Dual inheritance theory

Based on Wikipedia: Dual inheritance theory

Here's a puzzle that troubled biologists for decades: why do most adults in Sweden happily drink milk, while most adults in China cannot digest it without discomfort? The answer reveals something profound about human evolution—we are not shaped by our genes alone, nor by our culture alone, but by an intricate dance between the two that has been running for millions of years.

This is the core insight of dual inheritance theory, sometimes called gene-culture coevolution. It's a framework for understanding how humans became the strange creatures we are—the only species that cooks its food, builds cities, writes symphonies, and argues about politics on the internet.

Two Inheritance Systems, One Species

Every living thing inherits genetic information from its parents. That's evolution as Darwin described it: genes vary, some variants survive and reproduce better than others, and over time populations change. Simple enough.

But humans have a second inheritance system running in parallel. We inherit culture—beliefs, skills, habits, and knowledge—from the people around us. You learned your language not from your DNA but from listening to others speak. You learned to tie your shoes by watching someone demonstrate it. You acquired your food preferences, your moral intuitions, even your sense of what's funny, largely from your social environment.

Here's where it gets interesting: these two systems don't operate independently. They feed into each other in a continuous loop. Genetic changes can alter how we learn culture. Cultural changes can alter which genes get selected. The result is a feedback system that can produce rapid, dramatic changes in both our biology and our behavior.

The Milk Revolution

Let's return to that milk puzzle, because it's one of the clearest examples of gene-culture coevolution in action.

For most of human history, and for most mammals today, the ability to digest lactose—the sugar in milk—switches off after weaning. This makes biological sense. Why waste energy producing lactase, the enzyme that breaks down lactose, when you're no longer nursing? Adult lactose intolerance isn't a disorder; it's the ancestral default.

But around seven thousand five hundred years ago, something changed in certain human populations. Groups in Northern Europe and parts of Africa began keeping cattle not just for meat, but for milk. This cultural innovation created a new selection pressure. Individuals who happened to carry genetic mutations allowing them to produce lactase into adulthood suddenly had access to a rich source of calories and nutrition that their neighbors couldn't use.

The selection was remarkably strong. Geneticists estimate the selection coefficient—a measure of how much an advantage a trait provides—was between nine and nineteen percent in Scandinavian populations. That's enormous by evolutionary standards. It means that over just a few thousand years, lactase persistence went from rare to nearly universal in these populations.

The cultural practice of dairying came first. The genetic adaptation followed. Culture changed our genes.

The Cooking Ape

If the milk story is striking, the story of cooking is revolutionary. It may explain why you have such a large brain sitting inside your skull.

Compare yourself to a chimpanzee. Your teeth are smaller. Your jaw is weaker. Your stomach is reduced. Your intestines are shorter. For a primate of your size, your digestive system is surprisingly puny. But your brain? It's three times larger than a chimp's.

This trade-off didn't happen by accident. Both brains and guts are metabolically expensive tissues—they burn through calories even when you're sitting still. If you're going to grow an enormous brain, something else has to give. For humans, that something was our digestive system.

How did we manage this? By outsourcing digestion to technology.

Cooking is, in essence, external digestion. When you heat food, you break down its molecular structure before it ever enters your body. Proteins denature. Starches gelatinize. Cell walls rupture. The result is food that's softer, easier to chew, and far more digestible. Studies show that cooked carbohydrates are roughly thirty percent more digestible than raw ones.

The time savings alone are staggering. Chimpanzees spend over six hours every day chewing. American teenagers spend less than two hours. All that extra time becomes available for other activities—hunting, toolmaking, socializing, thinking.

Consider what this means for brain evolution. A larger brain requires more calories. Cooking provides more calories from the same amount of food. It also frees up time to acquire more food. And it allows the digestive system to shrink, redirecting metabolic resources to the brain. Every factor pushes in the same direction: toward bigger brains.

But bigger brains mean greater capacity for cultural innovation. Including innovations in food processing. Which enable even bigger brains. You can see the feedback loop forming.

This is why some researchers argue that humans have become biologically dependent on cooking. A 2005 study of over five hundred people eating long-term raw food diets found that the higher the proportion of raw food and the longer they maintained the diet, the lower their body mass index dropped—often to unhealthy levels. Even with access to modern processing techniques like blending and gentle heating, a raw diet struggles to provide adequate nutrition. We have evolved to require cooked food.

How Cultural Evolution Works

To understand gene-culture coevolution, you need to understand how culture itself evolves. And yes, "evolves" is the right word—culture changes through a process that's genuinely Darwinian, though it differs from genetic evolution in important ways.

The basic requirements for evolution are variation, differential survival, and inheritance. Culture has all three.

Variation arises from errors in learning, creative modifications, and simple mistakes. When you try to copy a behavior you've observed, you rarely replicate it perfectly. These imperfect copies are cultural mutations.

Differential survival happens because some cultural variants spread more effectively than others. A catchy song gets shared more than a forgettable one. A useful technique gets adopted more widely than a useless one. A compelling belief attracts more adherents than a boring one.

Inheritance occurs through social learning—watching, listening, imitating, being taught. You acquire most of your cultural traits from other people, just as you acquired your genes from your parents.

But cultural inheritance differs from genetic inheritance in crucial ways. You inherit your genes from exactly two people, your biological parents. You can inherit cultural traits from anyone—parents, peers, teachers, celebrities, strangers on the internet. This changes everything about how culture spreads and changes.

The Biases That Shape Culture

When you learn something from another person, you're not randomly sampling from all possible cultural models. You're biased. Everyone is. Understanding these biases helps explain why certain ideas spread while others die out.

