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Summary of Boyd and Richerson's Chapter 5, Biased Transmission and the Sociobiology Debate

Updated: Mar 20

Chapter 5 Biased Transmission and the Sociobiology Debate

The aim of this book is to develop a theory of cultural evolution and how it explains behavior. This chapter argues against sociobiology and its appeal to biased transmission in explaining the role of culture in human behavior. According to one prediction from sociobiology, agents use biased transmission (Boyd and Richerson’s jargon) when adopting cultural traits; that is, agents evaluate cultural traits and choose the ones that maximize their genetic fitness. For example, an individual will either adopt or deviate from a dating norm depending on whether it maximizes genetic fitness. If the sociobiologist is correct about biased transmission, then culture will play a secondary role in explaining human behavior. What plays a primary role in explaining human behavior is the criterion used to select among cultural variants, namely, genetic fitness maximization. To begin evaluating this hypothesis from the sociobiologist, the authors do two things. First, they provide a taxonomy of biased transmission, focus on direct bias, and build evolutionary models of direct bias. They conclude that direct bias can evolve when it is not too costly and when it reliably selects for adaptive cultural variants. Secondly, the authors provide empirical evidence from psychology that direct bias has played a minimal role. Other biases are required, and so they are investigated in subsequent chapters.

Transmission Biases

In the previous chapter, the authors discussed guided variation, by which they mean, “...the cultural transmission of behavior as modified by learning, where learning is taken to include a whole range of processes from simple conditioning to cognitively complex rational calculation” (136). Direct bias uses the same learning mechanisms except that the cultural traits adopted by the agent depend on what is available in one’s culture.

Models of Direct Bias

The authors begin with two kinds of models: one with dichotomous traits cultural traits and another with quantitative, i.e., admitting degrees, cultural traits. They complicate the models by including transmission from non-parents and considering biased modeling, i.e., parents preferring to model one type of behavior over the other. They do not develop many models considering biased modeling and teaching because they expect the lessons from those models to be similar to those of guided variation (chapter 4) and biased transmission. [I think this is an interesting point because some authors (e.g., Sterelny, 2012) have really focused on the importance of biased modeling or teaching, but they do not usually develop formal models].

Next the authors consider whether direct bias is adaptive in various kinds of environments. First, they assume an unchanging environment. In an unchanging environment, natural selection alone increases the frequency of favorable traits. In such a case, an unbiased rule will work. Natural selection won’t select for a direct bias since that would incur unnecessary costs. Second, the authors examine direct bias in a changing environment. Then they consider a direct bias that functions in all habitats, and one that is habitat-specific. An example of a general purpose direct bias is a person that has two preferences. One is a preference for balanced amino acid intake. Another is a preference for a food preparation practice with the least effort. In environments where one already has balanced amino acids, one won’t prefer alkali treated corn to untreated corn because the person already has enough amino acids. If the person does not have a balanced amino diet, they will prefer alkali treated corn.

Boyd and Richerson conclude that a general purpose bias may evolve in a varying environment, but not a habitat specific bias. Here is a quote explaining why the habitat specific bias would not evolve.

“When migration between different habitats is weak, natural selection increases the frequency of the locally favored cultural variant so that biases can have little additional beneficial effect. When migration is strong it swamps the genetic differentiation that is necessary if genetically transmitted habitat-specific biases are going to be effective” (166).

Four Theses of Sociobiology

Boyd and Richerson turn to four different theses of sociobiology consistent with the models discussed above and with the idea that individuals engage in rational calculation and direct bias (which applies rational calculation to cultural variants) to maximize genetic fitness. Each of the theses vary depending on the importance they put on the environment, genes, and culture. These four theses aim to account for the role of culture in explaining human behavior. At this point, the authors add that what needs to be explained is that human behavior exhibits an impressive array of diversity.

First off is the pure environment view. This view holds that genetic variation is not so important. Instead, environmental differences largely explain the diversity of human behavior. The authors note that this view will be mostly correct when it is easy to determine what the best behavior is in an environment. People will simply adopt the most suitable cultural practice. However, the authors note two problems with this view. First, culture seems to act like an inheritance system, and the environment view leaves this out (they argue this in chapter 3). Second, in many circumstances, it will be too costly to figure out what the best practice is–imagine trying to figure out for yourself the best way to farm or hunt.

Next we have the culture plus environment view. According to this view, cultural inheritance makes an important difference to the behavioral diversity of each generation. However, in the long run, over many generations, rational calculation and direct bias play an important role in shaping the trajectory of culture and behavior. This view shares what seems correct about the pure environment view (that people engage in rational calculations) while avoiding its problems. It gives cultural inheritance a role, and it does not assume that agents can always pay the full costs of rational calculation–traditions are changed slowly. The authors do not register any objections to this view, but I think they would argue that there are other important cultural forces.

