Science is often described as objective. Given the same evidence, anyone should reach the same conclusion. In reality, things are more complicated. Even in the most rigorous sciences, researchers’ perspectives influence how they interpret evidence. This influence is even stronger in the social sciences. Psychologists, for example, cannot fully set aside their personal views when designing studies, interpreting findings, or writing textbooks. That is why it may help for you to know a little about the author of this book. 

This textbook explores fundamental questions about human nature:

• How much are people alike, and how much do they differ?
• To what extent is behavior influenced by situations (social norms, conformity) versus personality (values, dispositions)?
• How much of personality is shaped by nature (genes) and how much by nurture (culture, socialization, parenting)?

Psychologists disagree about the answers to these questions. Biologically oriented psychologists emphasize evolution and genetics. Developmental psychologists highlight parenting. Social psychologists stress the power of situations. These perspectives are sometimes called paradigms. A paradigm is like a research culture with its own fundamental beliefs and research practices. Each perspective adds valuable insights, but paradigms also create blind-spots and biases. 

Behaviorism is a good example. Behaviorism denied the existence of personality traits. Everybody was just the product of a different reinforcement schedule. It also ruled out the study of emotions and forbade self-reports. For this reason, research on personality and emotions with self-report measures only emerged in the 1980s, when the behavioristic paradigm lost its influence. I would not be a psychologist, if behaviorism had lasted another couple of decades. Instead, I attended a conference in 1990, where Skinner gave his last speech to a large audience and only a handful of psychology clapped when he criticized cognitivism. The behavioristic paradigm was dead. At another a conference, an older psychologist described himself as a prisoner of the behavioristic paradigm. That phrase stuck with me. I did not want to look back at my career and realize that I had been a prisoner.  This does not mean I am without biases, but it does mean that I am not trying you to sell the personality paradigm that has many limitations that card-carrying personality psychologists like to ignore. 

The Origin of My Perspective

My journey began in 1966, in a small town in northern (West) Germany. Too young for the student revolutions of the late 1960s, I nevertheless grew up in their aftermath, surrounded by cultural shifts that reshaped much of the Western world. I was raised in a comfortable middle-class family, with a natural affinity for math and a growing interest in social issues. Once I discovered that psychology was a science—not just speculation about dreams—I knew it was the right field for me.

In 1988, I moved to West Berlin to study psychology, just one year before the fall of the Berlin Wall—an event that profoundly shaped my worldview and my appreciation of free societies. My early academic interests were in emotion research. I studied with Professor Rainer Reisenzein, who introduced me to theories of emotion, and with Professor Hubert Feger, who focused on measurement and group processes. At that stage, personality psychology did not appeal to me. The field was dominated by grand theories, such as Freud’s, that seemed disconnected from evidence. Other approaches emphasized genetics and biology in ways that, to me, echoed the dark history of Nazi eugenics. As a young student, I rejected this line of thought. 

In 1996, I began my dissertation research on how people recall their emotions: How do you know how happy you were last month, and how accurate is that judgment? That same year, I received a scholarship to study with Ed Diener at the University of Illinois, one of the leading figures in happiness research. Working with him and his students was an extraordinary experience. After defending my dissertation in 1997, I was fortunate to secure a two-year fellowship from the German Science Foundation (DFG), which allowed me to continue working with Ed Diener in Illinois. My focus shifted from emotions to personality and well-being: Why do some people consistently experience more positive and fewer negative emotions than others? Why are some people happier? Over time, my perspective expanded. Feeling good is important, but it is not the whole story. A full picture of well-being requires asking people how satisfied they are with their lives overall. Life satisfaction became the central theme of my research, and Chapter 14 of this book summarizes some key findings in this area. 

Since 2000, I have been a faculty member at the University of Toronto, Mississauga, a unique campus that reflects the cultural diversity of Toronto. Most of my research focused on happiness (subjective well-being), but since 2011, I have been working on examining the research practices of psychologists. This work was motivated by increasing awareness that many results in psychology journals that end up in textbooks are not replicable. This scientific study of scientists’ behavior is called meta-science or meta-psychology. With Rickard Carlsson in Sweden, I co-founded a journal with the title “Meta-Psychology.” My awareness of the replication crisis helped me to select only credible results for this textbook. Another benefit for students is that this makes the book a lot shorter because some research areas have few replicable findings. For example, we still know very little about the neurological differences between people that shape their personalities. 

Writing this textbook as an active researcher comes with both strengths and weaknesses. On the one hand, I can bring you closer to the science itself—critiquing studies, highlighting controversies, and even sharing my own analyses. On the other hand, professional textbook writers are often more skilled at producing polished narratives. The problem with polished narratives, however, is that they often gloss over controversies and discourage critical thinking. They present findings as if they were unshakable facts. In reality, personality psychology is an emerging science, barely 50 years old, and many findings rest on shaky foundations. The aim of this book goes deeper. It introduces students to scientific thinking, critical evaluation of empirical findings, and quantitative reasoning about personality. That is why the word science appears in the title. I will make a clear distinction between empirical facts (e.g., monozygotic twins are more similar than dizygotic twins for most traits) and inferences or implications (e.g., genetic differences cause personality differences). Facts should not be denied. Inferences can and should be questioned.

As I said before, I did not want to believe in genetic differences, but the evidence became impossible to ignore. Rather than resisting it, I learned to see it differently. Genetic differences do not mean that some people are born with better genes. They mean people are different—and good societies allow everyone to be who they are. Genetic variation is a strength. This principle is true in human evolution and in human societies. Understanding differences, and understanding people who differ from us, is essential for modern life.

The scientific study of personality can also help people avoid chasing unrealistic goals rooted in social norms of perfection. Instead, we can learn to accept ourselves and become our best unique selves. This non-judgmental approach aligns with science’s aim to be objective. Whether there are truly bad, evil, or pathological personalities is a difficult question, but psychology’s history shows how dangerous it can be to label some variations as pathological. Only 50 years ago, homosexuality was considered a disorder. Today, it is accepted as a normal variation in human sexuality.

Finally, I must mention political orientation. Like sexual orientation, it has some genetic roots. Some people are drawn to familiar, traditional values; others to different cultures and new ways of living. Universities are often criticized as leftist and “woke,” accused of indoctrinating students. In reality, students’ political beliefs are largely established before they enter the classroom, and professors have little power to change them. Moreover, many conservative critiques ignore the fact that some conservative ideas are directly opposed to science. At the University of Padua, where Galileo taught, it took the Catholic Church 500 years to accept that the Earth revolves around the sun.

The conflict between traditional values and science is especially sharp in psychology. Psychological science is still concentrated in a handful of mostly secular countries in Western Europe, North America, and East Asia. In the United States, science is currently under attack by right-wing conservatives. Learning about psychology as a science will expose students to progressive ideas that challenge traditional beliefs about human nature, sexuality, gender, and race. At the same time, most topics in psychology are not political, and personality psychology is less politically charged than social psychology. As you will see in Chapter 1, however, personality psychology does have its own dark history—one that is important to confront as we move forward.