Why do some people have double-jointed thumbs?

A “double-jointed” thumb is a common way to describe hyperextension at the thumb joints. Humans do not actually have double the number of thumb joints; instead, the joint surfaces, ligaments, and surrounding soft tissues allow a greater-than-usual range of motion. In many cases this trait runs in families and reflects normal genetic variation. It can be harmless, though very loose joints may be more prone to pain or injury if they are overstretched. The thumb’s special mobility is important because the human thumb is a key tool for grasping and precision tasks.