Third is the pure genes hypothesis. This holds that the diversity of human behavior is explained by differences in genetics. The authors note that this is a minority position, and that it is hard to find people believing it. Two problems raised with this approach is that it assumes selection is very strong and that migration probably undermines the amount of genetic variation required.

The fourth hypothesis is the genes plus culture hypothesis. As before, it is unlikely that any scientist holds this view. But the idea is that in the short run (one or two generations), behavior is explained by cultural inheritance. However, the culture that one adopts is ultimately the product of genetics. Some people might have genes that predispose them towards patrilineal society and others towards matrilineal society. Boyd and Richerson reply by reminding us of their models. They found that a genetically based general purpose direct bias that functions across habitats may evolve, so the gene-culture view may be true here. But they found that the bias cannot evolve when it is habitat specific, so the view won’t always be true.

Empirical Evidence

Lastly, the authors consider empirical evidence to show that guided variation and direct bias are weak forces. The first data point concerns how innovations are diffused. Here the authors note that when the benefits of the innovation are obvious, adoption easily occurs. When the benefits are not obvious, innovation adoption is slow. For example, the authors note a case where water boiling in Peru was resisted on the basis of government distrust, the inconvenience of boiling, dislike of the taste, and the counter intuitiveness of the practice. Consistent with the models, direct bias works when the consequence of the practice is discernible. Otherwise, adoption depends on individuals paying high learning costs and others copying.

Next, Boyd and Richerson think that the data on reasoning suggests that the forces of guided variation and direct bias are weak. First, the benefits of many cultural adaptations are statistical in nature. Since humans are not naturally good at this kind of reasoning, we should expect that guided variation and direct bias are not responsible for these adaptations. Second, since humans tend to make inferences from small samples, the choices individuals make will be based on noisy data. So, if cultural adaptations are useful, there is likely some other source besides direct bias explaining its adoption. Lastly, the fact that humans are unlikely to change their mind puts a constraint on direct bias.


Sociobiology is a useful point of comparison when developing a theory of how culture explains behavior. Sociobiology holds that, in the end, the criteria guiding culture is genetic. This chapter has drawn out hypotheses, developed models and terminology, and provided some key data. The authors suggest that hypotheses of sociobiology are only true to an extent. They go on in subsequent chapters to argue for other ways by which culture can play a role in explaining human behavior.

My Personal Analysis

What I love about this chapter is that it starts with a good set of questions. Why is human behavior so diverse? How does culture explain human behavior? It then sets as a point of comparison sociobiology, which holds that all behavior aims to maximize genetic fitness. Moreover, the authors do not rely on simple counter examples to refute this claim. They take it seriously and try to develop concepts and models where it is strong and where it is weak. Moreover, I appreciate their slow, piecemeal approach. The authors handle the view they critique carefully and through various chapters try to develop an alternative.

There are a few reflections I have. First, while the models they build are rigorous, there is the real question of whether they are sufficiently robust. Do they hold under various different assumptions? This can only be answered by looking at the wider literature.

Second, of the four sociobiological hypotheses that they considered, the strongest one seems to be the culture plus environment view. This view, we saw, gets what seems right about the environment view and avoids its problems. The other genetic variety views seemed to lack strong scientific support. The environment plus culture view says that human diversity is explained by inheriting culture in a generation and its slowly changing by rational calculation. If this is the theory to critique, then what can we say? I think (1) we still need a theory of how culture is inherited. (2) We need to specify the exact role between culture and rational calculation/direct bias. And we need to do so in light of empirical evidence.

The authors drew on some empirical evidence towards the end (see above). However, two ideas they noted from psychological studies (but did not elaborate on) are relevant to this second question. One is the idea that people inherit various domain specific cues and causal models from culture. The second is that many problems are solved collectively so that human reasoning errors are reduced. So, the question is, “how does rational calculation/direct bias relate to culture?” The sociobiologist's answer is that these are used to select cultural traits that maximize fitness. If the two ideas noted are correct, then we see the sociobiologist can be only partly correct. If we inherit domain specific cues and causal models, if we tend to collectively reason in order to reduce human reasoning inefficiencies, then culture plays an important role in enabling humans to engage in rational calculation and thus in direct bias.


Boyd R. & Richerson P. J. (1985). Culture and the evolutionary process. University of Chicago Press.

Sterelny K. (2014). The evolved apprentice : how evolution made humans unique. MIT Press.

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