Population thinking is the key to building a causal account of cultural evolution. We are largely what our genes and our culture make us. In the same way that evolutionary theory explains why some genes persist and spread, a sensible theory of cultural evolution will have to explain why some beliefs and attitudes spread and persist while others disappear. The processes that cause such cultural change arise in the everyday lives of individuals as people acquire and use cultural information. Some moral values are more appealing and thus more likely to spread from one individual to another. These will tend to persist, while less attractive alternatives tend to disappear. Some skills are easy to learn accurately, while others are more difficult and are likely to be altered as we learn them. Some beliefs make people more likely to be imitated, because the people who hold those beliefs are more likely to survive or more likely to achieve social prominence. Such beliefs will tend to spread, while beliefs that lead to early death or social stigma will disappear. In the short run, a population-level theory of culture has to explain the net effect of such processes on the distribution of beliefs and values in a population during the previous generation. Over the longer run, the theory explains how these processes, repeated generation after generation, account for observed patterns of cultural variation. The heart of this book is an account of how the population-level consequences of imitation and teaching work.

Taking a population approach does not imply that cultural evolution is closely analogous to genetic evolution. For example, population thinking that does not require cultural information takes the form of memes, discrete, faithfully replicating, genelike bits of information. A range of models are consistent with the facts of cultural variation as they are presently understood, including models in which cultural information is not discrete and is never replicated. The same goes for the processes that give rise to cultural change. Natural selection–like processes are sometimes important, but processes that have no analog in genetic evolution also play important roles. Culture is interesting and important because its evolutionary behavior is distinctly different from that of genes. For example, we will argue that the human cultural system arose as an adaptation, because it can evolve fancy adaptations to changing environments rather more swiftly than is possible by genes alone. Culture would never have evolved unless it could do things that genes can’t!

…To ask whether behavior is determined by genes or environment does not make sense. Every bit of the behavior (or physiology or morphology, for that matter) of every single organism living on the face of the earth results from the interaction of genetic information stored in the developing organism and the properties of its environment… genes are like a recipe, but one in which the ingredients, cooking temperature, and so on are set by the environment. Different traits do vary in how sensitive they are to environmental differences. Some traits aren’t much affected by the normal range of environments—humans develop five fingers on each hand in almost all environments—while others are highly sensitive—genetically similar people may end up with very different body sizes depending on nutrition and health during their childhood.

…lumping culture with other environmental influences leads people to ignore the novel evolutionary processes that are created by culture. Selection shapes individual learning mechanisms so that interaction with the environment produces adaptive behavior. For example, many plants contain toxic substances. Selection makes these chemicals taste bitter to herbivores so that they learn not to consume the toxic plant species. Culture adds something quite new and different to this scenario. Like other animals, humans normally use bitter taste as a signal that a plant is inedible. However, some bitter plant compounds (like salicylic acid in willow bark) have medicinal value, so we also learn from others that we can override the aversive bitter taste of certain plants when we have the need to cure an ailment. The genes making the plant taste bitter don’t change at all, but the behavior of a whole population can change anyway as the belief in the bitter plant’s medicinal value spreads. We take our medicine in spite of its bitter taste, not because our sensory physiology has evolved to make it less bitter, but because the idea that it has therapeutic value has spread through the population. In the distant past, some inquisitive and observant healer discovered the curative properties of a bitter plant. Then a number of processes that we describe in this book might cause this belief to increase in frequency, despite its horrible taste. You can’t understand this process by asking how individuals interact with their environment. Instead, you have to understand how a population of individuals interact with their environments and each other over time.

Thus, culture is neither nature nor nurture, but some of both. It combines inheritance and learning in a way that cannot be parsed into genes or environment.

