Evolutionary Psychology: An Exchange
by Steven Pinker
volutionary
psychology is the attempt to understand our mental faculties in light of the
evolutionary processes that shaped them. Stephen Jay Gould [NYR, June 12 and
June 26] 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 byproduct 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-panselectionists 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 byproducts 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 byproduct with an adaptation. If anything is a byproduct,
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 byproduct 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 nonadaptive
features "outside the compass of evolutionary psychology"; nor was Gould the
first to call attention to them. The original arguments for recognizing
nonadaptive features came from the founding document of evolutionary psychology,
George Williams's 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 nonadaptive 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 cliché 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.[1]
Gould's hostility to this exciting field is a missed opportunity for both.
Steven Pinker
Director, McDonnell-Pew
Center for Cognitive Neuroscience
Massachusetts Institute of Technology
Cambridge, Massachusetts
Werner Kalow and Harold Kalant
To the Editors: In his recent two-part article on
"Darwinian Fundamentalism" Stephen Jay Gould makes the important point that
natural selection is not the only element in evolution, though it is
undoubtedly an important one. He quotes Darwin himself in support of this
argument, and coins the memorable dictum "Variation proposes and selection
disposes." Professor Gould also makes the extremely important argument that
many variants are carried forward in the progeny despite conferring no
survival advantage, and applies to these neutral variants the amusing term
"spandrels." This term, borrowed from architecture, is appropriately used in
Professor Gould's field of paleobiology, which for obvious reasons rests
heavily on the architecture of fossil remains.
As pharmacologists, we would suggest that this argument can
derive even stronger support from the genetic studies of variation in drug
response among present-day living organisms. Pharmacogenetics is a specialized
area of genetics, more familiar to many physicians and chemical companies than
to most geneticists. In medicine, pharmacogenetic variation accounts for many
differences between people with respect to their responses to a given drug, even
to the point that some individuals have been fatally poisoned by a drug that was
curative to most of those who received it. Such dramatic variations can be due
to genetically determined differences either in the metabolism of a drug or in
the cellular mechanisms on which the drug acts. Pharmacogenetic variability is
also manifested in the responses of insects to chemical insecticides, and of
bacteria to many different antibiotics.
Many (though not all) of these variations in response to drugs
or toxins arise from random gene mutations. Some of these mutations are clearly
disadvantageous, decreasing the reproductive fitness of the mutant individuals or
decreasing the survival of the progeny, and are therefore eliminated by the
forces of natural selection. Others are advantageous in these respects,
conferring a survival advantage, and therefore lead to gradual evolution of the
species in accord with the classical Darwinian concept. Professor Gould's
argument is strongly supported, however, by the fact that most of the mutations
that survive in the offspring are more or less neutral with respect to
reproductive advantage,[2] or perhaps cause slightly
reduced fitness.[3] Why, then, do they survive? The
answer appears to be that they constitute a sort of biological insurance policy
for the species, rather than for the individual. Like Professor Gould's
"spandrels," they have no particular use when they arise, but may acquire a use
later on.
For example, a particular pharmacogenetic variation in an
insect species may make certain individuals in a given insect population extremely
resistant to a new insecticide. However, such variations arise long before the
insecticide appears on the scene, and in many instances are somewhat maladaptive
in the absence of a poison, so that the frequency of the variant gene remains low
in the population. When the new chemical appears, however, the variant individuals
have a much better chance of surviving, and thus enable the species as a whole to
survive. Once the chemical assault has passed, the variant individuals are again
at a reproductive disadvantage with respect to the typical population in the
normal environment, and their numbers again decrease to the previous low
"insurance" level.[4] Only if the chemical stress is
maintained over generations does the mutant type eventually become the most
prevalent one, through the death of the previously dominant type that was not
resistant to the new insecticide. In that case, the otherwise disadvantageous
mutation then becomes the basis of an evolutionary change.
In short, pharmacogenetic variation operates for the benefit
of a population, but not necessarily for the overall benefit of the variant
individual. Whether or not a given pharmacogenetic variant will ever be used
cannot be known in advance. The "ultra-orthodox" Darwinian view, as Professor
Gould has argued, is therefore a marked oversimplification, and ignores the
importance of temporary or localized environmental factors in determining whether
a given mutation is or is not a survival advantage, independently of its effect
on general reproductive fitness.
