How rare is it for someone to have double jointed fingers unless marphans?

Marfans is linked to. joint instabilty but there are many similar diseases yhat lead to joint instability. Many younger ( non marfanoid) have joint looseness that they also grow out of.
Many reasons. Hypermobility generally results from one or more of the following:misaligned joints, abnormally shaped ends of one or more bones at a joint , type 1 collagen or other connective tissue defect (found in ehlers-danlos syndrome, abnormal joint proprioception (an impaired ability to determine where in space parts of the body are, and how stretched a joint.