I've been thinking about this one and I'm inclined to agree with AerBabe. In the original example referred to the instructor was sitting behind the student, but most of us learn in PA38s or C152s, where we sit side by side, and there's not a lot of room. My memories when I was learning are of the instructor being too close, just by being there. I could practically feel what he was thinking, as it were, or so it felt. If I was feeling bad, or close to tears, physical contact could easily have made it worse. There are other ways of showing that you understand and care which can't be construed as invasive; maybe just telling the student they're doing OK, or asking if they'd like you to take over for a minute or two. I too was a female student with male instructors; it might be a bit different with two people of the same sex, but I'm not sure it's appropriate anyway, unless you've been flying with the person for quite a while and know them quite well. Maybe when you're on the ground; I don't think I'd have minded a friendly hug now and again after a lesson when it all seemed to be going haywire, but not at 3000 ft please!