"In a very real sense human beings are machines constructed by the nucleic acids to arrange for the efficient replication of more nucleic acids. ... We are, in a way, temporary ambulatory repositories for our nucleic acids."  Carl Sagan, The Cosmic Connection, Garden City, NY: Anchor Press, 1973.

Having described some basic tenets of genetics in the previous two chapters, we are now ready to undertake theoretical understandings of human behavior. It is important to keep in mind throughout the rest of this book that all behavior is the outcome of a competition among gene alleles for representation at specific locations on chromosomes.  Thus, the "macro behaviors" at the individual level, which will be the subject of the rest of this book, are the result of "micro motives" at the gene level.


An individual's "phenotype" is "the way it is"   its anatomy, physiology and behavior. An individual's "genotype" (inherited genes) interacts with environment to produce the "phenotype." Thus, a person's phenotype is who the person has become, as opposed to who they might have become had their environment been different. This powerful concept (Symons, 1979) can be referred to by the equation: G + E = P, or GEP.

Anatomy includes, for example, stature, the height a person achieves in adulthood. In a society where food is plentiful, as for most people in the United States, stature is determined almost entirely by the genes. But in a society where food is scarce for some people and plentiful for others, stature is determined most strongly by food availability (the environment, unless the genes determine access to food).

Consider the case of Japanese stature before and after World War II. Children after the war grew taller than their parents. Between the generations the disparity in the availability of food was large, and this diet difference was large "in relation to" the genetic variation between individuals. Stature differences were determined almost entirely by environment. This example illustrates that if the variation of environment is large, environment can be the dominate cause of phenotype variation, whereas if the variation of environment is small, making the variation of genotype more important, genotype can be the dominate cause of phenotype variation.

Physiology includes, for example, immune response. The genes create an immune system that includes a repertoire of responses to specific pathogen stimulations. A virus will elicit an immune response that is appropriate if our ancestors were survivors of the same (or similarly shaped) virus. The repertoire is limited by ancestor experience, so an individual is likely to be vulnerable to new viruses with novel shapes.

If the environment harbors the same viruses and bacteria that our ancestors survived, and if we are considering a stable population with no immigrants from other regions, then essentially all people who get sick will recover, and it will not be apparent that genotype is affecting phenotype (vulnerability to disease). But if the population includes immigrants from distant places, where there has been a different virus exposure history, there may be dramatic differences in who recovers and who dies from local sicknesses. The immigrants will be at a disadvantage when infected by local viruses and the native population will be vulnerable to any viruses brought by the immigrants. For dramatic illustration, old world explorers came to the new world and brought diseases that killed most people in the new world (Diamond, 1996).

The "behavior" component of phenotype is the most interesting, and the most challenging to understand. The "immune system's response repertoire" is a useful starting analogy for understanding behavior (Gazzaniga, 1992; Gazzaniga, 1997; Jerne, 1967). A "stimulus" in the environment can produce a "behavioral response," as when an abrupt approach of something to the face produces an eye blink. We blink our eye because we have ancestors who survived more successfully than those who didn't have the eye blink response.

A reductionist will want to employ the "stimulus/response" (S/R) explanation for behavior as much as possible. With effort, this approach at understanding behavior is broadly successful more often than is conventionally acknowledged. Since it is the simplest possible explanation type, it should be invoked as a first hypothesis.

S/R fails to account for behaviors that are self initiated, i.e., motivated behaviors. For example, this morning I decided to hike in the mountains in the afternoon. Planning a day's activities, as with life goals, requires something that in humans is identified as "prefrontal" cerebral activity, and a general sub cortical "drive" mediated by a "reticular activating system" (as described in Chapters 7 and 8). The prefrontal cortex initiates broad goals, such as a career path, it initiates behavioral programs, such as preparing a speech, and it also initiates specific behaviors, such as talking.

I will argue that the "shape" of behavioral programs, and the "shape" of life paths, are initiated by brain circuits that we inherit. The specific behavioral programs, and specific life paths, are the product of an interaction between inherited brain circuits and the environment.

