To the Editors:

Evolutionary psychology is the attempt to understand our mental faculties in light of the evolutionary processes that shaped them. Stephen Jay Gould calls its ideas and their proponents "foolish," "fatuous," "pathetic," "egregiously simplistic," and some twenty-five synonyms for "fanatical." Such language is not just discourteous; it is misguided, for the ideas of evolutionary psychology are not as stupid as Gould makes them out to be. Indeed, they are nothing like what Gould makes them out to be.

Evolutionary psychology often investigates the adaptive functions of cognitive and emotional systems -- how natural selection "engineered" them to solve the kinds of problems faced by our ancestors in their struggle to survive and reproduce. The rationale follows from two premises Gould himself states nicely:

(1) "I ... do not deny either the existence and central importance of adaptation, or the production of adaptation by natural selection. Yes, eyes are for seeing and feet are for moving. And, yes again, I know of no scientific mechanism other than natural selection with the proven power to build structures for such eminently workable design."

(2) "The human brain is the most complicated device for reasoning and calculating ... ever evolved on earth."

Quite so. First, adaptive design must be a product of natural selection. Complex organs like eyes have many precise parts in exacting arrangements, and the odds are astronomically stacked against their having arisen fortuitously from random genetic drift or as a by-product of something else. Second, the brain, like the eyes and the feet, shows signs of good design. The adaptive problems it solves, such as perceiving depth and color, grasping, walking, reasoning, communicating, avoiding hazards, recognizing people and their mental states, and juggling competing demands in real time are among the most challenging engineering tasks ever stated, far beyond the capacity of foreseeable computers and robots. Put the premises together -- complex design comes from natural selection, and the brain shows signs of complex design -- and we conclude that much of the brain should be explained by natural selection.

So where's the controversy? Gould claims his targets invoke selection to explain everything. They don't. Everyone agrees that aspects of the living world without adaptive complexity -- numbers of species, nonfunctional features, trends in the fossil record -- often need different kinds of explanations, from genetic drift to wayward asteroids. So yes, we all should be, and are, pluralists. But we should not be indiscriminate pluralists. Gould blurs his own distinction when he writes,

We live in a world of enormous complexity in organic design and diversity -- a world where some features of organisms evolved by an algorithmic form of natural selection, some by an equally algorithmic theory of unselected neutrality, some by the vagaries of history's contingency, and some as byproducts of other processes. Why should such a complex and various world yield to one narrowly construed cause?

It shouldn't, of course, but then most researchers aren't trying to explain the entire "complex and various world." Many of them are trying to explain "complexity in organic design" -- the remarkable natural engineering behind the ability of creatures to fly, swim, move, see, and think. Now, complex design should yield to one "narrowly construed cause" -- Gould knows of no scientific mechanism other than natural selection with the proven power to build it, remember? Those blinkered, narrow, rigid, miserly, uncompromising ultra-pan- selectionists whom Gould attacks are simply explaining complex design in terms of its only known cause.

In the case of the human brain, Gould accuses evolutionary psychologists of ignoring an alternative:

Natural selection made the human brain big, but most of our mental properties and potentials may be spandrels -- that is, nonadaptive side consequences of building a device with such structural complexity.

Evolutionary psychologists are not ignorant of this hypothesis. They have considered it and found it to be unhelpful.

First, it is rooted in a false dichotomy between "conventional natural selection working in the engineering mode" and "spandrels," the nonadaptive by-products that are "sources for later and fruitful reuse" and which "may later be co-opted" for useful purposes. What is missing from these phrases is the subject of the verb. Reuse by whom? Co-opted by what? Most snails have a spandrel formed by the space around their shell axis; what allows some species to use it to brood their eggs? Are they generally more clever and dextrous? No; their anatomy and nervous systems have been altered in an adaptive way to take advantage of the spandrel. So the re-user and co-opter are none other than: natural selection. Not only do co-opted spandrels implicate selection, but selection implicates spandrels. We evolved from organisms without eyes, feet, and other complex organs. The organs must have originated in precursors that were spandrels for some ancestral organism. The distinction in which spandrels work "in addition (and sometimes even opposed to)" natural selection is spurious.

