The process by which genetic evolution and cultural evolution appear to be linked is usually called gene-culture coevolution. The theory of gene-culture coevolution incorporates the two levels of approach I cited earlier as the core of modern biology. Put as briefly as possible, they are that living processes are physicochemical and also self-assembled by natural selection. The first level is composed of proximate explanations, which describe the structures and processes by which an organism responds. The question of interest in any proximate explanation is, How does the phenomenon occur? The second level is composed of ultimate, or evolutionary, explanations, which account for the origin of the structures and processes, usually by the adaptive advantage they confer on organisms. The question of interest at this level is, Why does the phenomenon occur? In the case of hereditarily based incest avoidance, the proximate causes are emigration and the Westermarck effect. The ultimate cause is the deleterious effects of inbreeding, which by natural selection has driven the species toward emigration and the Westermarck effect.

The theory of gene-culture coevolution is still spotty and largely untested. Nevertheless, I believe that most researchers on the subject would agree with the following outline of the present form of the theory: People survive and leave offspring to the degree that they learn and adapt to the culture of their society, and the societies themselves flourish or decline in proportion to the effectiveness of their adaptation to their environment and surrounding societies. For hundreds of millennia certain aptitudes and cultural norms have arisen that are consistently adaptive in this Darwinian sense.

They include language facility, cooperativeness within the group, exogamy and incest avoidance, rites of passage, territoriality, male polygyny, and parent-offspring bonding. Hereditary epigenetic rules have evolved that pull individual preference, and hence cultural evolution, toward these norms. They comprise the elements of what we subjectively call human nature. The genes prescribing them also increase in frequency as a result of the same process. Spreading through the population, maintained by the edge they give most of the time in survival and reproduction, they have secured the stability of human nature across societies and generations.

To conclude my synopsis of the theory, cultural evolution is much faster than genetic evolution. One result is nongenetic cultural diversity, which scatters particular cultural variants around each central, genetic trend to a degree determined by the strength of the epigenetic rules affecting them. The products of cultural evolution, multiplying rapidly through the population, can improve the fitness of individuals and societies, or they can reduce them. But only if the advantage or disadvantage is sustained for many generations–population genetics theory would suggest at least ten–can the epigenetic rules and the genes prescribing them be replaced. That is why human nature today remains Paleolithic even in the midst of accelerating technological advance. Thus corporate CEOs impelled by stone-age emotions work international deals with cellular telephones at thirty thousand feet.

If it is granted that the human condition is subject to consilient explanation from genes to mind to culture, even as a working hypothesis, the consequences to follow will be considerable. The first is support across the great branches of learning for what can appropriately be called “gap analysis” as a research strategy. Already a mainstay of the natural sciences, gap analysis is the systematic attempt to identify domains of phenomena in which important discoveries are most likely to be made. Its most productive method is reduction, the search for novel explanations of phenomena already known, by examination of the next level of organization down. Successful reduction confirms the existence of elements in the lower level that interact to create the higher level. In this manner, molecular biology was created de novo from the basic chemistry of macromolecules, and the study of cells and tissues was revolutionized by molecular biology.

The social sciences, I believe, will advance more rapidly if they adopt a consilient worldview and the gap analysis suggested by it leading to reductionist analysis. They have failed to give this approach a try, except in a few sectors such as biological anthropology, largely because of their aversion to biology. The reasons for the aversion are complex, stemming partly from the effort of the social science disciplines–anthropology, economics, political science, and sociology–to maintain intellectual independence, partly from the daunting complexity of the subject, and partly from fear of the misuse of biology to support racist ideology.

Still, biology is the logical foundational discipline of the social sciences. I mean by this assessment biology as broadly defined, including much of contemporary psychology, especially cognitive psychology, which is in the process of being subsumed by neurobiology and the brain sciences. A great majority of social scientists, including the most influential theoreticians in economics, build their models as if this information does not exist. Their conceptions of human behavior come either from folk psychology–intuitive notions that seem right but are often factually wrong–or from notions of the mind as an optimizing device for rational choice. They ignore contrary signs from genetics, neurobiology, cognitive psychology, and the many quirky properties of human nature. For them history began a few thousand years ago with the rise of complex societies, overlooking the fact that it began hundreds of thousands of years ago with the evolutionary origins of human nature in hunter-gatherer bands.

In summary, it is hard to imagine how the social sciences can unite and achieve general, predictive theory without taking a reductionist approach to the phenomena of human nature, both their proximate causes in the machinery of the brain and their ultimate causes in deep, evolutionary history.

The theory and criticism of the arts can also benefit in the same fashion. Let me cite several examples already in hand. We now know, from neurobiology and the brain sciences, how the brain breaks down and classifies the continuously varying wavelength of visible light into four basic colors, namely blue, green, yellow, and red. The process has been tracked in segments from the base sequences in the DNA that prescribe the cone pigments of the photosensitive retinal cells to the nerve-cell sequences that lead from the retina to the primary visual cortex at the extreme rear of the brain. From anthropological and linguistic studies we know that people in societies around the world fix their color terms toward the centers of the primary colors in the spectrum and away from the intermediate and hence ambiguous wavelengths.

Finally, we know that as societies increase their color vocabularies, in the course of cultural evolution, they tend to employ up to eleven basic terms, usually accumulating them in the following sequence: Languages with only two basic color terms use them to distinguish black and white; languages with only three terms have words for black, white, and red; languages with only four terms have words for black, white, red, and either green or yellow; languages with only five terms have words for black, white, red, green, and yellow; and so on until all eleven terms are included, as exemplified in the English language. The sequence cannot be due to chance alone. If the terms were combined at random, there would be 2,036 possible combinations. But for the most part they are drawn from only 22. Surely this is the kind of information needed to produce a coherent theory of aesthetics in the visual arts.

In another domain relevant to visual aesthetics, neurobiological measurements have shown that the brain is most aroused by abstract designs in which there is about 20 percent repetition of elements. That is the amount of redundancy found in a simple maze, two turns of a logarithmic spiral, or an asymmetrical cross. It seems hardly a coincidence that roughly the same property is shared by a great deal of the art in friezes, grillwork, colophons, and flag designs. Or that it crops up gain in the glyphs of ancient Egypt and Mesoamerica as well as the pictographs of Japanese, Chinese, Thai, Bengali, and other Asian languages. The response appears to be innate: Newborn infants gaze longest at figures with about the same amount of redundancy.

In yet another topic of aesthetics, ideal female facial beauty as judged in at least two cultures, European and Japanese, has recently been found to follow some surprising principles. Using blended and artificially altered photographs, psychologists have discovered that the most admired facial features are near the anatomical average of the population but with heightened cheekbones, reduced chin size, enlarged eyes, and shortened distance between nose and chin. The cause of this effect, if upheld as inborn by further cross-cultural and developmental studies, is unknown. It could represent an innate recognition of the signs of youthfulness and hence greater reproductive potential.

The creative arts themselves, in literature, the visual arts, drama, music, and dance, may not be affected significantly by such knowledge from the natural sciences. The purpose of the arts is to transmit personal experience and emotion directly from mind to mind while avoiding explanation of the logic behind the creative work; thus, ars est celare artem, it is art to conceal art. But theory and criticism of the arts, which does attempt this mode of explanation, cannot help but be strengthened by the new information. If the greatest art is indeed that which touches all humanity, as commonly said, it follow that consilient cause-and-effect accounts of human nature will become increasingly foundational to sound theory and criticism.

Editor’s note Frank Method, director of UNESCO Washington, has suggested a further article on consilience in Science by Edward O. Wilson to any group or organization concerned with crossing disciplines to further knowledge.