Werner Kalow
Harold Kalant
Professors Emeriti
Department of Pharmacology
University of Toronto
Stephen Jay Gould replies:
f
we define poetic justice as defeat by one's own favored devices—Robespierre
before the guillotine or Midas in golden starvation—then we might be
intrigued to find Steven Pinker, a linguist by training, upended by his own use
of words.
He begins by unjustly characterizing my two recent articles on
"Darwinian Fundamentalism" as a
misguided attack on the nascent field of evolutionary psychology. I can't imagine,
first of all, what thesaurus could cast such a broad net for synonyms of "fanatic."
More importantly, I cite evolutionary psychology as just one illustration within a
much wider critique—and I devote only the last part of my second article to
the subject. My objections, however forceful, are clearly offered with constructive
intent, for I praise the field's goal, while arguing that a truly evolutionary
psychology cannot arise when leading practitioners so strongly exaggerate an
adaptationist style of explanation that represents but one mode of evolutionary
causation among many legitimate alternatives. I wrote:
Humans are animals and the mind evolved; therefore, all curious
people must support the quest for an evolutionary psychology.
I also stated my central critique:
Evolutionary psychology could, in my view, become a
fruitful science by replacing its current penchant for narrow, and often barren,
speculation with respect for the pluralistic range of available alternatives that
are just as evolutionary in status, more probable in actual occurrence, and not
limited to the blinkered view that evolutionary explanations must identify
adaptation produced by natural selection.
Pinker then follows his false opening charge with a three-part
argument overturned by its own illogic and verbal inconsistency. The first third
denies that evolutionary psychologists rely exclusively on adaptation. The second
third (as I shall document below) shows how Pinker's restrictive focus upon
adaptationist thinking leads him to misunderstand the concept of spandrels. The
closing third then extols the power and range of adaptationist explanation, but
gives the game away by equating this limited mode with "evolutionary reasoning"
in general.
But the first and third parts contradict each other. Which
claim does Pinker want to make: that pluralism reigns in evolutionary psychology
(and I characterized the field unfairly), or that adaptationism reigns as a
synonym for "evolutionary reasoning" (and my warnings are sterile)? He can't have
them both. (My true position, of course, holds that adaptationism rules wrongly
and too restrictively.)
Pinker then centers the second part, the guts of his critique,
upon another verbal error that exposes the depth of his commitment to
adaptationist logic, and his consequent inability to conceptualize the
alternatives properly, if at all. (Words and taxonomies often exert a tyranny
over thoughts. If you have neither a term nor a category for something, you may
not be able to see it—no matter how largely or evidently it looms.)
Pinker quotes me correctly in noting that I accept natural
selection as the only known cause of "eminently workable design"—and he then
writes, again correctly (although I would add the restrictive adjective "complex"
to the beginning of the phrase), that "adaptive design must be the product of
natural selection." But, two paragraphs later, and now in the sarcastic mode, he
ridicules me with a very different claim that he regards as equivalent:
Those blinkered, narrow, rigid, miserly, uncompromising
ultra-panselectionists whom Gould attacks are simply explaining complex design
in terms of its only known cause.
I'm astonished that Pinker doesn't see the key fallacy here
(and he states the point several times, so he has not just made a careless slip):
"complex design" does not equate with "complex adaptive design" (or what I
preferred to call "eminently workable design"). Complex design forms a much
broader category than adaptive design—and has many other potential
evolutionary causes. Which brings us to the subject of "spandrels"—just one
of the nonadaptive ways to build crucial parts of complex designs (but
incomprehensible as a concept to Pinker because he conflates complexity with
adaptation).
Spandrels are architectural byproducts, or automatic
consequences, of building something in a certain way (and I am happy to allow
that natural selection usually sets the mode of building in biology—not at
all the same thing as saying that every part of the building is an adaptation!)[5] Pinker then makes a truly strange move to deny the importance
of spandrels—one that lays bare his adaptationist bias. He argues that when
an ancestral spandrel becomes modified for an adaptive purpose in a descendant
species, then natural selection is the agent of modification. Sure—and I have
said so, prominently, in all my papers on the subject.[6] But
so what? The origin of the spandrel remains nonadaptive as an automatic
architectural byproduct. The secondary modification for utility is, well,
secondary—and therefore not a criticism of the claim for nonadaptive origin
of the original feature.
