Woven throughout this book are four of psychology’s big ideas.

Let’s consider these four ideas, one by one.

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Whether reading an online article or swapping ideas with others, critical thinkers ask questions. How do they know that? Who benefits from this? Is the conclusion based on a personal story and gut feelings, or on evidence? How do we know one event caused the other? How else could we explain things?

In psychology, critical thinking has led to some surprising findings. Believe it or not . . .

This same critical thinking has also debunked some popular beliefs. When we let the evidence speak for itself, we learn that . . .

In later chapters, you’ll see many more examples in which psychology’s critical thinking has challenged old beliefs and led us onto new paths.

Each of us is part of a larger social system—a family, a group, a society. But each of us is also made up of smaller systems, such as our nervous system and body organs, which are composed of still smaller systems—cells, molecules, and atoms.

If we study this complexity with simple tools, we may end up with partial answers. Consider the many ways of explaining horrific school shootings. Is it because the shooters have brain disorders or genetic tendencies that cause them to be violent? Because they have observed brutality and mayhem in the media or played violent video games? Because they live in a gun-toting society? Each of these is a partial truth, but none is a full answer. For the best possible view, we need to use many levels of analysis. The biopsychosocial approach integrates three levels: biological, psychological, and social-cultural. Each level’s viewpoint gives us a valuable insight into a behavior or mental process. Each asks different questions and has limits, but together they offer the most complete picture.

Suppose we wanted to study gender differences in a group of people. Gender is not the same as sex. Gender refers to the traits and behaviors we expect from a boy or girl, or a man or woman in a specific culture. Sex refers to the biological characteristics people inherit, thanks to their genes. To study gender similarities and differences, we would want to know about biological influences. But we would also want to understand how the group’s culture—the shared ideas and behaviors that one generation passes on to the next—views gender. Critical thinking has taught psychologists to be careful about making statements about people in general if the evidence comes from studies done in only one time and place. Participants in many studies have come from the “WEIRD” cultures—Western, Educated, Industrial, Rich, and Democratic (Henrich et al., 2010). We are also increasingly aware that the categories we use to divide people—including “gender” and “sex”—are socially constructed. As we will see in Chapter 4, many individuals’ gender identity differs from their biological sex.

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Even with this much information about people’s gender and culture, our view would be incomplete. We would also need some understanding of how the individuals in a group differ from one another because of their personal abilities and learning.

Studying all these influences in various people around the world, researchers have found some gender differences—in what we dream, in how we express and detect emotion, and in our risk for alcohol use disorder, eating disorders, and depression. Psychologically as well as biologically, we differ. But research shows we are also alike. Whether female or male, we learn to walk at about the same age. We experience the same sensations of light and sound. We remember vivid emotional events and forget everyday details. We feel the same pangs of hunger, desire, and fear. We exhibit similar overall intelligence and well-being.

Psychologists have used the biopsychosocial approach to study many of the field’s big questions. One of the biggest and most persistent is the nature–nurture issue: How do we judge the contributions of nature (biology) and nurture (experience)? Today’s psychologists explore this age-old question by asking, for example:

In most cases, nurture works on what nature provides. Our species has been graced with a great biological gift: an enormous ability to learn and adapt. Moreover (and you will read this over and over in the pages that follow), every psychological event—every thought, every emotion—is also a biological event. Thus, depression can be both a brain disorder and a thought disorder. (You’ll learn more about this in Chapter 13.)

Mountains of new research reveal that our brain works on two tracks. Our conscious mind feels like our body’s chief executive, and in fact we do process much information on our brain’s conscious track, with full awareness. But at the same time, a surprisingly large unconscious, automatic track is processing information outside of our awareness. Thinking, memory, perception, language, and attitudes all operate on these two tracks. Today’s researchers call it dual processing. We know more than we know we know.

Vision is a great example of our dual processing. As science often reveals, truth can be stranger than fiction.

During a stay at Scotland’s University of St. Andrews, I [DM] came to know research psychologists Melvyn Goodale and David Milner (2004, 2006). A local woman, whom they studied and call D. F., was overcome by carbon monoxide one day. The resulting brain damage left her unable to consciously perceive objects. Yet she acted as if she could see them. Slip a postcard into a mail slot? She could do so without error. Report the width of a block in front of her? No, but she could grasp it with just the right finger-thumb distance. How could a woman who is perceptually blind grasp and guide objects accurately? A scan of D. F.’s brain revealed the answer.

The eye sends information to different brain areas, and each of these areas has a different task. A scan of D. F.’s brain revealed normal activity in an area concerned with reaching for and grasping objects, but not in another area concerned with consciously recognizing objects. A few other patients have a reverse pattern of damage. As you might expect, their symptoms are the reverse of D. F.’s. They can see and recognize objects, but they have difficulty pointing toward or grasping them.

We think of our vision as one system: We look. We see. We respond to what we see. Actually, vision is a two-track system. Our visual perception track enables us to think about the world—to recognize things and to plan future actions. Our visual action track guides our moment-to-moment actions.

This big idea—that much of our everyday thinking, feeling, sensing, and acting operates outside our awareness—may be a strange new idea for you. It was for me [DM]. I long believed that my own intentions and deliberate choices ruled my life. In many ways they do. But as you will see in later chapters, there is much, much more to being human.

Psychology’s first hundred years focused on understanding and treating troubles, such as abuse and anxiety, depression and disease, prejudice and poverty. Much of today’s psychology continues the exploration of such challenges. Without slighting the need to repair damage and cure disease, Martin Seligman and others (2002, 2005, 2011) have called for more research on human flourishing. These psychologists call their approach positive psychology. They believe that happiness is a by-product of a pleasant, engaged, and meaningful life. Thus, positive psychology focuses on building a “good life” that engages our skills, and a “meaningful life” that points beyond ourselves. Positive psychology uses scientific methods to explore

Will psychology have a more positive mission in this century? Can it help us all to flourish? An increasing number of scientists worldwide believe it can.