Think of all the ways human beings are alike. Everywhere, no matter what their backgrounds or where they live, people love, work, argue, dance, sing, complain, and gossip. They raise families, celebrate marriages, and mourn losses. They reminisce about the past and plan for the future. They help their friends and fight their enemies. They smile with amusement, frown with displeasure, and glare in anger. Where do all these commonalities come from?

Think of all the ways human beings differ. Some are extroverts, always ready to make new friends or speak up in a crowd; others are shy and introverted, preferring the safe and familiar. Some are ambitious and enterprising; others are placid, content with the way things are. Some take to book learning like a cat to catnip; others struggle in school but have plenty of street smarts and practical know-how. Some are overwhelmed by even petty problems; others remain calm and resilient in the face of severe difficulties. Where do all these differences come from?

Years ago, psychologists addressing these questions tended to fall into two camps: On one side were the nativists, who emphasized genes and inborn characteristics, or nature; on the other side were the empiricists, who focused on learning and experience, or nurture. Edward L. Thorndike (1903), one of the leading psychologists of the early 1900s, staked out the first position when he claimed that “in the actual race of life . . . the chief determining factor is heredity.” But his contemporary, behaviorist John B. Watson (1925), insisted that experience could write virtually any message on the tabula rasa, the blank slate, of human nature: “Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any type of specialist I might select—doctor, lawyer, artist, merchant-chief and yes, even beggar-man and thief, regardless of his talents, penchants, tendencies, abilities, vocations, and race of his ancestors.”

In this chapter, we will examine the contributions of both nature and nurture in shaping our human commonalities and our individual differences. We will focus largely on findings from two related areas, evolutionary psychology and behavioral genetics. Scientists in these areas study much the same topics, including language learning, attention, perception, memory, sexual behavior, cooperation, helpfulness to others, emotion, reasoning, personality, and many other aspects of human psychology, but they emphasize different mechanisms to explain human differences.

Virtually no one, however, argues in terms of nature versus nurture anymore. Scientists today understand that heredity and environment constantly interact to produce our psychological traits and even most of our physical ones. This interaction works in two directions. First, genes affect the kinds of experiences we have. A teenager with a genetic aptitude for schoolwork may be more likely than other kids to join a chess team and get books and science kits as birthday presents. These experiences reward and encourage the development of academic skills, turning what began as a small intellectual advantage into a large one. Conversely, although most people don't realize it, experience affects our genes: Stress, diet, emotional events, and hormonal changes can all influence which genes are active (“expressed”) over a person's lifetime (Fraga et al., 2005; Mischel, 2009). Try, then, as you read this chapter, to resist the temptation to think of nature and nurture in either–or terms.