Content bias means some ideas are simply more appealing than others because of what they contain. Humans have evolved preferences for certain kinds of information. Stories about social relationships grab our attention more than abstract principles. Information about threats spreads faster than information about safety. Foods that are sweet or fatty are more memorable than bland ones. These preferences shape which cultural variants stick in our minds and get passed along.

Prestige bias means we preferentially copy people who seem important or admired. If a celebrity endorses a product, sales increase—not because the celebrity has any expertise, but because we're wired to imitate high-status individuals. This bias probably evolved because high-status people have often figured out something useful, so copying them is a reasonable shortcut.

Success bias is similar but more targeted. Here you copy people who are demonstrably successful at something specific. If you want to learn to hunt, copy the best hunter. If you want to learn to code, copy successful programmers. This bias is more rational but also more demanding—you need to actually evaluate performance rather than just notice status.

Conformist bias means we disproportionately adopt behaviors that are common in our group. If everyone around you is doing something, you're likely to do it too, even without much thought. This bias can stabilize useful cultural norms, but it can also lock populations into suboptimal practices.

Similarity bias means we prefer to learn from people who are like us in some way. This can accelerate cultural divergence between groups, as similar individuals reinforce each other's traits while different groups drift apart.

When Culture and Genes Conflict

Usually, culture and genes push in the same direction. Cultural practices that help people survive and reproduce spread more effectively, and genes that enhance beneficial cultural learning are favored by selection. The two systems reinforce each other.

But not always.

Consider the demographic transition—the dramatic fall in birth rates that accompanies industrialization. In pre-industrial societies, wealthier families typically had more children who survived to adulthood. More resources meant more reproductive success, exactly as evolutionary theory predicts.

But in modern industrial societies, this pattern reverses. The wealthiest, most educated people often have the fewest children. They invest enormous resources in education and career advancement, delaying or forgoing reproduction. This is, from a purely genetic standpoint, bizarre. Why would natural selection favor traits that lead to fewer offspring?

Dual inheritance theory offers an explanation: it wouldn't, but cultural selection might. In industrial societies, prestige often comes from professional achievement rather than family size. If people preferentially copy high-prestige individuals, and high-prestige individuals tend to have few children, then low-fertility norms will spread through the population even though they're genetically maladaptive.

Culture, in other words, can hijack our evolved biases and send us in directions that don't serve our genes at all. We're still copying the successful and prestigious, just as our ancestors did. But success and prestige now mean something very different.

Cultural Drift and Random Change

Not all cultural change is adaptive. Some of it is just noise.

Consider baby names. There's no obvious reason why "Jennifer" should have been popular in the 1970s or "Emma" popular in the 2010s. These names don't confer survival advantages. They're not endorsed by prestigious figures. They just... drift in and out of fashion.

This process resembles genetic drift—random changes in gene frequencies due to chance sampling effects. In a small population, rare variants can become common or disappear entirely just by luck. The same happens with culture. In small groups, unusual practices can catch on simply because the few people who happened to adopt them were the ones others ended up copying.

Researchers have found that cultural drift models do a surprisingly good job of predicting changes in baby names, pottery styles in archaeological records, and even patent applications. Much of cultural change may be essentially random, shaped more by accident than adaptation.

Songbirds provide a nice parallel from the animal kingdom. Different populations of the same species often have distinct song dialects—variations that arise from copying errors as young birds learn from adults. These dialects aren't adaptive; they're just accumulated mistakes that happened to get transmitted. Culture drifts in birds just as it drifts in humans.

What Makes Us Human

Dual inheritance theory suggests that the capacity for cumulative cultural evolution is itself an evolved adaptation. At some point in our lineage, social learning became so advantageous that natural selection refined the cognitive architecture underlying it. We evolved to be cultural animals.

This created a feedback loop. Better cultural learning allowed more sophisticated culture. More sophisticated culture created selection pressure for even better cultural learning. The result was runaway evolution of both our cultural capacities and our cultural complexity.

Other animals have culture—tool use in chimpanzees, songs in whales, foraging techniques in dolphins. But none show the cumulative, ratcheting character of human culture. A chimpanzee might learn to fish for termites by watching its mother, but chimpanzee termite fishing hasn't improved over thousands of generations. Human technology, by contrast, builds relentlessly on itself. Each generation inherits the innovations of all previous generations and adds its own.

This capacity for cumulative culture may be what truly distinguishes us from other species. Not just the ability to learn from others, but the ability to improve on what we learn and pass those improvements along. It's a second evolutionary system layered on top of the first, running faster and reaching further than genes alone ever could.

The Dance Continues

Gene-culture coevolution didn't stop when humans developed agriculture or writing or cities. It's still happening.

Recent analyses suggest that human genetic evolution has actually accelerated over the past ten thousand years, driven partly by cultural changes. Agriculture changed our diets, creating selection for new digestive capabilities. Dense settlements created selection for disease resistance. Complex societies created selection for traits suited to living among strangers.

We tend to think of culture as something we create and control, separate from our biology. Dual inheritance theory suggests a different picture. Culture is part of our evolutionary heritage, as deeply embedded in our nature as our upright posture or our opposable thumbs. And like those physical traits, it continues to shape the genetic evolution of our species.

The feedback loop runs on. Our genes shape what culture we can create. Our culture shapes which genes get selected. Neither can be understood without the other.

Next time you drink a glass of milk, or cook a meal, or learn something new from someone you admire, you're participating in a process that has been shaping humanity for millions of years—and that continues to shape us still.