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

There can be little doubt that the human brain too is fragile, and not just because it routinely commits the cognitive errors we've already discussed, but also because it is deeply vulnerable both to minor malfunctions and even, in some cases, severe breakdown. The mildest malfunctions are what chess masters call blunders and a Norwegian friend of mine calls "brain farts" — momentary lapses of reason and attention that cause chagrin (d'Oh!) and the occasional traffic accident. We know better, but for a moment we just plain goof. Despite our best intentions, our brain just doesn't manage to do what we want it to. No one is immune to this. Even Tiger Woods occasionally misses an easy putt.

At the risk of stating the obvious, properly programmed computers simply don't make these kinds of transient blunders. My laptop has never, ever forgotten to "carry the one" in the midst of a complicated sum, nor (to my chagrin) has it "spaced out" and neglected to protect its queen during a game of chess. Eskimos don't really have 500 words for snow, but we English speakers sure have a lot of words for our cognitive short circuits: not just mistakes, blunders, and fingerfehlers (a hybrid of English and German that's popular among chess masters) but also goofs, gaffes, flubs, and boo-boos, along with slips, howlers, oversights, and lapses. Needless to say, we have plenty of opportunities to use this vocabulary.

The fact that even the best of us are prone to the occasional blunder illustrates something important about the neural hardware that runs our mental software: consistency just isn't our forte. Nearly everything we carbon-based units do runs some chance of error. Word-finding failures, moments of disorientation and forgetfulness, each in its own way points to the imperfection inherent in the nerve cells (neurons) from which brain circuits are made. If a foolish consistency is the hobgoblin of little minds, as the American writer Ralph Waldo Emerson (1803-1882) once said, a foolish inconsistency characterizes every single human mind. There's no guarantee that any person's mind will always fire on all cylinders.

Yet random gaffes and transient blunders are just a tiny piece of a larger, more serious puzzle: why do we humans so often fail to do what we set out to do, and what makes the mind so fragile that it can sometimes spiral out of control altogether?

Plenty of circumstances systematically increase the chance of making mental errors. The more that's on our mind, for example, the more likely we are to fall back on our primitive ancestral system. Bye-bye, prefrontal cortex, signature of the noble human mind; hello, animal instinct, short-sighted and reactive. People committed to eating in a healthful way are, for example, more likely to turn to junk if something else is on their mind. Laboratory studies show that as the demands on the brain, so-called cognitive load, increase, the ancestral system continues business as usual — while the more modern deliberative system gets left behind. Precisely when the cognitive chips are down, when we most need our more evolved (and theoretically sounder) faculties, they can let us down and leave us less judicious. When mentally (or emotionally) taxed, we become more prone to stereotyping, more egocentric, and more vulnerable to the pernicious effects of anchoring.

No system, of course, can cope with infinite demands, but if I had been hired to design this aspect of the mind, I would have started by letting the deliberative, "rational" system take priority, whenever time permits, favoring the rational over the reflexive where possible. In giving precedence instead to the ancestral reflexive system — not necessarily because it is better but simply because it is older — evolution has squandered some of our most valuable intellectual resources.

Whether we are under cognitive strain or not, another banal but systematic failure hampers our ability to meet mental goals: most of us — at one time or another — "space out." We have one thing that we nominally intend to accomplish (say, finishing a report before a deadline), and the next thing you know, our thoughts have wandered. An ideal creature would be endowed with an iron will, sticking, in all but the most serious emergencies, to carefully constructed goals. Humans, by contrast, are characteristically distractible, no matter what the task might be.

Even with the aid of Google, I can neither confirm nor deny the widespread rumor that one in four people is daydreaming about sex at any given moment,* but my hunch is that the number is not too far from the truth. According to a recent British survey, during office