In houses with truss roof construction. It has to do with the top chords of the trusses becomming cold in the winter and the bottom chords staying warm because they are covered with insulation.
Usually the cracks that form are 1/8 to 3/16 inch in size at the maximum. 1/2 half inch seems excessive, unless this is an unusually large span or odd configuration.
Additional securing of the trusses to the top wall plates will help, especially in the one that is moving 1/2 inch. However, I doubt you be able to get rid of the problem completely. What I did in a home I lived in that had an interior wall opening a crack of 1/16 inch, was to caulk the crack with flexible paintable caulk. This hid the "problem" even though it didn't go away.