In fact, Lewontin and I coined the term "spandrel" precisely to
make this crucial distinction between nonadaptive origin and possible later
utility.[7] We did this in order to expose one of the great
fallacies so commonly made in evolutionary argument: the misuse of a current
utility to infer an adaptive origin.
Reasons for origins must not be confused with alterations for
later use. Since evolutionary biologists are primarily interested in the origins of
features, such an error becomes crucial. The snail umbilicus is, I admit, a fairly
trivial example—but it illustrates the point and fallacy particularly well.
The umbilicus arises nonadaptively as a spandrel—a necessary geometric
consequence of growth by winding a tube around an axis. The fact that a very
few species later adapt this space secondarily as a brood chamber doesn't
challenge the claim for a nonadaptive origin of the space itself. After all,
thousands of snail species have umbilici and do not brood their young (or
do much of anything) in the necessary space.
Similarly, many universal features of human cognition—the
primary data of evolutionary psychology—probably arise as spandrels of a
general consciousness evolved for other reasons (almost surely adaptive). Freud
argued that our fear of death acts as a key inspiration for the universal human
institution of religion (for which many adaptationist explanations have been
proposed, largely in the speculative mode). But I don't see how a biologist could
argue that the human brain evolved consciousness in order to teach us that we must
die. Knowledge of death is therefore probably a spandrel—an ineluctable
consequence of consciousness evolved for other reasons. But this spandrel may
then have inspired one of our defining institutions.
Pinker then appends two specific errors to this general
fallacy—both further illustrating his failure to conceptualize the centrality
of spandrels and other forms of nonadaptation. First, in trying to argue further
that spandrels are adaptations (or intrinsically bound with adaptations), Pinker
errs in writing that "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." Here Pinker confuses spandrels with the fascinating
and well-known notion—so important for understanding the quirky and
unpredictable nature of evolutionary pathways—of "functional shift," a concept
stressed by Darwin himself, and often identified with the unfortunate and confusing
name of "preadaptation."
Structures evolved as adaptations for one function often get
co-opted for a different role in a descendant lineage. (In the classic case,
feathers evolved for thermoregulation in small running dinosaurs get co-opted later
for flight in birds.) I don't think that eyes or legs originated as spandrels, but
they did arise for one function and get co-opted for another (proto-eyes for light
sensitivity, later co-opted for image forming; legs (as fins) for balancing in
fishes, later co-opted for locomotion in terrestrial vertebrates)—whereas
spandrels arise nonadaptively, and may then be co-opted for later utility.
The distinction between spandrels and preadaptations couldn't be
more crucial—for preadaptation is an important and subtle concept within the
adaptationist program (the co-optation of one adaptive design for another and quite
different function), while a spandrel is a nonadaptive architectural byproduct that
might (but also might not, as in most snail umbilici) be co-opted later for an
adaptive use.
Pinker, I assume, doesn't grasp the distinction because his
viewpoint only admits arguments about adaptation into the domain of "evolutionary
reasoning" (his words)—so he cannot see beyond the single common feature of
secondary co-optation in spandrels and preadaptations, while he misses the key
distinction that spandrels originate as nonadaptive side consequences, and
therefore differ fundamentally from preadaptations.
Second, Pinker makes a serious, and false, charge about our
integrity by claiming that Lewontin and I failed to credit George Williams for
formulating "the original arguments for recognizing non-adaptive features" in "the
founding document of evolutionary psychology." I love Williams's book and cite it
frequently—but not in our spandrels paper because neither he, nor I, nor
anyone else in our century invented the idea. The concept has always been part of
evolutionary theory. It was stressed most prominently by William Bateson, the
inventor of the term "genetics," in his 1894 book, Materials for the Study of
Variation. Darwin also discussed the concept (under the phrase "correlations of
growth")—as Lewontin and I explored at length in our original paper, in a
section entitled "The master's voice reexamined." The problem does not lie in full
ignorance, but in the tendency of strict adaptationists to treat this inconvenient
exception as a trivial oddity at best—one that is then swept under the rug of
their favored and exclusive mechanism. Moreover, while I greatly value (and quote)
Symons's support for the nonadaptive status of clitoral orgasm, I derived the
argument from Alfred Kinsey's physiological studies. Interestingly, before Kinsey
switched his life's work to the source of his iconic notoriety, he spent twenty
years working on the taxonomy of the gall wasp Cynips, writing two famous
monographs well known for their iconoclastic doubts about adaptationist
explanations for the origin of new species.
