Grey (50 Shades of)

I think we can find some parallels in Frank's work to the ideas Khadija pointed out in Sen. As Khadija said, "people can behave very unpredictably or 'irrationally,' when making decisions," making it difficult to normalize rational behavior universally. I think that Frank's adaptive rationality framework works to address this concern. Whereas individual conceptions of rationality will often differ, this doesn't necessarily prevent us from creating common standards, while obviously expecting some deviations. I found it interesting how he constructed the "cooperator and defector" example, illustrating that our differences in rationality, as well as how we discern these differences, are actually a vital part of maintaining our population. 

When cooperators and defectors are indistinguishable, defectors might rationalize coercive or immoral behaviors that have larger payoffs than the behaviors of the more beneficent cooperators. In this "complex interaction of genes and culture" defectors reign supreme, and are able to rationalize their pursuits of self-interest at the expense of those cooperators around them, who, due to their lower material payoffs relative to the defectors, would likely go extinct (49). When the two types of individuals can be easily distinguished, cooperators unite away from self-interested defectors, and "reap the benefits of mutual cooperation" because this higher order pursuit of self-interest is combined with a sense of altruism (49). Defectors eventually go extinct in this situation. What this example illuminates is the shortcomings of existing, binary models of rationality. It does not pay to be exclusively self-interested nor exclusively benevolent and trusting. This is why Frank suggests the adaptive rationality framework, whereas individuals possess an internal "equilibrium in which each individuals experiences both selfish and altruistic motives to varying degrees" (50). It is impossible to define rationality without grey area, so Frank makes the grey area the focal point of his definition, which I think addresses your initial concerns. There will likely never be a universal definition of rationality that can be applied in all situations in all contexts, but this framework applies to many. 

I also like the connection between Sen and Frank where both mention the application of rationality models to individuals rather than groups. With Frank, he suggests that because populations will showcase differing conceptions of rationality, and "a stable balance" must be struck between those forces in order for the most members of the population to thrive, it would make the most sense for individuals to possess various conceptions and motives, and to various degrees. Just as it is unlikely that all defectors or all cooperators in a population would go extinct, it is unlikely that individuals would only be able to work exclusively within one conception of rationality. "We should not expect a world populated exclusively by the homo economicus caricature," Frank warns, for "they hardly represent a sensible basis on which to ground a universal science of human behavior" (50). People are rationally nuanced just as populations are, and we must work within this framework of grey area.