Unlike snails, of course, we humans are clever enough to co-opt our spandrels in our lifetimes, as when we use our noses to hold up eyeglasses. But how did our brains get clever enough to do that? This is exactly what a theory of brain evolution must explain. Explaining the evolution of the human intellect in terms of humans' ability to co-opt spandrels is circular.

Second, Gould casually slides from saying that natural selection made the brain "big" to saying that the brain was built with "structural complexity," as if bigness and complexity were the same thing. As Gould himself has argued, bigger brains aren't necessarily more complex or smarter brains. Worse, the suggestion that humans were selected for bigger brains is a perfect example of the sin Gould attributes to others, the confusion of a by-product with an adaptation. If anything is a by-product, it is the size of the human brain, which guzzles nutrients, makes us vulnerable to blows and falls, compromises the biomechanical design of the woman's pelvis, and makes childbirth dangerous. Bigness of brain is surely a by-product of selection for more complex (and hence hardware-demanding) computational abilities, ones that allowed our ancestors to deal with tools, the natural world, and one another.

A rejection of Gould's theory does not put non-adaptive features "outside the compass of evolutionary psychology"; nor was Gould the first to call attention to them. The original arguments for recognizing non-adaptive features came from the founding document of evolutionary psychology, George Williams' Adaptation and Natural Selection, long before Gould and Lewontin reiterated them (without attribution) in their "Spandrels" paper. Nonadaptive explanations have been commonplace in the field ever since, as Gould must be well aware, for in one column he touted a non-adaptive explanation of the female orgasm taken from another founder of evolutionary psychology, Donald Symons. According to the most popular view in the field, many other important human activities are spandrels, including art, music, religion, science, and dreams. Gould's accusation is not even close to being accurate.

Evolutionary psychology is "even more fatuous," according to Gould, for thinking seriously about the environment in which our ancestors evolved. That is "outside the primary definition of science," he says, because claims about that environment "usually cannot be tested in principle but only subjected to speculation." Really? Then what makes Gould so certain that our ancestors' environment lacked written language -- the basis for his argument that reading is a spandrel? Obviously it is the archeological record, which shows that writing is a recent invention, and the ethnographic record, which shows that writing is absent from cultures not in contact with any of the inventors. It is precisely such evidence that leads evolutionary psychologists to infer that the ancestral environment lacked agriculture, contraception, high-tech medicine, mass media, mass-produced goods, money, police, armies, communities of strangers, and other modern features -- absences with profound implications for the minds that evolved in such an environment.

Gould is uninformed when he repeats the cliche that evolutionary reasoning is just cocktail-party speculation. The standards of the field require a good empirical fit between the engineering demands of an adaptive problem and the facts of human psychology. The former is grounded in game-theoretic and other optimality analyses, in artificial intelligence and artificial life simulations, and in relevant sciences such as genetics, physiology, optics, or ecology. The latter is based on converging evidence from experiments with children, adults, and neurological patients and from survey, historical, ethnographic, paleoanthropological, archeological, and economic data. Far from being "barren," the adaptationist approach has, for over a century, driven the most rigorous, elegant, and empirically rich branch of psychology, perception. Today it is spawning new insights and intensive modeling and data-gathering on every other aspect of the mind, including reasoning, mental imagery, memory, language, beauty, sexual desire, autism, emotions such as fear and disgust, violence, the numerical abilities of children and animals, and the shaping of personality.(Note 1) Gould's hostility to this exciting field is a missed opportunity for both.

NOTES (1)For recent reviews, see The Adapted Mind, edited by J. Barkow, L. Cosmides, & J. Tooby; The Moral Animal, by R. Wright, and How the Mind Works, by S. Pinker.