Both Caltech and MIT students know exactly what they’re supposed (and in fact, encouraged) to collaborate on and what they’re supposed to do completely independently. Cheating does occur but is very rare. If caught, it’s probably the worst type of the humiliation a student there could ever experience. Besides, if you cheat to pass a course, what about the next set of courses that’re even more difficult? You’re likely to either fall behind very quickly or be caught cheating at some point.