Bone Rotation Skews Mesh

Rigging my first character (on and off for several months now) and I have come to another problem that I can’t seem to figure out. I have the arm rig setup (based on the tutorials in the Character Animation in Blender book) but when I move the hand it shrinks. It rotates the direction I want it to, but it moves slower than the bone and also skews the mesh as it approaches 90 degrees as so:

Does anyone know what I am doing wrong?

You possibly have all the Verts in the hand assigned to some other Bone as well. It could be each Vertex Group, it could be the whole hand as a seperate VG or it could be all the verts in the mesh, and the other bone is probably far away from the hand itself.