Over the seven years we have been in the house, all of the exterior vertical corners in my house have opened up about 1/16" or more in the middle part of the corner (starting about 18" up from the floor and extending to about 18" short of the ceiling). The framing is 2x6, of course. Because of the layout of the house, there are three such corners in the house; there are two corners in the attached garage, which is 2x4 construction, and these corners have not cracked to any significant degree. None of the interior partition corners, nor corners where partitions meet the exterior walls, have opened up. Two possibilities occur to me: the nature of an exterior corner post assembly is different from other corners, and 2x6 lumber moves 60% farther under moisture changes than does 2x4 lumber.
So, what is likely going on with these corners, and what might I have done with the drywall to prevent this? By now, probably not much can be done to fix it, since the corners open and close with the seasons to some extent (it will be less noticeable when summer humidity and heat return). I am going to clean out and fill the three corners with flexible white caulk, but I expect there will still be some cracking, at least of the paint layer.