Not only is an individual's environment a changing thing (career opportunities, available books, current beliefs, etc), but the human environment can change dramatically from one generation to the next, and especially from one millennium to another. It is very likely that everyone was capable at birth of adapting to a hunter gatherer lifestyle, as was common in the AE (ancestral environment).
A human trait that seems to be common, such as "greed," may be expressed in only specific environments. A person born into a tribal hunter gatherer setting, where there are few possessions (because it's hard to carry things from camp to camp) may grow up without expressing greed. The same person growing up in an agrarian society might be greedy. In other words, the person's "genotype" will interact with "environment" to produce a lesser or greater amount of "greed" (phenotype), which illustrates G + E = P.

There are limits to an environment's influence. This is easiest to illustrate using dramatically different genotypes, such as individuals of different species. We cannot make a snake behave like a cat just by cuddling it while it grows up. The snake is limited in what it can become. No amount of environmental adjustment will ever make a snake cat-like, because snake genes do not have cat behavior in their repertoire. Instead of Gsnake + Ecat = Pcat we are limited to Gsnake + Ecat = Psnake. Although snakes and cats are different species, the GEP equation still holds. Illustrating this concept using such different critters the point is easier to understand.

In any population there is a variation of genetic predispositions, or genotype. Thus, given a fixed environment, there will still be a variation in phenotypes, and in this case it will be due entirely to the variation of genotypes. Where there is wide variation in the environment, even a uniform genotype will produce a variation of phenotype, and such variation will be due entirely to environment. The normal situation, of course, is for variations in both genotype and environment, which obscures the sources of observed phenotype variation.

Every population must have unfortunate cases of bad genotype coupled with bad environment. Whereas either one might produce a bad adult, together they could produce a really bad adult. (Do you think Attilla the Hun may have been the product of bad genes and a bad environment? Was he maladapted from the perspective of his genes?)

IF/THEN Brain Circuits

It is reasonable to assume that each person inherits genes that pre wire their brain to recognize situations that elicit appropriate behaviors (S/R) for situations that our ancestors repeatedly encountered and survived. Like any computer program with many IF/THEN sections of code, some of the IF/THEN code will not be used during normal experience. Indeed, most IF/THEN code that exists may only be used in response to rare experiences, especially so for humans, who have an unusually large repertoire of conditional behaviors and personality development paths.

Once a specific piece of IF/THEN code comes into existence, in response to a sustained period of selective pressure in which a recognizable situation occurs and responses have reliable benefits or costs, this piece of code can remain in the genome almost forever. If it later comes into conflict with a similar situation requiring different responses, then this old code will be modified or may disappear. Since the code that elicits a behavior is almost always produced by a combination of genes, if any of these genes are modified in response to other adaptive pressures the original code could inadvertently be modified. If this occurs, the gene pool would have to be exposed to the original adaptive pressures again to restore the original, or equivalent, IF/THEN code.

If it someday becomes possible to list the IF/THEN circuits in a typical human brain, we may wonder when and where each evolved. If this were possible it would probably turn out that most code sections were created during specific eras in our ancestry, with few (or no) recurrences. Each distaste may thus owe its existence to a time when we lived among a specific inedible plant. The plant might have existed for only a few centuries, during the past 2.5 million years of the human past, yet its IF/THEN legacy stays with us.

Many of our ancestors were nomads, living in wandering tribes. Behaviors required by nomadic tribal life are part of our repertoire, and if a "modern" non nomad were raised in nomadic setting he might develop with the same nomadic functionality as present day nomads. Settled farmers, living as single family units, are also part of our ancestry, probably confined to parts of the past 10,000 years. Each of us probably has the code necessary to grow up into fully functional, single family farmer. Large settlement living must have been a part of some other of periods of our ancestry, confined perhaps to the past 5000 years. Each of us is presumably capable of becoming functional urban dwellers. Our large brains are "ready" for many lives that cannot all be lived!

