The more science discovers how the mind works, the less rational humans are revealed to be.

We make many important decisions, including choosing a mate and buying a car, based more on emotion than reason. On top of that, our reason is often fatally flawed — even for the smartest among us — as outlined by psychologist Daniel Kahneman in his fascinating 2011 book, "Thinking, Fast and Slow."

He shows that the structure of our thinking, guided by what he labels System 1 and System 2, makes it impossible for us to be impartial actors guided strictly by empiricism. System 1 works without our even knowing it: answering simple arithmetic questions, turning us toward a loud noise or reading the emotion in someone's voice. System 2 handles more complex intellectual tasks like analyzing two arguments, driving on a busy highway or solving long division.

Everyone's brain functions this way, no matter our intellectual capability.

And as Mr. Kahneman, the winner of the 2002 Nobel Prize in Economics, shows, we make errors without being aware of them as a result of these mechanics.

Some examples:

•Frequent advertising leads to a more favorable opinion of the person or thing advertised.

•A need for "coherence" makes us jump to illogical conclusions.

•Mood can determine whether you make a right or wrong logical decision.

•People, including scientists, often search for information that confirms their own beliefs.

Part of the explanation lies in our narrow life experiences, which influence our intuition, expectations and belief systems in ways that cause our System 1 to make the wrong inferences.

In one telling experiment, Mr. Kahneman turns scrutiny on himself. He started to suspect that he was grading exams incorrectly by reading each student's answers in successive order. He was right. He found that the strength or weakness of the first essay held a disproportionate sway over his opinion of subsequent essays. So, for example, he would forgive flaws in second pieces when a student's first answer was strong because he wanted to give the student the benefit of the doubt. As a result, he stopped reading each exam from start to finish and instead broke them up. As he writes, "By allowing myself to be strongly influenced by the first question in evaluating subsequent ones, I spared myself the dissonance of finding the same student doing very well on some questions and badly on others."

The phenomenon he describes is readily apparent in how we ascribe uniformly positive or negative traits to people (including politicians), regardless of facts. Think of Republicans who easily believed President Barack Obama was not born in the United States, or Maryland Democrats who supported slots when Gov.Martin O'Malley wanted them but not when Republican Gov.Robert L. Ehrlich Jr. proposed them.

The structural deficiencies in how we think should make each of us more humble, and — as Mr. Kahneman hopes — spend time double checking the processes that lead to a decision.

While Mr. Kahneman describes how organizations can use knowledge of human fallibility to run more justly and more capably analyze risk, why shouldn't our knowledge of the human condition also apply to the size and scope of government?

If even the smartest regularly make egregious mistakes, often without knowing it, why should we trust 15 people to unequivocally decide what Medicare should and should not pay for? (I am referring to the Independent Payment Advisory Board, part of 2009's Affordable Care Act.) Or why should we trust the executive branch of government to decide which American citizens can be killed overseas without due process (or any public vetting), as Attorney General Eric Holder'sinterpretation of the Constitution allows? Or in Maryland, why should Annapolis be granted ever-increasing power to decide how and where counties can develop their land and how much they must spend on education?

A more powerful government is destined to more often make bigger and worse mistakes, silence its critics, and repeat its errors. It doesn't matter if those who run it have the best intentions.

It follows that all politicians, particularly liberals and progressives who profess an evangelical belief in science, should apply its findings on human nature to their own political aspirations. But I do not have a lot of hope they will. It would mean a shattering of the worldview that holds social policy can remake man in a better image and demand they diminish government control of people's lives. Now, expecting that would truly be irrational.