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Coin tosses don't really give you fifty-fifty odds

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The coin toss, the great equalizer of all odds, is not as random as it seems. When flipped by a human being, the odds are slightly stacked towards one side.

One famous Futurama episode showed a universe where everything is the same, except the result of each coin toss. If a tossed coin came up heads in one universe, it came up tails in the other. The comedy in the episode came from the changes that this random event - entirely up to the physical laws of the universe - caused. It turns out, though, that this mirror universe is impossible. While coin tosses represent fifty-fifty odds, they don't actually work out that way.


As usual, it's humanity that messes things up. When flipped by a machine, coins come up heads a solid fifty percent of the time, and tails the other fifty percent. Put the fate of the coin in grubby human hands and the odds tip slightly in favor of the side that faces up just before the coin is flipped. The side that was face up at the beginning of the flip has a fifty-one percent chance of landing face-up at the end. Humans are not as precise as machines, and so the coin rotates around several axes instead of one. The extra rotation favors the side the original position, to a measurable degree. This is independent of the material that the coin is made out of. Scientists have tried the experiment using coins made out of balsa wood (and probably the labor of some very tired interns), and gotten the same results.

So if the initial placement of the coin matters to the flip, how did that alternate Futurama universe work out the difference? Did a character who liked to place the coin on their hand heads-up in one universe switch to tails-up in another? Suddenly that universe was not just about the effect of random chance on people's actions, but the effect of subtle psychological decisions that had an effect one percent of the time.


Via The Washington Post.

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