April H. Crusco and Christopher G. Wetzel, “The Midas Touch: The Effects of Interpersonal Touch on Restaurant Tipping,” Personality and Social Psychology Bulletin 10, no. 4 (December 1984): 512–17; Céline Jacob and Nicolas Guéguen, “The Effect of Physical Distance Between Patrons and Servers on Tipping,” Journal of Hospitality & Tourism Research 36, no. 1 (February 2012): 25–31.