The Temperamental Thread: How Genes, Culture, Time and Luck Make Us Who We Are



Temperament is the single most pervasive fact about us and our fellow travelers in life. We notice it; we gossip about it; we make judgments based on it; we unconsciously shape our lives with it.

In The Temperamental Thread, developmental psychologist Jerome Kagan draws on decades of research to describe the nature of temperament—the in-born traits that underlie our responses to experience. Along the way he answers such questions as, How does the temperament we are born with affect the rest of our lives? Are we set at birth on an irrevocable path of optimism or pessimism? Must a fussy baby always become an anxious adult?

Kagan paints a picture of temperament as a thread that, when woven with those of life experiences, forms the whole cloth of personality. He presents solid evidence to show how genes, gender, culture, and happenstance contribute to temperament as well as influence and shape a mature personality. He explains how temperament sets the stage for the myriad of personality variations, from the dazzling to the desperate, that we see all around us.

Temperament research, powered by the new tools of neuroscience and psychological science, will be an important source of tomorrow’s ideas, as well as enriching our understanding of others in every context, from our closest relationships to those in workplaces, schools, and even causal encounters. In a highly readable and enjoyable style, Jerome Kagan shows us how.


Table of Contents

Chapter 1:           Introduction: What Are Human Temperaments?

Chapter 2:           Reacting to the Unexpected

Chapter 3.           Experience and Inference

Chapter 4.           Temperament and Gender

Chapter 5.           Temperament and Ethnicity

Chapter 6.           Temperament and Mental Illness

Chapter 7.           What Have We Learned?



“In this marvelous book, one of the world’s most distinguished psychologists synthesizes cutting-edge research to illuminate how biology and environment jointly shape human psychology. The book reveals deep erudition, yet is written in an engaging and accessible style. Anyone keen to learn what science has revealed about human nature will be captivated.”

— Richard J. McNally, Ph.D., Department of Psychology, Harvard University, Author of Panic Disorder: A Critical Analysis and Remembering Trauma

 “An excellent read—highly informative and accessible. Jerome Kagan provides a broad overview of the importance of individual differences early in life in the formation of adult personality. The book encompasses the worlds of psychology, genetics, and neuroscience…in a manner that is readily understood to readers of varied backgrounds. In addition, Professor Kagan provides in-depth discussion of the influence of temperament on achievement and psychopathology.”

— Professor Nathan A. Fox, University of Maryland-College Park

"Jerome Kagan, with his usual brilliance, separates the biology and psychology of temperament to unravel the complexity of why we act and react the way we do. If you want to understand why you or your child is anxious, easy-going, a worrier, compulsive, feels guilty or entitled and more, you must read Kagan’s discoveries about the brain’s role and the factors that influence it."

— Susan Newman, Ph.D., Social psychologist, and author of Parenting An Only Child: The Joys and Challenges of Raising Your One and Only.



Introduction: What Are Human Temperaments?

…The mothers brought their rested and recently fed infants to my Harvard laboratory where a woman presented them with a series of unfamiliar sights, sounds, and smells. The infants saw mobiles of brightly colored objects and stuffed animals move slowly back and forth in front of their faces, heard voices speaking sentences coming from a speaker baffle without the presence of a person, and experienced the subtle odor of dilute alcohol on a cotton swab placed under their nose. Lisa’s arms and legs remained relatively still to all of these events. Although she occasionally babbled or smiled, she rarely thrashed her arms and legs, twisted in her seat, or cried. Marjorie, by contrast, began to thrash her limbs to the third presentation of the colored mobiles and became so distressed by the fourth presentation she lifted her back from the seat and began to cry. I will explain later why this reaction is a significant sign of a temperament. After Marjorie was soothed the examiner continued. On about a third of the presentations of the mobiles, taped sentences, and the smell of alcohol, Marjorie repeated the pattern of vigorous limb movements, fretting or crying, and occasional arching of the back.

Because none of these episodes was frightening or frustrating, why did these two infants react so differently to these benign events? One reasonable explanation was that Marjorie and Lisa inherited a biology that affected their threshold of arousal to unfamiliar events that were unexpected. If the tendency to become highly or minimally aroused by unfamiliarity were temperamental biases that were preserved we should see manifestations of these properties in the older child’s reactions to strangers, unfamiliar rooms, and novel objects. That is exactly what we found. Marjorie became a timid, anxious adolescent but Lisa developed a more relaxed, spontaneous profile.