Men Bear More of Evolution's Burden

Paternity success, as measured by offspring per male, exhibits a wider range than maternity success (offspring per female). Every tribe will have some males who don't reproduce, whereas it is rare to find women who are childless. For sexually mature women, after weaning an offspring she is likely to become pregnant soon after menstrual cycles return.

As with any species, whenever a dominant male controls access of other males to females, there will be a large disparity in breeding success among males. Harems were common in human history, and presumably pre history as well. Even when males do not dominate other males, females are prone to prefer to breed with specific males.

Whereas women typically give birth to about 6 or 7 babies during their lifetime, with little difference between women, men may sire from zero to hundreds of babies! Why is there such a disparity?

The human ancestral environment is presumed to have been exclusively tribal. The men of most tribes, it is thought, engaged in hunting expeditions. It was also probable that they engaged in brief raids of neighboring tribes, as well as more dangerous inter tribal warfare. Such male activities entail an extra burden of mortality. A man could die not only from combat, but from a mismatch of anatomy or physiology to climate. A man could also die by formulating less successful strategies in warfare, or by not adhering to a planned strategy requiring careful social coordination.

An extreme view of this situation is to state that the purpose for men is to go out and be measured. Those who come back, and especially those who come back as heroes, will have survived the measurement test, and the women shall deem them more valuable as potential fathers for their children. All women will prefer to mate with heroes than with the others. (It goes without saying that they won't mate with those who died in the process of being measured.)

Any man who refused to "play this game," to go out and be measured, could expect to be shunned by women   as well as by men. Such a man may even have been banished from the tribe (leading eventually to death). It has only been during the present interglacial (during the past 12,000 years) that alternative niches proliferated for the less adventurous man. Although there might have been a small, exempt class of weapon makers, most men could not have escaped the high mortality life style.

Consider the first people who migrated from Africa to the mid latitudes. Dark skinned men would have fared less well, all other things being equal, than slightly lighter skinned men, as the dark skinned men would not synthesize as much Vitamin D. Vitamin D deficient men would be at higher risk of succumbing to the physical demands of traveling, hunting and warfare. On average, the lighter skinned men would be more likely to return from exploits. The women, who stayed home, would be less affected by lowered Vitamin D, so their mortality would have been less affected than their male counterparts. After no more than 200 to 400 generations (based on the New World immigration experience), the entire group's skin would have evolved to a new, more adaptive color. This process would have been achieved by a differential survival of men, combined with a differential breeding success of men. Thus, the burden of adapting would have been borne more heavily by men.

Takeover Infanticidal Males

Male lions kill a female's young lion cubs after they overpower the male lions in a pride. Not only does this remove lion competitors for the male's offspring, but the female soon stops nursing, becomes fertile, and is available for mating with the killer male. We humans might think that a female lion would be upset to see her cubs killed by the new males, but amazingly, the female quickly makes the best of a bad situation by becoming coquettish with the killers. By these actions, the female increases the prevalence of the very genes that thwarted her initial reproductive investment; by favoring this behavior, which humans find so repugnant, the lioness helps shape the male genotype.

Infanticide by males has been documented for species of birds, fish, insects and mammals   such as rodents, carnivores and primates (Wrangham and Peterson, 1996). The following description is for primates.

"Hrdy noticed an invading male charge after a mother, attempting to snatch away her baby. For several days, the other females in the group tried to defend the mother and her baby. But the male persevered, and finally managed to deliver a slash to the infant's stomach that left the intestines exposed. Taking the wounded infant to her breast, the mother looked up at the sky, as though in despair. 'It was the only time in my professional career that I wept.' //  Because females are usually outmatched in the physical war between the sexes, they are helpless to protect their offspring against an infanticidal male. //  Female gorillas respond to infanticide ... they leave the father who allowed their baby to be killed and run off with the murderous male. Infanticide, along with various female defenses, has been seen in 13 primate species. (Angier, 1983.)

