CeeDee Lamb of the Dallas CowboysAssociated Press/Alamy

We may perceive sports players to be bigger if they are wearing a bigger number on their shirt.

Some American football players reportedly choose lower numbers because they think they look slimmer, but until now there has been no research to back this up.

Ladan Shams at the University of California, Los Angeles, and her colleagues tested 37 people by showing them cartoon images of American football players. The players, who were different sizes as well as having different skin and shirt colours, either wore a number between 10 and 19 or a number between 80 and 89. The participants were shown one player at a time on a screen and asked to quickly judge using a sliding scale how slender they considered the athlete to be.

During the experiment, the participants saw the exact same player twice – once when wearing a shirt number between 10 and 19 and once when wearing a number between 80 and 89. “As we had a huge variety of jersey colours, sizes and skin tones, the participants didn’t notice this,” says Shams.

The team found that the same player was considered to be thinner when wearing a smaller number than they were when wearing a bigger number. “The effect size was not huge, but it was a robust finding,” says Shams.

The researchers then repeated the experiment with 147 participants, but this time used shirt numbers that included the same digits, such as 17 and 71 or 19 and 91. This ensured that the effect wasn’t due to the actual width of the written number – for example, the number 8 looks bigger than the number 1. They found the same effect again.

“Our brains keep track of almost everything that we experience,” says Shams. “They keep track of all kinds of statistical regularities in the world around us and this happens to another regularity, which is the association between numbers that are written and the size of the objects.”

“You may not be aware of it, but every time you go to the gym and you see that the weights with the larger numbers are heavier, your brain is subconsciously tracking this,” she says.