And to qualify as "hypermobility", that added flexibility must arise without any practice or stretching. Some athletes and dancers, for example, can eventually train their bodies to be more and more flexible, but the sort of hypermobility with which we concern ourselves right now is something you’re born with.

Understanding how a joint can be naturally more or less flexible requires a quick lesson in human anatomy. There are two things that limit the motion of a joint: the contours of the bones and cartilage, or the ligaments. "You can have a physical hard stop – it can't go any further because something hits something – or you can have a ligament holding it in place," says USC anatomist and vertebrate palaeontologist Michael Habib.

Hooks and grooves

When someone is able to pull their thumb back to their arm, that's usually because their ligaments have afforded them a little extra motion. "If you're born with looser ligaments, you'll be able to move it more," says Habib.

Some double-jointed individuals, on the other hand, owe their flexibility to their bones. A common form of hypermobility involves the elbow. Some people can bend this joint in the wrong direction so that their forearm swings beyond 180 degrees. There's a knob of bone that forms the pointy part on the back of the elbow called the olecranon. "It's got this little hook on it. Actually, a big hook," explains Habib. That hook fits into a small groove on the back of the humerus, the upper arm bone. And when you're extending your arm, that hook slides into that little groove. When it hits the end of the groove, you can't bend your elbow any further. "If your olecranon is small, or your groove is deep, you'll be able to go past 180 degrees," says Habib. "So individuals who can hyperextend their elbows can do that simply because their bones are a little bit differently shaped."