Reason is the slave of the passions but the passions are ultimately rational — a deep rational

There is increasing scientific evidence debunking the classical economic model of behavioral “irrationality” and the contra-emotion view of behavioral drives and decision-making.  The challenges come from adaptive/evolutionary biological and neuro/brain processes models, experiments and research.  In our work, we find this view much more practical and productive than the “irrationality” one. 

from Prof. Doug Kenrick video presentation at UCLA Center for Behavior, Evolution and Culture on iTunes.