We humans find such behaviors repugnant, so surely men do not act this way. Alas, they do! Studies of infanticide in Canada (Daly and Wilson, 1988a) and the United States (Daly and Wilson, 1988b) reveal that step fathers are 75 and 100 times more likely to commit infanticide than are biological fathers.

In any species where males "take over" a female by overpowering the resident male and kill the offspring, females should evolve an aversion to males who cannot protect her and her offspring from takeover males. Thus, women should find weak and low status males unattractive, in relation to strong and high status males. This should be especially true for attractive women, who are more likely targets for takeover males. These predictions are borne out by "common knowledge."

Monogamy and Cuckolding

Monogamy, and the associated female faithfulness which monogamous husbands require, give every man a more or less equal influence on the next generation's genetic pool. This must retard the potential speed of evolutionary adaptation of that gene pool. Thus, the stronger the forces of evolution, the greater the reward for polygamy.

It would be surprising if the genes have remained blind to this. Women, for example, should sense when evolutionary forces are strong, and in response, they should seek consort with men who are "successful." If monogamy were the norm (which would have been more likely only during the past 12,000 years), then women should be expected to try to cuckold their husband (secretly mate with a man who is not the husband) in order to bear children carrying the more "adapted" man's genes. Since monogamy was probably rare before 12,000 years ago, in the human ancestral environment the need to cuckold was also probably unimportant before that time. Cuckolding, I suggest, is therefore a "recent" tool in women's behavioral repertoire.

Blood tests of Canadian and American families reveal a cuckoldry rate that ranges from 15% to 25% (see Christenfeld and Hill, 1995 for additional material). Presumably, cuckoldry rate varies with time and conditions in accordance with some optimizing algorithm created during the AE.

Knowing the optimal time for a wife to cuckold her husband would have evolved during the time that societies became monogamous. Refinements in a woman's cuckolding wisdom would have improved the most when evolutionary forces were greatest. If a gene pool underwent a period of polygamous evolution, the previously gained cuckolding wisdom would have remained "ready" but not expressed until monogamy was restored. This is similar to the way an immune system accumulates a repertoire of immune responses, each specific one of which remains "ready" for expression when exposed to a pathogen that is "recognized."

Recent studies (Hazelton, 2006) show that women have a heightened interest in cuckolding their husbands when they are most fertile. This logical female strategy is matched by an equally logical male strategy of exhibiting a higher level of mate-guarding at the same time. The real challenge for women is to recognize when it is appropriate to cuckold, and with whom. As for “when,” being in a monogamous relation is one precondition.  Sensing that "times are tough" would be another (i.e., evolutionary forces are strong).

As for “with whom” a woman must be capable of measuring her husband against other men. One measure of the successful man is "fashion"; the man who is sought by other women is likely to produce boys who will grow up to also be sought by the next generation of women   regardless of the intrinsic worth of the type. Another way to identify a good candidate is to determine who is dominant over whom. Men live much of their life within a male society, and the men who are most successful in male activities, such as the cooperative hunt, will be accorded privileged positions by other men. Women are sure to notice how men sort themselves while establishing the male hierarchy, and those who are esteemed by other men are good candidates for a cuckolding episode. Women who are fascinated by men’s sporting events might be “doing their homework” for optimizing their future cuckolding.

The main effect of this increased attention by women to male worth is to increase the imbalance of reproductive activity among men; fewer men will account for a greater fraction of a generation's paternity. A secondary effect of this enhanced reward for whatever the forces of evolution deem important is to reduce genetic diversity. In a one harem society all offspring will resemble the harem master. If he is vulnerable to a specific disease, most children of the next generation will be similarly vulnerable.

Thus, there are risks to tribal organizations that give excessive reproductive rewards to small numbers of men. It is in a woman's genetic interest to not succumb totally to fashion; but it is always in a man's interests to be the most successful man and to dominate male reproductive activity.

