1. A system comprising: a data acquiring section which acquires a cost caused as a result of adopting each of a plurality of behaviors to a target, as training data for generating a behavioral decision function, the plurality of behaviors having already been adopted to a target; and a function generator which generates, based on the training data, the behavioral decision function to minimize expected shortfall of the cost to be obtained as a result of adopting the behavior to a target, wherein the system is a system which supports a decision on a treatment policy for a patient having a certain test result, wherein the behavioral decision function is a function which decides a treatment policy for a patient, the data acquiring section acquires the scale of a loss, which is caused on each of a plurality of patients, as the training data, the plurality of patients having already been treated, and the function generator generates, based on the training data, the behavioral decision function to minimize expected shortfall of a loss caused by a treatment.