Flipping a coin may not be the fairest way to make a decision. František Bartoš, who studies psychology at the University of Amsterdam in the Netherlands, had 48 volunteers perform 350,757 coin tosses over multiple weekends. When Bartoš analyzed the data, he found that the probability of getting heads or tails wasn’t 50/50. The odds were slightly in favor of the side that was facing up before the coin was flipped. 

Researchers call this “same-side bias.” When someone flicks a coin upward, they typically introduce a slight wobble. This wobble causes the airborne coin to spend more time on the side it started over the course of the toss—giving it a greater chance of landing same side up.