The popular – and sensible – thought is that we call coin flips because there are only two outcomes, and each are equally likely.

Except it turns out they’re not.

A whole bunch of European researchers have put together some research and found what they claim is “overwhelming evidence” that there’s a side all coins are slightly (but significantly) more likely to land on.

“Despite the widespread popularity of coin flipping, few people pause to reflect on the notion that the outcome of a coin flip is anything but random.”

These researchers flipped a coin 350,757 times and found that, a majority of the time, it landed on the same side it started on.

Don’t get too excited, though – it’s about a 51% chance the coin will behave like this, so it’s only slightly over half.

They put it down to the fact that when you flip a coin off your thumb it wobbles, which causes the same side to spend slightly more time facing up – which means there’s a slightly bigger chance it will be in that position when it lands.

It might seem like a small percentage, but František Bartoš, the lead author of the study, says it adds up.

“This is more than the casino advantage for six deck blackjack against an optimal player, but less than that for single-zero roulette.”

If you bet a dollar on the outcome of a coin toss 1000 times, knowing the starting position of the coin toss would earn you 19$ on average. This is more than the casino advantage for 6deck blackjack against an optimal player (5$) but less than that for single-zero roulette (27$).

— František Bartoš (@BartosFra) October 9, 2023

It’s not a new supposition. In 2007, Stanford mathematician Persi Diaconis published a paper that claimed the same thing.

It’s not nothing, in other words.

So definitely take this into account the next time you’re the one responsible for calling the flip of a coin.