The first tale concerns a familiar psychological quality: intelligence. Consider this question:

Which is the more important cause of individual differences in intelligence: (1) inheritance (nature) or (2) experience (nurture)?

Once you learn to think like a psychologist about nature and nurture, you won’t answer simply “#1” or “#2.” Here’s a study that shows why.

In 2003, a team of psychologists (Turkheimer et al., 2003) studied intelligence in a large population of twins. They measured intelligence in pairs of (1) identical or monozygotic (MZ) twins, who have the same genes, and (2) fraternal or dizygotic (DZ) twins, who share only half their genes. By comparing MZ and DZ twins, they could determine the degree to which genes influence intelligence. (We’ll explain later exactly how this comparison is done.)

123

In addition, the researchers measured each twin pair family’s socioeconomic status (SES), which is an index of family wealth, parents’ level of education, and the quality of jobs held by the parents. Because socioeconomic status shapes the environment twins experience once they are born, it is an aspect of nurture.

Why did the psychologists measure SES? They did so thanks to a critical insight into the question stated above; they realized that the question “Which is more important, nature or nurture?” might not be a good one. Its wording suggests that there is a single correct answer that applies to all people, in all places. But there may be no single correct answer; rather, genes might be more important in some environments and less important in others.

To find out, the researchers calculated the effects of genes on intelligence among people living at each of various SES levels (Figure 4.1). In high-SES environments, genes were the primary cause of individual differences in intelligence. But in economically poor environments, genes explained hardly anything at all. “In the most impoverished families,” the genetic effect was “essentially zero” (Turkheimer et al., 2003, p. 626).

In this case, “thinking like a psychologist” means recognizing that nature and nurture are not independent forces, each with a fixed size. It means recognizing, instead, that nature and nurture are interdependent; that is, the effect of one may depend on the other (Ridley, 2003; Russell, 2011). Our simple question above—“Which is more important?”—is simplistic; it’s not a good question.

You’ll learn a similar lesson in our second tale of nature and nurture.

124

“Which came first, the chicken or the egg?” That’s a tricky question. But here’s one that sounds easier: Which came first, human biology or human culture?

Most people would say biology. The biology of all present-day species is a product of evolution, the process through which species change across generations (as we discuss below). You might expect that our biological features evolved as ancestral humans adapted to Earth’s physical challenges: the various floods, famines, droughts, diseases, predators, and ice ages that threatened the survival of our species. Then, after our biological nature was in place, humans began to develop the social practices and beliefs that make up human culture (Figure 4.2). In this conception, people’s biological nature develops first, and then, later in human history, it runs into culture’s newfangled rules and practices.

Evidence suggests, however, that this conception is wrong. Biology did not evolve first, prior to culture. Rather, biology and culture evolved together, or coevolved. Coevolution refers to processes through which biology and culture interacted in the course of human evolution (Durham, 1991; Figure 4.2). In coevolutionary processes, human biology evolves, in part, in response to the cultural practices that generation after generation of humans encounter.

Coevolution is evident in a behavior common to many—but not all—of us: drinking milk. To many people, drinking milk seems as natural as breathing air. Shouldn’t almost everyone be able to digest the nutrients in milk, especially its key sugar molecule, lactose? Take a look at the map in Figure 4.3. In some parts of the world, more than 9 out of 10 people can digest lactose, as you might expect. But in others, more than 9 out of 10 people cannot! In fact, in East Africa, where humans originally evolved, almost everyone is lactose intolerant; that is, they cannot digest lactose (Bloom & Sherman, 2005; Durham, 1991). Although people in all regions of the world can digest lactose in infancy, in a great many regions, most people lose that biological ability later in life.

125

At a biological level, genes explain variations in lactose tolerance (Laland, Odling-Smee, & Myles, 2010). To digest lactose, people need genes that produce a protein required for its digestion. In some parts of the world, most people possess these genes; in other parts, most do not.

What accounts for this biological difference of having the genes or not? Culture! Cultural differences created the differences in biology (Durham, 1991; Laland et al., 2010). Here’s how it worked. Across thousands of years of evolution, different cultures developed different ways of producing and preparing food. One major difference involved milk, for which there were three main cultural practices:

In regions of the world with cultural practice #1, people who could digest lactose had an evolutionary advantage. They got nutrients from milk, grew stronger, and thus were more likely to survive and reproduce. Across generations, the population contained more and more people who were lactose tolerant (because more and more of them were surviving and reproducing). Today, almost everyone is lactose tolerant in those regions of the world.

126

In regions with cultural practice #2, the ability to digest lactose was less of an advantage. People who had trouble digesting milk could still get nutrition from cheese. Today, in these areas of the world, about 50% of people are lactose tolerant.

In regions with cultural practice #3, the ability to digest lactose had no evolutionary advantage (because there was no milk). As a result, that biological ability did not evolve. Today, only a very small number of people in these parts of the world are lactose tolerant.

We see, then, that in the course of human evolution, neither biology nor culture “came first.” Biology and culture were intertwined. Biology evolved, in part, in response to cultural practices.

Once again, “thinking like a psychologist” does not mean settling on one versus another answer to our question (“Which came first, human biology or human culture?”). It means rejecting the question as simplistic. As you learn about the way genes and environments interact, you’ll learn to ask smarter, more sophisticated questions about nature and nurture. You, too, will learn to think like a psychologist.

These two stories, about intelligence and lactose tolerance, have the same moral: Nature and nurture interact. People’s psychological and biological characteristics reflect a combination of the two. Although we haven’t yet analyzed genetic mechanisms at a detailed biological level (that happens later in this chapter), you can already see what that analysis has to explain: the interaction of genes and environmental influences.

Before we return to the psychology of humans, we note that gene–environment interactions are evident in simpler organisms, such as plants. The plants shown in Figure 4.4 are seven sets of genetically identical triplets. For each set of triplets, researchers planted one plant at each of three elevations: high, medium, and low.

Which was more important to plant growth, nature or nurture? Looking at the plants, that question hardly makes sense. Different environments (nurture) made little difference to some plants (e.g., #4) but greatly affected others (e.g., #1). Some genetically based differences seen in low elevation (e.g., plant #3 taller than #6) reverse at high elevation (#6 taller than #3). At both low and high elevations, plant #1 was the tallest of the seven, but at medium elevations, it was the second shortest. To predict the height of the plants, you need to know about their genes and their environments.

For some characteristics, genetic influences are predominant and environmental influence is negligible. Eye color, for instance, is determined overwhelmingly by genes (Zhu et al., 2004). For other characteristics, the reverse is true. As you saw earlier, among people living in poverty, environmental effects on individual differences in intelligence are large and genetic effects are small (Turkheimer et al., 2003). Yet, in general, both nature and nurture are influential and, to understand their influence, must be considered together.

127

With that lesson in place, let’s look at nature, nurture, and individual differences in psychological characteristics.