system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do fancy people touch shoulders when they shake hands?
gpt
There's a lot at play psychologically. Crusco and Wetzel (1984) found that a touch on the hand or shoulder significantly increased patrons' tipping. The effect was particularly strong when the touch was relatively unobtrusive. This was dubbed "the Midas Touch". The influential aspects of touch also include getting things for free. Researchers have demonstrated that bus drivers are more likely to give a passenger a free ride, if they touch the driver while making the request (Gueguen & Fischer-Lokou, 2003). Touch is very persuasive. 

Placing a hand on one's shoulder, or cupping one's hand in both of yours during a hand shake, are subconscious alpha signals which show superiority. People who try to shake hands with their hand coming from a high position down to your position, with the hand pointing down, are also asserting superiority. Think "Big Hoss" on Pawn Stars. He is subconsciously but physically showing that he is an aggressor personality, prone to egotism and likely to crave control. Body language speaks volumes whether we know it or not! It's a good practice to return the gesture when someone touches your shoulder with a handshake, otherwise you physically assert that they are dominant in that situation. Likewise, take the hand of the "Big Hoss" handshake and turn it directly parallel with your own hand. Mimic a hand on your bicep or hand on your shoulder handshake in the same way, it levels the playing field whether you know it or not.