Kids believed that 'double-jointed' people had literally doubled their number of joints, which is why they could bend their fingers and limbs backward.
Double-jointed people (hypermobile individuals) have the same number of joints as everyone else. They simply have greater range of motion due to differences in ligament and muscle flexibility, often related to connective tissue properties like collagen makeup. Some have conditions like Ehlers-Danlos syndrome that increase flexibility, but this is a variation in tissue properties, not an additional joint.
Reception
Sources
- Mayo Clinic: Joint Hypermobility PRIMARY
- Wikipedia: Hypermobility REFERENCE
- Verywell Health: Double Jointed REFERENCE