According to the parsimonious theory of mind, emotions are the modifications of neural activity that animate and focus the scenarios. An act of decision is the prevalence of certain future scenarios over others; those that prevail are most likely to be the ones most conformable to instinct and reinforcement from prior experience. What we think of as meaning is the linkage among neural networks. Learning is the spreading activation that enlarges imagery and engages emotion. The self (to continue the parsimonious theory) is the key dramatic character of the scenarios. It must exist, because the brain is located within the body, and the body is the constant intense focus of real-time sensory experience and decision making.

The primary environment in which the mind develops is culture. This highest level of human activity was defined in 1952 by Alfred Kroeber and Clyde Kluckhohn, out of a review of 164 prior definitions, as follows: “Culture is a product; is historical; includes ideas, patterns, and values; is relative; is learned; is based upon symbols; and is an abstraction from behavior and the products of behavior.” It comprises the life of a society, the totality of its religion, myths, art, technology, sports, and all the other systematic knowledge transmitted across generations.

Throughout this century scholars in all the branches of learning have treated culture as an entity apart, comprehensible only on its own terms and not those of the natural sciences. By this conception culture stands apart even if the mind has a reducible, material basis; it must do so first because the fine details of the cultures comprise phenomena too complicated, too flickering through time, and too subtle to be subject to natural scientific analysis.

A fixed belief in the independent nature of culture has contributed to the isolation of the social sciences and humanities from the natural sciences throughout modern history. It is the basis of the discontinuity famously cited by C. P. Snow in 1959 as separating the scientific culture from the literary culture. Now there is reason to believe that the difference is not a true epistemological discontinuity, not a divide between two kinds of reality, but something far less forbidding and yet much more interesting. The boundary between the two cultures is instead a vast, unexplored terrain of phenomena awaiting entry from both sides.

The terrain is the interaction between genetic evolution and cultural evolution. We know that culture is learned. At the same time, evidence is mounting that learning is genetically biased; it is becoming increasingly accepted that culture is influenced by human nature. But what exactly is human nature? It is not the genes, which prescribe it, or the cultural universals, which are its most obvious products. It is the epigenetic rules, the hereditary biases that guide the development of individual behavior. There are several examples of epigenetic rules that can be cited in this early stage of investigation.

The facial expressions denoting the elementary emotions of fear, loathing, anger, surprise, and happiness are human universals and evidently inherited. They are adjusted by cultural evolution within individual societies to project particular nuances of meaning. The smile, one of the basic elements of emotive communication, appears at two to four months in infants everywhere, virtually independent of environment. It occurs on schedule in deaf-blind infants and even in thalidomide-crippled children who cannot touch their own faces.

The tendency to fear snakes is another human universal. It is furthermore widespread, if not universal, in all other Old World primate species. Snakes are among the few stimuli that easily evoke true phobias in people–the deep and intractable visceral reactions acquired with only one or two frightening experiences. They share their power with heights, closed spaces, running water, spiders, and other ancient perils of humanity; a similar degree of sensitivity does not exist for knives, guns, electric sockets, automobiles, and other modern sources of risk. The cultural consequences of the response to snakes, combining fear and intense curiosity, are manifold. Snakes are among the animals most commonly experienced in dreams, even among urbanites who have never seen one in life. They play prominent mythic roles in cultures around the world, taking new forms variously as demons, dragons, seducers, magical healers, and gods.

Automatic incest avoidance is universal in primate species studied to date, including Homo sapiens. The generally accepted adaptive explanation is the heightened risk that inbreeding poses of producing defective offspring, and that evolutionary inference is well supported by the evidence. The closer the genetic relationship of parents, the more likely they will bring together matching recessive genes that are deleterious in a double dose. Children of full siblings and of fathers and daughters, for example, have twice the early mortality rate of outbred children. Among those that survive, ten times more suffer genetic defects such as heart deformities, deaf-mutism, mental retardation, and dwarfism.

The epigenetic rules, or hereditary developmental biases that prevent incest, are two-layered in apes, monkeys, and other non-human primates. First, all species so far studied for the trait (nineteen worldwide) practice the equivalent of human exogamy: Young individuals leave the parent group and join another before they attain full maturity. Second, all species examined for the possible existence of the Westermarck effect also display that phenomenon. This means that individuals are sexually desensitized to individuals with whom they have been closely associated while very young, normally their parents and siblings. The critical period for the effect in human beings is the first thirty months of life. Out of the Westermarck effect have apparently risen incest taboos with all their supporting arsenal of legends and myths. The effect is enhanced in some but not all societies by a third barrier: the direct observation and correct rational understanding of the ill effects of incest.

Similar examples of epigenetic rules have multiplied in the literature of biology and the behavioral sciences during the past several decades. They have been found in virtually all categories of human behavior, including sexual and parental bonding, the acquisition of language, and even the cardinal role of trust during contract formation. They leave little doubt that a true hereditary human nature exists, and that it includes social behaviors held in common with nonhuman primate species and others that are diagnostically human.