Dr. Jensen's questions and snowball effect

In considering "human nature," that is -- if we were to assume there are predictable, regulatory and quantifiable patterns to how human beings think or behave, I think we a priori have to accept the premise that "human beings are rational."  

As a quantitative researcher myself, I think such a positivist ontology is what allows for theoretical deduction based on empirical testing (after all, what would be the point of studying human behaviors if we believe human beings are fundamentally irrational and unpredictable?); however, such assumption is also what may become troublesome for studies of economics, for example, because as rational as we would like to be, we are by and large not rational beings -- our emotions often get in the way of our rationality; our utility-based evaluation of short term benefits versus long term consequences (and vice versa) is not always clear-cut, and our seemingly "rational" decisions are often based on things that we cannot parse out or articulate.

This is not to say that human beings are essentially unpredictable and there is no future for quantitative research in social sciences. In fact, I think most of us do share some fundamental traits that are predictable and may constitute "human nature." For example, most of us prefer being rewarded than punished in most cases, and most of us are creatures of habit under normal circumstances.  Nonetheless, the question is -- at which level of specificity does our assumption of "human nature" begin to falter due to too much variance in all kinds of known and unknown factors that influence our decision making? 

Going back to Dr. Jensen's questions about human nature and the design of an economy, which I clearly don't have the answers to... I think a question that may be also worthy of discussion or examination is --  At which point do the "noises"  (e.g., emotional responses, short vs. long term considerations, etc.) outweigh the underlying predictability we model after in studies and designs of economics? Recent history in domestic and world economy suggests that we haven't found the answer yet, but one thing is certain: "insanity is doing the same thing over and over again and expecting different results" (Einstien).

Perhaps asking different questions is not the solution nor proper response to the questions that have already been raised, but for someone like me who often have more questions than answers, I can only pray that some of my endless questions will somehow lead to the right way of seeking the answers...