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This paper, by psychologist / cognitive scientist Roger Shepard [1], appeared as a chapter in The Adapted Mind: Evolutionary Psychology and the Generation of Culture (Barkow, Cosmides, and Tooby 1992).

This may be one of the best academic papers I’ve ever read, in any field. As nothing more than a clear and concise description of the problem, it would have great value. The paper enumerates certain surprising (on a naive view) properties of colors (as perceived by humans):

  1. The perceptual constancy of colors. The light that strikes our retinas after being reflected from an object viewed at noon on a cloudless day, and the light that strikes our retinas after being reflected from the same object at dusk, has very different spectral composition. Yet we perceive the object to have the same color in both cases.

  2. The three-dimensional structure of colors. Despite varying in an unlimited number of dimensions (one dimension per wavelength, the number of which has continuum cardinality [modulo quantum effects, probably]), colors, as experienced by humans, vary in only three dimensions.

  3. The circular structure of spectrally pure colors. Spectrally pure red and violet, despite being maximally distant from each other in wavelength, are perceptually closer to each other than either is to spectrally pure green.

  4. The universal organization of colors into categories and prototypes. How the color space is organized perceptually is remarkably constant across cultural, ethnic, and linguistic variations. (I can’t do justice, here, to this quite surprising, and little-appreciated, point; read the paper for the full treatment.)

The paper then asks: Whence these specific properties? Are they merely accidental, historically contingent on random chance? Or perhaps there is something in our evolutionary history that caused our color perception to be exactly as it is, some reason why it could not be otherwise?


The paper then goes on to consider answers to these questions, comprehensively, taking care to note methodological challenges, consider counterexamples, etc. (The punchline, as it were, is given away in the title; the devil, as usual, is in the details, and I highly recommend reading the whole thing—it’s an easy read, written in concise and clear language.)


Finally, Shepard, in the introduction and the conclusion of the paper, sets out a general case, first, for evolutionary psychology as an indispensable tool for psychological investigation, and second, for looking to universal (rather than particular) properties of our environment as explanatory factors. I can do no better than Shepard’s own words, which are worth quoting at length:

Those taking an evolutionary approach to the behavioral, cognitive, and social sciences have been emphasizing the natural selection of mechanisms and strategies that are specific to particular species, genders, and problem domains. This emphasis on the particular is understandable as a reaction against the tendency, long dominant in these sciences, to proceed as if human and animal behavior could be explained in terms of just two things: (a) general laws of learning and cognitive constraints that hold across species, genders, and domains, and (b) the particular set of environmental (including cultural) circumstances to which each animal can adapt only by learning through its own individual experience. Evolutionary theorists’ emphasis on specific adaptations may also reflect the tendency of such adaptations to catch our attention through the very diversity of their specificity. Adaptations to universal features of our world are apt to escape our notice simply because we do not observe anything with which such adaptations stand in contrast.

For over a century, psychological researchers have been probing the structures and processes of perception, memory, and thought that mediate the behaviors of humans and other animals. Typically, this probing has taken the form of behavioral experiments suggested by evidence from one or more of three sources: (a) introspections into one’s own experience and inner processes, (b) information gleaned about the anatomy or physiology of the underlying physical mechanisms, and (c) results obtained from previous behavioral studies. More recently, in seeking to understand not only the nature but also the origins of psychological principles, some of us have been turning to a fourth source for guidance—namely, to the ecological properties of the world in which we have evolved and to the advantages to be realized by individuals who have genetically internalized representations of those properties.

Taken by themselves, findings based on introspective, behavioral, and physiological evidence alike, however well established and mutually consistent they may be, remain as little more than “brute facts” about the human or animal subjects studied. What such findings reveal might be merely arbitrary or ad hoc properties of the particular collection of terrestrial species investigated. Even our own perceptual and cognitive capabilities, as much as our own bodily sizes and shapes, may be the products of a history of more or less accidental circumstances peculiar to just one among uncounted evolutionary lines. Certainly, these capabilities do not appear to be wholly dictated by what is physically possible.

The following are just a few of the easily stated and well known of our perceptual/cognitive limitations, as these have been demonstrated under highly controlled but nonnaturalistic laboratory conditions:

… [list of human limitations and capabilities] …

To the extent that psychological science fails to identify nonarbitrary reasons or sources for these perceptual/cognitive limitations and for these perceptual/cognitive capabilities, this science will remain a merely descriptive science of this or that particular terrestrial species. This is true even if we are able to show that these limitations and capabilities are consequences of the structures of underlying neurophysiological mechanisms. Those neurophysiological structures can themselves be deemed nonarbitrary only to the extent that they can be seen to derive from some ultimately nonarbitrary source.

Where, then, should we look for such a nonarbitrary source? The answer can only be, “In the world.” All niches capable of supporting the evolution and maintenance of intelligent life, though differing in numerous details, share some general—perhaps even universal—properties. It is to these properties that we must look for the ultimate, nonarbitrary sources of the regularities that we find in perception/cognition as well as in its underlying neurophysiological substrate.