…Under the right conditions, selection can favor a psychology that causes most people most of the time to adopt behaviors “just” because the people around them are using those behaviors. The last 800,000 years or so have seen especially large, rapid fluctuations in world climate; the world average temperature sometimes changed more than 10 degrees Celsius in a century, leading to massive shifts in ecosystem structure. A group of hominids living in a habitat something like contemporary Madrid could find themselves in a habitat like Scandinavia one hundred years later. You might think that such rapid and extreme environmental changes would put a premium on individual learning over imitation. Odd as it may seem, in many kinds of variable environments, the best strategy is to rely mostly on imitation, not your own individual learning. Some individuals may discover ways to cope with the new situation, and if the not-so-smart and not-so-lucky can imitate them, then the lucky or clever of the next generation can add other tricks. In this way the ability to imitate can generate the cumulative cultural evolution of new adaptations at blinding speed compared with organic evolution. A population of purely individual learners would be stuck with what little they can learn by themselves; they can’t bootstrap[highlight by me] a whole new adaptation based on cumulatively improving cultural traditions. This design for human behavior depends on people adopting beliefs and technologies largely because other people in their group share those beliefs or use these technologies. When lots of imitation is mixed with a little bit of individual learning, populations can adapt in ways that outreach the abilities of any individual genius.

Thinking about the population properties of culture helps us understand the psychology of social learning. For example, we will see that selection can favor a psychology that causes people to conform to the majority behavior even though this mechanism sometimes prevents populations from adapting to a change in the environment. Evolution also favors a psychology that makes people more prone to imitate prestigious individuals and individuals who are like themselves even though this habit can easily result in maladaptive fads. These psychological mechanisms in turn give rise to important patterns of behavior, like the symbolic marking of social groups that would not evolve unless their culture had certain population-level consequences.

However, not all of the processes shaping culture do arise from our innate psychology—culture itself is subject to natural selection. Much as a child resembles her parents, people resemble those from whom they have acquired ideas, values, and skills. Culturally acquired ideas, values, and skills affect what happens to people during their lives—whether they are successful, how many children they have, and how long they live. These events in turn affect whether their behavior will be culturally transmitted to the next generation. If successful people are more likely to be imitated, then those traits that lead to becoming successful will be favored. Even more obviously, if living people are more likely to be imitated than the dead, then ideas, values, and skills that promote survival will tend to spread. Consequently, a culture of honor arises, at least in part, because in lawless societies, men who are not aggressive in protecting their herds and their families tend to fall victim to tough, ruthless predators. If these advantages to a culture of honor have disappeared in the modern South, the higher death rate of those who cling to the custom will eventually extinguish it.

Such selective processes can often favor quite different behaviors from those favored by selection on genes. For example, beliefs and values that lead to prestige and economic success in modern societies may also reduce fertility. Such beliefs spread because the prestigious are more likely to be imitated, even though this lowers genetic fitness…

Natural selection acting on culture is an ultimate cause of human behavior, just like natural selection acting on genes. Consider an example we will return to repeatedly. Much cultural variation exists at the group level. Different human groups have different norms and values, and the cultural transmission of these traits can cause such differences to persist for long periods of time. Now, the norms and values that predominate in a group plausibly affect the probability that the group is successful, whether it survives, and whether it expands. For the purposes of illustration, suppose that groups having norms that promote group solidarity are more likely to survive than groups lacking this sentiment. This creates a selective process that leads to the spread of solidarity. Of course, this process may be opposed by an evolved innate psychology that biases what we learn from others, making us more prone to imitate and invent selfish or nepotistic beliefs rather than ones favoring group solidarity, like patriotism. The long-run evolutionary outcome would then depend on the balance of the processes favoring and disfavoring patriotism. Again for the sake of illustration, let us suppose that net effect of these opposing processes causes patriotic beliefs to predominate. In this case, the population behaves patriotically because such behavior promotes group survival, in exactly the same way that the sickle-cell gene is common in malarial areas because it promotes individual survival. Human culture participates in ultimate causation.

…genetic elements of our evolved psychology shape culture—how could it be otherwise? But at the same time, natural selection acting on cultural variation shaped the environments in which our psychology evolved (and is evolving). The coevolutionary dynamic makes genes as susceptible to cultural influence as vice versa. We will argue that the phenomenon of group selection on cultural variation described above could have produced institutions encouraging more cooperation with distantly related people than would be favored by our original evolved psychology. These cooperators would have discriminated against individuals who carried genes that made them too belligerent to conform to the new cooperative norms. Then the cultural rules could expand cooperation a bit further, generating selection for still more-docile genes. Eventually, innate elements of human social psychology became tolerably well adapted to promote living in tribes, not just families.

from this excerpt.