Who gets
the last slice of pizza? Flip a coin. On second thought ... maybe not. The coin
toss is a time-honored solution to indecision, but, weirdly, it's not a truly
fair method. Better to just split that slice.
Imagine
all the qualms in history ever settled by the flip of a coin. I’ll bet few of
those involved knew the odds weren't quite even. It turns out that a spinning
penny will land tails-up a staggering 80% of the time, according to
Persi Diaconis from Stanford University's Department of Statistics, who
performed a mathematical analysis on the matter in 2004. That's because a
penny is slightly heavier on the heads side, skewing the center of mass and
making the coin more likely to turn up tails. Coins with unserrated edges (like
a nickel) tend to be slightly more biased too. (Insider info: Magicians will
often shave the tails side down so the weight discrepancy even greater, making
it even more likely to land tails-up.)
Flipping a coin could
get you a similar bias, too. If you flip a coin into the air and let it fall
onto a hard surface to reveal the result, the coin often ends up spinning
before settling, anyway. And, as we learned earlier, a spinning coin is, way
more often than not, a tails-up coin. Note that older pennies may not give you
quite as pronounced a bias as newer ones. It's gross, but you have to account
for all the dirt, grime, and other junk that can build up on coins over time
and throw off the center of mass.
Catch or Drop?
Forget
heads or tails, let's ask a better question: Catch or drop? It may seem silly,
but according to Diaconis, it is fairer to catch a coin that was flipped into
the air than to let the coin bounce and spin on the ground until it lies flat.
Your hand isn't a hard, flat surface like the ground, so it'll land wherever
you stick your hand in its descending path. But--you guessed it-- even that is
not truly random.
Diaconis holds that any flipped coin will still be slightly biased
toward landing tails-up, at about 51-49 odds to be precise. In 1986,
mathematician Joseph Keller proved that one fair way to toss a coin is to
throw it so that it spins perfectly around a horizontal axis through the coin's
center. Doing that would require superhuman ability, so don't count on it. For
us mere mortals, just stick to calling it tails. You'll probably be right.
(Adapted from curiosity.con/Joanie Faletto)