Some of the properties that I have in mind here are the following (see Shepard, 1987a, 1987b, 1988, 1989): Space is three-dimensional, locally Euclidean, and endowed with a gravitationally conferred unique upward direction. Time is one-dimensional and endowed with a thermodynamically conferred unique forward direction. Periods of relative warmth and light (owing to the conservation of angular momentum of planetary rotation) regularly alternate with periods of relative coolness and darkness. And objects having an important consequence are of a particular natural kind and therefore correspond to a generally compact connected region in the space of possible objects—however much those objects may vary in their sensible properties (of size, shape, color, odor, motion, and so on).

Among the genes arising through random mutations, then, natural selection must have favored genes not only on the basis of how well they propagated under the special circumstances peculiar to the ecological niche currently occupied, but also, as I have argued previously (e.g., Shepard, 1987a), even more consistently in the long run, according to how well they propagate under the general circumstances common to all ecological niches. For, as an evolutionary line branches into each new niche, the selective pressures on gene propagation that are guaranteed to remain unchanged are just those pressures that are common to all niches.

Motivated by these considerations, much of my own recent work on perception and cognition in humans has sought evidence that our perceptual/cognitive systems have in fact internalized, especially deeply, the most pervasive and enduring constraints of the external world. …

I began this chapter with the claim that natural selection should lead to the emergence not only of perceptual, behavioral, and cognitive mechanisms that are adapted to the specific circumstances faced by particular species, genders, and domains, but also of mechanisms that are adapted to the general circumstances faced by all so-called higher organisms. Color vision offers a less obvious case for supporting such a claim than the phenomena that I have usually used for this purpose, namely, those of spatial representation (e.g., Shepard, 1984) and of generalization (e.g., Shepard, 1987b). Color pertains to a single sensory modality (vision), whereas space and generalization are not modality specific. Moreover, the properties in the world that might determine the organization of colors are less obvious than those that might determine the representation of space. For example, whereas the three-dimensionality of perceptual space derives directly from the fundamental three-dimensionality of physical space, the three-dimensionality of color space does not so directly or obviously derive from a three-dimensionality in the world. The case for a nonarbitrary basis for the structure of colors, to the extent that it can be made, has the advantage, however, of suggesting that psychological constraints may correspond to regularities in the world even when the correspondence is not initially obvious.

[emphasis mine]


[1] Roger Shepard is one of the most important cognitive scientists of the 20th century. In addition to this research, he and Jacqueline Metzler conducted the brilliant research that established the angular velocity of mental rotation (60°/sec, as it happens). He also discovered the Shepard tone.

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    At the end of the paper, Shepard also speculates on the subject of xeno-evo-psych, so to speak:

    I conclude with a brief consideration of the sense, if any, in which a tendency toward the properties of color representation I have been considering may be general—or even universal. Throughout, I have spoken of the possibility of selective pressures to which highly visual terrestrial animals may be subject in the terrestrial environment. I used the qualification “terrestrial” because conditions of illumination in other, for exam-ple, aquatic environments can be quite different. Owing to the already noted selective absorption by water of the longer wavelengths, with increasing depth in a marine envi-ronment, the available solar light, in addition to being progressively reduced in overall intensity, becomes progressively restricted in spectral range to the shorter wavelengths. This blue shift and compression in range of available wavelengths is known to be matched by a corresponding blue shift and compression in the range of spectral sensitivity of deeper dwelling marine animals (Lythgoe, 1979).

    It does seem to me, however, that the wavelength dependencies of the variable height of a sun, presence of atmospheric aerosols, and direct versus indirect illumination might apply quite generally on the surfaces of planets capable of supporting the evolution of highly visual organisms. Such a planet is presumably likely (a) to circle a long-lived star that emits a stable, broad range of wavelengths, (b) to undergo regular rotation about its own axis (owing to the conservation of angular momentum), and (c) to possess an atmosphere that differentially filters the wavelengths of direct and scattered light depending on the (rotationally determined) angle at which the light enters the atmosphere and the size distribution of atmospheric particles. Moreover, the arguments for categorical representation of colors based on memorability and shareability do not depend on particular features of the terrestrial environment. In short, just as there may be universal selective pressures toward mechanisms for the representation of three-dimensional space and for an exponential law of generalization (Shepard, 1987a), there may be quite general selective pressures toward mechanisms for the representation of the surface characteristics of objects in a low-dimensional (perhaps even a three dimensional) color space, with a circular component of hue, and a categorical structure.

    Possibly, behavioral and cognitive theorists should aspire to a wider scope for their science. An evolutionary theory of mind need not confine itself to the particular minds of the more or less accidental collection of species we find on planet Earth. There may be quite general or even universal principles that characterize planetary environments capable of supporting the origin and evolution of increasingly complex forms of life. If so, there may be corresponding general or even universal principles of mind that by virtue of their mesh with the principles of these environments, are favored by a process of natural selection wherever it may be taking place.

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