What physical contact, if any, is acceptable between a supervisor and a student? I was rather surprised that in this answer which suggests that a supervisor giving a side-hug to a research assistant is acceptable behaviour. I try and limit the physical contact with my students/postdocs/assistants to hand shakes and high fives, however if they initiate a hug I tend not to block it. What physical contact, if any, is acceptable between a supervisor and a student? I guess a follow up question would be do the genders of the supervisor or student matter?
In general, I refrain from initiating any physical contact other than handshakes or "congratulatory" gestures such as high-fives or fist bumps. This allows the student to control the level of interaction if they so choose.

My suspicion about what would be considered "acceptable" is that it varies widely from country to country, depending on what is considered an "acceptable" level of contact between strangers or professional colleagues. For instance, in Muslim-dominated countries, the "side-hug" mentioned in the answer—or even shaking of hands—between a male boss and a female subordinate, or vice versa, would likely be frowned upon! Similarly, in other countries, I could see how a "side-hug" could be within the realm of acceptable contacts (although still on the somewhat "iffy" side).