Even though the large number of human temperaments has a foundation in the child’s biology, we do not yet know the underlying biology. Therefore, at least for now, a temperament is defined by behavior rather than by genes or brain states. Darwin admitted in the first edition of “Origin of Species” in 1859 that he did not know the mechanism that produced the variation in anatomy and behavior that nature acted upon.

This book summarizes what many scientists have learned about a small number of human temperaments, especially the forms they assume during infancy, their derivatives in later childhood, their possible biological origins, the experiences that shape each set of biases into various personality types or symptoms of mental illness, and their contribution to the psychological differences between males and females and ethnic groups. I will argue, however, that no temperament is the foundation of only one personality type. Each temperament must be regarded only as an initial tendency to develop one class of profiles from a large envelope of possibilities. Each bias makes it relatively easy or relatively difficult to acquire one family of behaviors, emotions, and beliefs rather than another. The concept of temperament, therefore, is an example of the notion of biological preparedness. Monkeys born and reared in a laboratory who had no exposure to snakes will show no signs of fear the first time they see a snake. However, monkeys acquire a fear of snakes quickly if they see another animal display fear to the sight of a snake, but do not learn to fear flowers if they see another monkey show fear behavior to a flower. It is easier to acquire a fear of snakes than a fear of flowers because the former is a biologically prepared tendency.

Marjorie’s temperament can be regarded as a biologically prepared tendency that made her vulnerable to worry when a blizzard is predicted or she has to strike up a conversation with a stranger. Lisa possessed the opposite set of biologically based biases. Tall youth find it easier to become varsity basketball players than short youth. I have played tennis for over 50 years but my biology prevents me from playing like Roger Federer.

Nonetheless, each child’s experiences are influential. Lisa might have become a melancholic, dour adolescent if she had been born to a depressed mother and alcoholic father who lost their tempers easily, even though these characteristics were incompatible with her initial temperament. Girls with Marjories’s temperament who grew up in poor, single parent families in neighborhoods with high rates of crime and teenage pregnancies are at risk for becoming delinquent or pregnant, even though these traits are uncharacteristic of this bias. John Hinckley, Jr., the young man who tried to assassinate Ronald Reagan in 1987, may have possessed Marjorie’s temperament for his mother described him as an extremely anxious, timid, child. It is rare that a boy with this temperament commits an aggressive act, but the frequent family moves made it difficult for him to establish lasting friendships and, in addition to other experiences we cannot know, contributed to a personality that is uncommon in boys with this temperamental bias.

I think the Nobel Laureate T. S. Eliot may have possessed Marjorie’s temperament for he was a shy, cautious, sensitive, child. Most boys with this temperament do not win Nobel Prizes. However, because Eliot possessed unusual verbal abilities, in addition to the good fortune of a supportive family and attendance at good schools, he was able to exploit his temperamental preference for an introverted, solitary life to become an outstanding poet. Marjorie is verbally talented and might become a successful writer.

Each temperamental bias can be likened to a particular anatomical or physiological feature of an animal. The adaptive advantage of the feature always depends on the local environment. A finch on a Galapagos island with a very large beak will have a distinct advantage over one with a small beak if most of the food supply consists of large seeds. But this advantage will be lost if the ecology contains mainly small seeds. Thus, the personalities that are generated by combinations of temperaments and life experiences can be likened to a gray cloth woven from extremely thin black and white threads. One sees only the gray cloth, not the separate black and white threads. Perhaps the best metaphor for a temperament is a block of stone in a sculptor’s studio. The hardness, color, and size of the stone restrict the possible forms the sculptor might create, while leaving the artist considerable freedom to produce any one of a large number of aesthetic products. The ancient Greeks were prescient when they proposed the four human temperaments they called sanguine, phlegmatic, choleric, and melancholic which they assumed were the result of the balance among the bodily substances of blood, phlegm, yellow bile, and the mysterious black bile. Contemporary scientists have replaced these body humors with the many molecules that affect brain activity….