Well, I've looked underneath, and the joists are ok. We recently had our basement worked on (sump pump, french drain, sealed inside and out, etc) and the bathroom was remodelled, including a glass block window in addition to an exhaust fan. So, now that I've fixed the moisture concerns (I hope), maybe that will correct any future problems. I personally think that since the house is 57 years old, and was a rental for awhile, that part of the reason could be from neglect. Oddly enough, the bathroom is the only room in the house with these cracks.
I've counted at least 7 layers of paint on the walls, and of course you can't assume that each paint job was done with care. I'll try repairing the damage and see what happens 6 months from now. If I'm getting new cracks, then I'll replace the drywall and see if there's anything else going on.
Thank you for your advice and I'll definitely keep it in mind =)