Women understand that husbands should be loyal "producers" even though they should favor other men for cuckolding. There are "husband material" men, and then there are "exciting affair" men. Women are attracted to both types, but in different ways. A faithful husband type is attractive at the time a commitment is to be made, and the exciting affair type is less attractive at this time. Some time after marriage, however, women's interest in "affair men" should increase. Thus, women regard some men as good on long timescales, serving as loyal husbands and fathers, whereas some men are good for short timescales, serving as cuckold consorts to provide offspring with genes specialized in victimizing the next generation.

Men likewise automatically categorize women as good for the long term, serving as loyal wives and mothers, while other women are good for short term consort. Men and women must automatically categorize each other as belonging to one or the other category.

Men and Women Shape Each Other

Men and women have made each other what they are!

Men have a greater variance in IQ, and we men also exhibit higher incidences of genetic deficiencies. For example, dyslexia (reading and writing problems) is most common among boys. This may merely be an effect of males exhibiting a greater genetic variance of recently evolved traits (i.e., men dominate both ends of the spectrum of most measures). Thus, there are more men geniuses, as well as more men among the learning disabled. Men are burdened by "high risk" mutation experiments that eventually benefit the larger population. Men appear to be more "expendable" than women.

To what extent are women responsible for making men genetically "fragile"? Women prefer men who "go out to be measured," and who come back with good measures. When women cuckold their husbands, they assure their male offspring a greater likelihood of being a cuckolding partner in the next generation. These women also assure that they will produce daughters who are prone to cuckolding their husbands. This occurs because the cuckolding males are likely to carry genes which predispose their girl offspring to cuckold when they are women, since they are likely to have been the result of women who cuckolded (this is a subtle argument).

Women shape men with every preference they express. If women favor men who are "travelers" (i.e., vagabonds, minstrels, pirates), then each succeeding generation of men will tend to resemble travelers.

Why would women be attracted to travelers?  When diseases are a principal cause of mortality, traveling men that women encounter are the ones who have immune systems with the best immunity to diseases beyond the village. This may account for girls going crazy for pirates, traveling musicians, and other itinerate roustabouts who have no long term parenting value.

Birth Order

Frank Sulloway (1996) has presented an immensely well documented case for the influence of family birth order on specific personality traits. The theoretical argument for such an influence begins with the fact that in the ancestral environment children often died before reaching adulthood (approximately 2/3 of children perished). Surviving childhood requires that the child adopt strategies for maximizing parental investment. By this logic, firstborns should ingratiate themselves to their parents, and gain their favor by appearing to be good prospects for their investment. Firstborns should be obedient, conscientious, hard working, and they should internalize the values of their parents.

Laterborns, noticing that there already exist firstborns who have acquired parental confidence by becoming what the parents want, must create for themselves a different identity. They must distinguish themselves from their older sibling by excelling in another endeavor, for if they tried to compete in the same arena, and became equally successful at comparable age, they would be destined to always be a worse investment prospect due to their age disadvantage. All other things being equal, older children are a better investment option because they've already survived more of the childhood risks, and they are closer to childbearing age. Thus, laterborns try to excel in things untried by the firstborn, and perhaps unfamiliar to the parents. Laterborns are more open to new experiences, and are more adventurous. As stated by Sulloway (pg 98), "...the addition that each child makes to the parents' inclusive fitness will tend to be proportional to the development of skills not already represented among other family members."

Firstborn boys followed by a laterborn girl are a congenial combination, since the girl is naturally inclined to have different interests than the boy. From the parents’ perspective, both children are like first-borns and represent good investments.

Firstborn boys feel threatened by a laterborn boy. They are likely to fight, and the younger brother must become proficient with wit, words or some other clever strategy to compensate for his smaller body. The older boy will become accustomed to dominating his younger brother, whereas the younger brother will become adept in the use of social skills for minimizing the disadvantages of being dominated by the older brother. These effects appear to be maximum when the age difference is about 3 years (close enough in age to be competing for similar age related niches, yet different enough that the younger is weaker and can be successfully dominated). Sulloway writes (pg 79) "Like the alpha males of primate societies, firstborns covet status and power. They specialize in strategies designed to subordinate rivals."

