I'm right-handed and left-thumbed. Interestingly, when I, just now, tried vigorously jiggling my fingers (while keeping my hands still) in mid-air, the fingers on my left hand seemed to move more readily than those on my right. Also, when I widen the spaces between my fingers, as if I were going to run them through my hair, the fingers on my right hand moved more awkwardly. Is this the same for everyone? If it is, then "thumb-dominance" seems to be associated with the hand possessing higher finger dexterity.