Hi Ross,
          Scientifically I know of no reasoning.
          However a B.S. story once related to me by the late Bill Old, was,
          Once the world rotated on its axis in the opposite direction than it
now does.
Hence left handed molluscs.When it turned in its present direction so did the
molluscs. Now if you believe that, have I got some stories for you.
                                              Mel S.