The interesting letter from Kalow and Kalant illustrates
(without so intending) the importance of maintaining pluralistic alternatives in
evolutionary explanation. Strict Darwinians would explain this
phenomenon—maintenance of neutral (or even slightly deleterious) variation
that may later prove of great value in offering fortuitous resistance to a new
insecticide—as a classical case of ordinary selection at the conventional
organismic (or even genic) level, rather than, as Kalow and Kalant maintain, "a
sort of biological insurance policy for the species, rather than for the
individual." Strict Darwinians argue that mutations arise fortuitously, and at a
low but dependable rate, due to the chemistry of nucleic acids. If slightly
deleterious, these mutations get eliminated from the population, but only slowly.
So populations naturally maintain mutational variation of this kind, not because
selection acts at the unconventional level of groups, or even species (as Kalow
and Kalant argue), but because mutations constantly arise, and only get removed
slowly. If one such mutation turns out to confer a lucky advantage in a new
situation (a blast of DDT, for example), then the population survives by good
fortune—and again, strictly by ordinary Darwinian selection on organisms
now fortuitously "blessed" with a suddenly crucial mutation.
Ironically perhaps, I suspect that this standard Darwinian
explanation is probably correct and adequate in most cases of this sort. But
Kalow and Kalant's alternative explanation—positive selection at the
species level, based on enhanced variability as the feature subject to
selection—does represent a possible and testable alternative. Genetic
variability is a trait of populations, not of organisms—so if selection
works by conferring greater geological longevity or higher speciation rates
upon more variable species, then Darwin's process also operates at the species
level, a form of "supra-organismic" selection much disfavored (and formerly
strongly anathematized, but no longer as evidence and renewed respect
accumulates) by strict Darwinians. It is surely preferable, and more within the
spirit of science, to work with such interesting and testable[8]
alternatives, rather than simply to assert by fiat, speculation, and a priori
satisfaction, that natural selection on genes and organisms builds all complex
form and pattern in the richly varied history of life.
Notes
- For recent reviews, see Jerome Barkow, Leda Cosmides, and John
Tooby, editors, The Adapted
Mind: Evolutionary Psychology and the Generation of Culture (Oxford University Press,
1995); Robert Wright, The
Moral Animal: Evolutionary Psychology and Everyday Life (Pantheon, 1994); and
Steven Pinker, How the
Mind Works (Norton, 1997).
- M. Kimura, "Evolutionary rate at the molecular level,"
Nature 217, pp. 624-626, 1968.
- M. Gell-Mann,
The Quark and the
Jaguar (W.H. Freeman, 1994), p. 68; D.N. Cooper, M. Krawczak, and S.E.
Antonarakis, "The nature and mechanisms of human gene mutation," in C.R. Scriver
et al., editors, The Metabolism and Molecular Bases of Inherited Disease
(McGraw-Hill, 1995),pp. 259-291.
- Dr. Neil W. Forrester, New South Wales (Australia)
Agricultural Research Station, unpublished data; W. Kalow, "Pharmacogenetics in
biological perspective," Pharmacological Reviews, in press, 1997.
- The concept of spandrels has been much debated in the
biological literature. I have tried to analyze and rebut these criticisms in a
technical article to be published this month in the Proceedings of the National
Academy of Sciences, and entitled:
"The exaptive
excellence of spandrels as a term and prototype."
- See S.J. Gould and E.S. Vrba, "Exaptation: a missing
term in the science of form," Paleobiology, 1984.
- See S.J. Gould and R.C. Lewontin,
"The
spandrels of San Marco and the Panglossian paradigm," Proceedings of the
Royal Society of London, 1979.
- See E. Lloyd and S.J. Gould,
"Species selection on
variability," Proceedings of the National Academy of Sciences, 1993.
[ S. Pinker, et al. "Evolutionary Psychology: An Exchange,"
New York Review of Books,
44 (Oct. 9, 1997): 55-56. ]
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