Kluge: The Haphazard Construction of the Human Mind (Houghton Mifflin; 2008)

Each of these examples of mental contamination — the focusing illusion, the halo effect, anchoring and adjustment, and the familiarity effect — underscores an important distinction that will recur throughout this book: as a rough guide, our thinking can be divided into two streams, one that is fast, automatic, and largely unconscious, and another that is slow, deliberate, and judicious.

The former stream, which I will refer to as the ancestral system, or the reflexive system, seems to do its thing rapidly and automatically, with or without our conscious awareness. The latter stream I will call the deliberative system, because that's what it does: it deliberates, it considers, it chews over the facts — and tries (sometimes successfully, sometimes not) to reason with them.

The reflexive system is clearly older, found in some form in virtually every multicellular organism. It underlies many of our everyday actions, such as the automatic adjustment of our gait as we walk up and down an uneven surface, or our rapid recognition of an old friend. The deliberative system, which consciously considers the logic of our goals and choices, is a lot newer, found in only a handful of species, perhaps only humans.

As best we can tell, the two systems rely on fairly different neural substrates. Some of the reflexive system depends on evolutionarily old brain systems like the cerebellum and basal ganglia (implicated in motor control) and the amygdala (implicated in emotion). The deliberative system, meanwhile, seems to be based primarily in the forebrain, in the prefrontal cortex, which is present — but vastly smaller — in other mammals.

I describe the latter system as "deliberative" rather than, say, rational because there is no guarantee that the deliberative system will deliberate in genuinely rational ways. Although this system can, in principle, be quite clever, it often settles for reasoning that is less than ideal. In this respect, one might think the deliberative system as a bit like the Supreme Court: its decisions may not always seem sensible, but there's always at least an intention to be judicious.

Conversely, the reflexive system shouldn't be presumed irrational; it is certainly more shortsighted than the deliberative system, but it likely wouldn't exist at all if it were completely irrational. Most of the time, it does what it does well, even if (by definition) its decisions are not the product of careful thought. Similarly, although it might seem tempting, I would also caution against equating the reflexive system with emotions. Although many (such as fear) are arguably reflexive, emotions like schadenfreude — the delight one can take in a rival's pain — are not. Moreover, a great deal of the reflexive system has little if anything to do with emotion; when we instinctively grab a railing as we stumble on a staircase, our reflexive system is clearly what kicks in to save us — but it may do so entirely without emotion. The reflexive system (really, perhaps a set of systems) is about making snap judgments based on experience, emotional or otherwise, rather than feelings per se.

Even though the deliberative system is more sophisticated, the latest in evolutionary technology, it has never really gained proper control because it bases its decisions on almost invariably secondhand information, courtesy of the less-than-objective ancestral system. We can reason as carefully as we like, but, as they say in computer science jargon, "garbage in, garbage out." There's no guarantee that the ancestral system will pass along a balanced set of data. Worse, when we are stressed, tired, or distracted, our deliberative system tends to be the first thing to go, leaving us at the mercy of our lower-tech reflexive system — just when we might need our deliberative system the most.

The unconscious influence of our ancestral system is so strong that when our conscious mind tries to get control of the situation, the effort sometimes backfires. For example, in one study, people were put under time pressure and asked to make rapid judgments. Those who were told to (deliberately) suppress sexist thoughts (themselves presumably the product of the ancestral reflexive system) actually became more likely than control subjects to have sexist thoughts. Even more pernicious is the fact that as evolution layered reason on top of contextually driven memory, it left us with the illusion of objectivity. Evolution gave us the tools to deliberate and reason, but it didn't give us any guarantee that we'd be able to use them without interference. We feel as if our beliefs are based on cold, hard facts, but often they are shaped by our ancestral system in subtle ways that we are not even aware of.