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

rest tends to stay at rest, and an object in motion tends to stay in motion). Evolution tends to work with what is already in place, making modifications rather than starting from scratch.

Evolutionary inertia occurs because new genes must work in concert with old genes and because evolution is driven by the immediate. Gene-bearing creatures either live and reproduce or they don't. Natural selection therefore tends to favor genes that have immediate advantages, discarding other options that might function better in the long term. Thus the process operates a bit like a product manager who needs his product to ship now, even if today's cut corners might lead to problems later.

The net result is, as Nobel laureate Fran?ois Jacob famously put it, that evolution is like a tinkerer 'who . . . often without knowing what he is going to produce . . . uses whatever he finds around him, old cardboards, pieces of strings, fragments of wood or metal, to make some kind of workable object... [the result is] a patchwork of odd sets pieced together when and where opportunity arose.' If necessity is the mother of invention, tinkering is the geeky grandfather of kluge.

In short, evolution often proceeds by piling new systems on top of old ones. The neuroscientist John Allman has captured this idea nicely with an analogy to a power plant he once visited, where at least three layers of technology were in simultaneous use, stacked on top of one another. The recent computer technology operated not directly, but rather by controlling vacuum tubes (perhaps from the 1940s), which in turn controlled still older pneumatic mechanisms that relied on pressurized gases. If the power plant's engineers could afford the luxury of taking the whole system offline, they would no doubt prefer to start over, getting rid of the older systems altogether. But the continuous need for power precludes such an ambitious redesign.

In the same way, living creatures' continuous need to survive and reproduce often precludes evolution from building genuinely optimal systems; evolution can no more take its products offline than the human engineers could, and the consequences are often equally clumsy, with new technologies piled on top of old. The human midbrain, for example, exists literally on top of the ancient hindbrain, and the forebrain is built top of both. The hindbrain, the oldest of the three (dating from at least half a billion years ago), controls respiration, balance, alertness, and other functions that are as critical to a dinosaur as to a human. The midbrain, layered on soon afterward, coordinates visual and auditory reflexes and controls functions such as eye movements. The forebrain, the final division to come online, governs things such as language and decision making, but in ways that often depend on older systems. As any neuroscience textbook will tell you, language relies heavily on Broca's area, a walnut-sized region of the left forebrain, but it too relies on older systems, such as the cerebellum, and ancestral memory systems that are not particularly well suited to the job. Over the course of evolution our brain has become a bit like a palimpsest, an ancient manuscript with layers of text written over it many times, old bits still hiding behind new.

Allman referred to this awkward process, by which new systems are built on top of old ones rather than begun from scratch, as the 'progressive overlay of technologies.' The end product tends to be a kluge.

Of course, explaining why evolution can produce kluge-like solutions in general is not the same thing as showing that the human mind in particular is a kluge. But there are two powerful reasons for thinking that it might be: our relatively recent evolution and the nature of our genome.

Consider, first, the short span of human existence and what it might mean. Bacteria have lived on the planet for three billion years, mammals for three hundred million. Humans, in contrast, have been around for, at most, only a few hundred thousand. Language, complex culture, and the capacity for deliberate thought may have emerged only in the past fifty thousand years. By the standards of evolution, that's not a lot of time for debugging, and a long time for the accumulation of prior evolutionary inertia.

Meanwhile, even though your average human makes its living in ways that are pretty different from those of the average monkey, the human genome and primate genomes scarcely differ. Measured nucleotide by nucleotide, the human genome is 98.5 percent identical to that of the chimpanzee. This suggests that the vast majority of our genetic material evolved in the context of creatures who didn't have language, didn't have culture, and didn't reason deliberately. This means that the characteristics we hold most dear, the features that most distinctly define us as human beings — language, culture, explicit thought — must have been built on a genetic bedrock originally adapted for very different purposes.

Over the course of this book, we'll travel through some of the most important areas of human mental life: memory, belief, choice, language, and pleasure. And in every case, I will show you that kluges abound.

Humans can be brilliant, but they can be stupid too; they can join cults, get addicted to life-ruining drugs, and fall for the claptrap on late-night talk radio. Every one of us is susceptible — not just Joe Sixpack, but doctors, lawyers, and world leaders too, as books like Jerome Groopman's How Doctors Think and Barbara Tuchman's The March of Folly well attest. Mainstream evolutionary psychology tells us much about how natural selection has led to good solutions, but rather less about why the human mind is so consistently vulnerable to error.

In the pages to come I'll consider why our memory so often fails us, and why we often believe things that aren't true but disbelieve things that are. I'll consider how it is that half of all Americans can believe in ghosts and how almost four million can sincerely believe that they've been abducted by space aliens. I'll look at how we spend (and often waste) our money, why the phenomenon of throwing good money after bad is so widespread, and why we inevitably find meat that is 80 percent lean much more appealing than meat that is 20 percent fat. I'll examine the origins of languages and explain why they are replete with irregularity, inconsistency, and ambiguity — and, for that matter, why a sentence like People people left left ties us in knots even though it's only four words long. I'll also look at what makes us happy, and why. It's often been said that pleasure exists to guide the species, but why, for example, do we spend so many hours watching television when it does our genes so little good? And why is mental illness so widespread, affecting, at one time or another, almost half the population? And why on earth cant money buy happiness?

Kluge, kluge, kluge. In every case, I'll show that we can best understand our limitations by considering the role of evolutionary inertia in shaping the human mind.

This is not to say that every cognitive quirk is without redeeming value. Optimists often find some solace in even the worst of our mental limitations; if our memory is bad, it is only to protect us from emotional pain; if our language is ambiguous, it is only to enable us to say no without explicitly saying 'no.'

Well, sort of; there's a difference between being able to exploit ambiguity (say, for purposes of poetry or politeness) and being stuck with it. When our sentences can be misunderstood even when we want them to be clear — or when our memory fails us even when someone's life is at stake (for example, when an eyewitness gives testimony at a criminal trial) — real human cognitive imperfections cry out to be addressed.

I don't mean to chuck the baby along with its bath — or even to suggest that kluges outnumber more beneficial adaptations. The biologist Leslie Orgel once wrote that 'Mother Nature is smarter than you are,' and most of the time it is. No single individual could ever match what nature has done, and most of nature's designs are sensible, even if they aren't perfect. But it's easy to get carried away with this line of argument. When the philosopher Dan Dennett tells us that 'time and again, biologists baffled by some apparently futile or maladroit bit of bad design in nature have eventually come to see that they have underestimated the ingenuity, the sheer brilliance, the depth of insight to be discovered in Mother Nature's creations,' he's cheerleading. In an era in which machines can beat