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It’s probably happened to you: a friend says, “I know this place like the back of my hand,” and then proceeds to get you hopelessly lost. Well, it could be that they really did know it like the back of their hand. Because researchers have found that people don’t actually know their hands as well as they think they do. The study appears in the Proceedings¹ of the National Academy of Sciences.

We all have “mental representations” of our body parts. So when our hands or feet are out of sight, we still know pretty much where they are and what they look like. But just how accurate is our mental picture?

Scientists had volunteers place one hand, palm down, under a board. And they asked the subjects to guess, by pointing to the board with their other hand, where their hidden knuckles² and fingertips lay.

The results? Participants thought their hands were about two thirds wider and one third shorter than they really were. And their finger length estimates got worse as they moved away from their thumbs. That’s interesting, because finger sensitivity declines as you head towards the pinky. So our mental body maps seem to reflect our ability to feel. Oh, and for their gripping result, let’s give the researchers a hand.

Thanks for the minute, for scientific American’s 60-second-science, I am Karen Hopkin.