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Flip a coin: Fifty per-cent of the time the coin will land heads and fifty per-cent of the time it will land tails—right?

Not according to a study which suggests that coin tossing is inherently biased; a coin is more likely to land on the same face it started out on.

This discovery is based on some 1986 work done by the mathematician Joseph Keller. He argued that the only fair way to toss a coin is to toss it so vigorously that it spins perfectly around the horizontal axis in the center. Since it’s impossible to throw it with such precision, every other toss becomes biased—because the coin spins around a tilted axis. The end result is a coin will land on the same face it started on 51 per-cent of the time.

Furthermore, in experiments, the researchers were surprised to find that it’s difficult to tell from watching a coin whether it has flipped. A coin toss typically takes just half a second, with the circumference of the coin whizzing around at 3 meters per second. What’s more, the coin’s spin makes it wobble, often creating the illusion that the coin has flipped.

“Sometimes we had the complete impression that the coin had turned over when it really hadn’t,“ Holmes says.

Magicians and charlatans may take advantage of this illusion. Keller observes, “Some people can throw the coin up so that it just wobbles but looks to the observer as if it is turning over.“

…and if you’ve ever tried to perform a coin-flip by spinning the coin on its edge on a table; watch out:

[These biases pale] when compared with that of spinning a coin on its edge. A spinning penny will land as tails about 80 percent of the time, Diaconis says, because the extra material on the head side shifts the center of mass slightly.

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