider argument: table height is set by human height, and human height is limited by the strength of bone and the pull of gravity, so butter-side-down toast follows from the fundamental constants of the universe. The paper won the 1996 Ig Nobel Prize for physics.

Each mechanism, cause, odds and memory, has its own everyday mishaps.

Each mechanism has a different remedy, so the counter is to separate them.

• Find the cause. If a mishap really does happen more often than chance, there is usually a reason with a direction, and once it is known it can be designed around. Toast lands butter side down; so carry the plate lower, or do not slide it.
• Count the options. When your lane, your line or your car seems always the slowest, ask how many alternatives there were. With five similar lanes and no way to tell them apart in advance, you lose to somebody about four times in five by arithmetic alone.
• Keep the four-cell record. For any pairing like rain and umbrella, there are four combinations, and the feeling of a jinx comes from counting only the one that hurts. A week of tallying all four usually ends it.
• Notice the mood. The sense that things are arranged against you is strongest when you are tired, late or stressed, which is also when mishaps cost the most. The frequency has not changed; the weight has.
• Use the law as a plan, not a prophecy. The engineers who coined it meant that any failure that can happen should be designed out in advance. That is foresight, and it is the law's one useful form.

Murphy's law is folk psychology, and several studied biases sit underneath it.

• It is not negativity bias, the general finding that bad events are weighted more heavily than good ones. Negativity bias is one of the reasons the law feels true; the law also runs on odds and on real physical causes.
• It is not the availability heuristic, which judges frequency by how easily cases come to mind. Availability explains the umbrella and the printer; it does not explain the toast, which really does land butter side down.
• It is not illusory correlation, the perception of a link between two things that are not linked. The rain-and-umbrella jinx is one, but the slow-lane feeling is a real lopsidedness read as a jinx.
• It is not pessimism bias, the expectation that outcomes will be worse than they are. Murphy's law can be held cheerfully, as a joke or a planning rule, by people who are not pessimists at all.