Like many non-specialist popularizers of psychology, Professor Sagan [in The Dragons of Eden] overestimates our physiological knowledge and underestimates our psychological knowledge. I'll get back to this point later. First, I must acknowledge that Professor Sagan has taken the first hard step in learning psychology. The first step in studying psychology is to convince yourself that there is something to study, above and beyond common sense and common knowledge. Professor Sagan likes the theory of the "triune brain," as formulated by a neurophysiologist named Paul MacLean in the early 1950's. Not an active theory in the technical literature these days, it nevertheless appeals to popularizers…. The theory depicts the human brain as combining in uneasy equilibrium our reptilian ancestry, our pre-human mammalian ancestry, and our rational, competent selves. A reptile, a mammal, and a human reason within each skin—with these wild cards, Professor Sagan can play just about any hand he wants.

Tripartite psychologies are hardly new…. I have not counted, but there must be at least a dozen major psychological systems based on tripartite divisions and very few based on any number other than three. Why is this? I'm not sure I know the answer, but I'd guess that it has little to do with the facts of matter. (pp. 67-8)

The first problem with this book's tripartite psychology is … its inherent weakness. Another is that, as psychology as distinguished from anatomy, it is no match for [Sigmund] Freud's or Sheldon's or even Aristotle's. It appeals to brain anatomy while trying to say something about human behavior. They are not the same, although non-psychologists and beginning psychology students looking for a scientific psychology almost always gravitate to physiology and anatomy. Anatomy and physiology are tangible and technical, and biologists generally enjoy higher status in the academic pecking order than psychologists. It takes conscious effort and a certain amount of sheer study to resist the lure of physiologizing and to see that a creature's psychology not only can, but must, be studied in its own right before scientific sense can be made of it. When he physiologizes his psychological hunches, Professor Sagan is investing his hunches with specious authority from a "harder" science.

The triune brain probably appeals to Professor Sagan because it seems to jibe with his psychological intuition. It captures in an evolutionary metaphor the idea that conflicting motives, conflicting values, and conflicting forms of knowledge are the human condition. Many of the other tripartite psychologies are also metaphors expressing more or less the same idea. Without further evidence, one metaphor is about as good as another, assuming we are after science, not poetry. The evidence must, moreover, be psychological. Except for a review of language in chimpanzees, The Dragons of Eden lacks psychological data. Besides anatomy, there is paleontology, geology, garnishes of astronomy, chemistry, and physics, but almost no psychology.

For psychology, Professor Sagan mostly relies on intuition. His intuitions are not unreasonable; he believes we possess both logical and alogical tendencies, that we are not as rational as we think we are, that we are conscious of only a fraction of the psychic forces that move us, that some of our psychology is adaptive for a world that no longer exists, that human relationships resemble to some extent relationships among other animals. But it is a pity that Professor Sagan has ignored the evidence that refines and transcends those sensible (though hardly original) hunches of his. Even more, it is a pity that Professor Sagan has ignored the rich, growing, often surprising and occasionally sobering discipline of psychology. But then, I have yet to stumble across a popularizer who knows much psychology, let alone bothers to popularize it. (p. 68)

Professor Sagan can rest easy; his message will trouble very few (except the occasional psychologist who looks at the book). For example, Professor Sagan comments in the course of his discussion of the frontal lobes of the brain:

Cassandric components of our nature are necessary for survival. The doctrines for regulating the future that they produced are the origins of ethics, magic, science, and legal codes. The benefit of foreseeing catastrophe is the ability to take steps to avoid it, sacrificing short-term for long-term benefits. A society that is, as a result of such foresight, materially secure generates the leisure time necessary for social and technological innovation.

Indeed, but did we need to know the gross anatomy of the brain to say this? And what, in fact, are the bases of ethics, magic; science, and legal codes? How does an organism, human or otherwise, learn to sacrifice short-term for long-term benefits? What are the relevant abilities, and what are the limitations on the abilities? Would it surprise Professor Sagan to learn that there are data, real data, about the psychology (rather than the pseudo-physiology) of ethics, magic, and all the rest in this little passage? It would certainly surprise those of his readers who are relying on him for finding out about human psychology. (p. 69)

Though he knows how profoundly science can change our picture of the world, Professor Sagan seems not to realize that the psychological shift in outlook promises to be deeper, broader, and at times more difficult to accept than the Copernican and Darwinian ones were. Once again, people are to confront a science that challenges their egocentricity, but this time it is their very sense of themselves and their personal destinies that are to be challenged, not such relatively secondary possessions as the planet on which, or the bodies in which, they live. The intensity of their resistance is already apparent in their alarmed reactions to B. F. Skinner's conditioning procedures or A. R. Jensen's assessments of individual differences in cognitive ability. None of this surfaces in, or even ripples the surface of, Professor Sagan's book. He is asking his readers to change their minds about almost nothing, though doing so with grace, humor, and style. (p. 70)