To the extent that Sulloway's birth order correlations are correct, women should prefer men who happened to be firstborns with younger brothers. They are more likely to be "masculine" and capable of protecting their wives from "takeover" males. On the other hand, these men are less likely to tolerate cuckolding wives, rendering cuckoldry a more dangerous option for the wife of a firstborn husband.

How confusing (at a subconscious level) this birth order "environmental monkey wrench" must be to women! Men who appear to be strong and domineering may be so merely because they had a brother 3 years younger. They can probably be counted on to protect them and their children from takeover males, but they will not necessarily provide "domineering" and "high status" genes to her offspring. Do women have methods for identifying "genotype produced" versus "birth order produced" dominant men? We await further studies in this young field.

Sexually Specific Morality

Women complain that men can philander with less consequence than women who cuckold. They attribute this disparity to the fact that men can get away with dominating women due to their greater physical strength.

The real reason for this duality of morality has to do with the difference in natural consequences for out of marriage mating. A philandering husband does not necessarily diminish his "paternal investment" value for his wife's children, whereas a cuckolding wife who produces illegitimate offspring necessarily does cause her husband's "paternal investment" to be squandered.

If it Feels Good, Beware!

What is the purpose of emotions? They are meant to influence behavior!

In the particular case of humans, emotions are meant to influence behavior in situations where rational thought is also likely to subvert the genetic agenda. Some behaviors are too important to be meddled with by rationality. The Australian redback spider, in which the male is prone to allow itself to be eaten by the female during copulation, is incapable of rational thought. A simple, automatic instinct suffices to assure that his gene serving deed be done. But what about humans?

Humans think, and are theoretically subject to influence by rational thought. The genes, in their infinite wisdom, have created emotions to safeguard behaviors that serve their interests. Emotions are employed to protect behaviors that are threatened by rational considerations of individual welfare!

Emotions symbolize the conflict between "outlaw genes" and a thinking, rational individual. Therefore, if something has a feel-good emotional payoff, beware!

This theory implies that only intelligent creatures have emotions. It suggests that emotions were "invented" by the genes as a quick solution to a fast evolving human intellect. The genes could not know what rational threats lay ahead, in untried environments, or even newly endowed rational brains, yet they "learned" from past experience that certain actions were at risk of being overturned by individuals who cared more for individual welfare than being an obedient tool of the genes (of course the genes didn't "know" anything; it merely happened that those that safeguarded important behaviors from the influence of other gene mutations prospered.)

We will return to this subject in a later chapter.


It is difficult, at this point in my argument, to avoid the problem of "consciousness."  It is tempting to speculate that C, as consciousness gurus refer to it, was invented by the genes to mediate conflicts between an old instinctual brain, and a new rational one. To the extent that C grew in power, emotions must also grow in strength.

A common sense theory for C is that it “exists” whenever a novel situation demands that brain modules compete for control of understanding and behaving. C almost certainly is generated by the prefrontal cortex, possibly on the left side. Measurements of brain activity show that this area is active when a novel task is being confronted, whereas tasks that have been mastered during previous encounters are not associated with the same level of activation. These cortical activity measurements may have been detecting something produced by C.

Are humans more conscious than chimpanzees? There is growing evidence that chimpanzees "think"   in the way that people commonly think of thinking (Goodall, 1986, and Wrangham and Peterson, 1996). Chimpanzees appear to have something called “theory of mind,” or knowing what other chimpanzees are likely to know, and this also would imply that they have C, at some level.

It is important that thinkers with a sociobiological approach address the consciousness problem, as most of the C literature is devoid of an appreciation that 1) genes construct brains, and 2) genes exist because they're good at surviving. Anyone else who tries to investigate C is handicapped at the outset.

Books that treat consciousness with an adequate respect for the reductionist paradigm include Consciousness Explained (Dennett, 1991), The Illusion of Consciousness (Wegner, 2002) and The Quest for Consciousness (Koch, 2004).

In the next chapter we will return to this issue, which deals with neuropsychology and evolution

Return to Table of Contents