Ask HN: Effect of touch interface on tipping behavior?

Square card readers have a feature where you can let the customer enter a tip on the touch screen. What impact does this have on tipping behavior? I couldn't find any research studies on it.

How does it compare to tips left when paying with cash? How about tips left on paper receipts? Are places that wouldn't be comfortable accepting tips (due to inconvenience) on receipts accepting tips on the touch screens? How much impact does tipping behavior have on business decisions?