Softmax is a mathematical function frequently used to model probabilistic choice between multiple alternatives associated with different values, e.g. two arms in a bandit task; See for example  and .
Where was this method first applied?
 Shteingart, Hanan, Tal Neiman, and Yonatan Loewenstein. "The role of first impression in operant learning." Journal of Experimental Psychology: General 142.2 (2013): 476.
 Egelman, David M., Christophe Person, and P. Read Montague. "A computational role for dopamine delivery in human decision-making." Journal of Cognitive Neuroscience 10.5 (1998): 623-630.