Rational Choice Theory.
My dad was a cop.
Sorry for taking so long to respond here. Aside from the problem that rational choice is an assumptive framework for predictive modeling as opposed to a robust ontology that fully explains human behavior, I see another problem with your explanation. Assuming rat choice is true, it still wouldn't explain the actions of a person who doesn't believe in an absolute morality.
You stated that "The only criteria that ultimately matter are the cost/benefit to the individual when deciding a particular course of action."
Rational choice theory doesn't assume individualism. The "Good" that is the object of the cost/benefit framework is empty of content. It could be an individual good, as you claim, or public Good that isn't consistent with the good of all individuals, or any other kind of Good. All that the theory states is that the actor will behave according to a rational cost/benefit analysis to achieve that goal. Someone like Milton Friedman may argue that the Good is inevitably going to be individualistic, but that itself is not part of rational choice theory, nor is it a fact. Anyway, to continue:
A person with absolute morality will consider the Good of that morality the object of their cost/benefit analysis.
A person without absolute morality has no Good to use as a goal for that cost/benefit framework. The injection of that Good is a purely contingent act. It could be that the person resorts to individualism, as you claim, but it could also be otherwise. Logically, there is no necessary connection between the loss of absolute morality and the adoption of individualism.
Thus, the absolute moralist and the person with no absolute morality are not necessarily the same.