The Human Spark

The Science of Human Development


By Jerome Kagan

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As infants we are rife with potential. For a short time, we have before us a seemingly infinite number of developmental paths. Soon, however, we become limited to certain paths as we grow into unique products of our genetics and experience. But what factors account for the variation — in skills, personalities, values — that results? How do experiences shape what we bring into the world?

In The Human Spark, pioneering psychologist Jerome Kagan offers an unflinching examination of personal, moral, and cultural development that solidifies his place as one of the most influential psychologists of the past century. In this definitive analysis of the factors that shape the human mind, Kagan explores the tension between biology and the environment. He reviews major advances in the science of development over the past three decades and offers pointed critiques and new syntheses. In so doing, Kagan calls out the shortcomings of the modern fad for neuroscience, shows why theories of so-called attachment parenting are based on a misinterpretation of research, and questions the field’s reflexive tendency to pathologize the behavior of the young. Most importantly, he reminds us that a life, however influenced by biology and upbringing, is still a tapestry to be woven, not an outcome to be endured.

A profound exploration of what is universal and what is individual in human development, The Human Spark is the result of a scientist’s lifelong quest to discover how we become who we are. Whether the reader is a first-time parent wondering what influence she, her genes, and the wider world will have on her child; an educator seeking insight into the development of her students; or simply a curious soul seeking self-knowledge, Kagan makes an expert and companionable guide.



Psychology’s Ghosts

The Temperamental Thread

The Three Cultures

What Is Emotion?

Three Seductive Ideas

The Second Year

An Argument for Mind

The Long Shadow of Temperament

Galen’s Prophecy

The Nature of the Child

Birth to Maturity


Social scientists who were trained in American universities during the first half of the twentieth century found it hard to escape the assumptions about human nature that history had bestowed on them. As that century began, large numbers of children from impoverished, illiterate immigrant families living in densely populated neighborhoods were doing poorly in school and disrupting civic harmony. The social scientists’ preferred explanation of such facts emphasized the power of experience to create these and other profiles. This unquestioned faith in the malleability of the mind, an idea not yet documented by research, sustained the hope that proper rearing within the family and proper instruction by conscientious teachers in the schools could transform all children into productive citizens.

Only a few decades earlier, many experts had assumed that the less-than-adequate adjustment of the children born to poor immigrants was attributable to inherited biological defects. This pessimistic explanation bothered liberal Americans who, believing in the power of experience to conquer all but the most serious deficiencies, hungered for scientific support of their belief. Freud and the behaviorists supplied the reassurance by announcing that variation in experience could account for most of the variation in children’s competences and behaviors. By the 1950s, a large majority of developmental psychologists were certain that the events of early childhood, especially in the home, were the primary determinants of adolescent and adult profiles. Each child’s biological features, which the psychologists did not deny, could essentially be ignored.

A rash of unexpected scientific discoveries after 1960 challenged this optimistic position. Stella Chess and Alexander Thomas described the contribution of infant temperaments to later personality at the same time that others were finding evidence for genetic contributions to many talents. These discoveries—combined with the failure to provide convincing evidence that experience alone could create an extremely shy, aggressive, or intellectually impaired child—forced the next cohort of psychologists to acknowledge biology’s influence.

I entered graduate school in 1950 committed to the older environmental position but sufficiently receptive to the biological perspective to take advantage of a chance event that led to a personal epiphany. This event occurred during the 1960s, when I visited Guatemala as a member of a team of American scientists charged with evaluating a research proposal on the effects of nutritional supplements on the health and cognitive talents of malnourished children living in poor, rural villages. Following our formal meeting, Robert Klein, the American psychologist who would direct the day-to-day operations of the research, took me to Lake Atitlan in the northwest part of the country. This exquisitely beautiful, cobalt-blue lake at the foot of a volcano was rimmed by a number of villages containing the descendants of Mayan Indians, some living under conditions that had not changed much over the previous two hundred years.

This scene provoked my curiosity about the development of children in this non-Western setting, and I spent my sabbatical year in 1972–1973 observing adults and children in one of the poorest, most isolated villages on the lake. It was there, after several months of study, that I was forced to acknowledge biology’s substantial contribution to psychological development through its control of brain maturation—an idea supported by evidence from other laboratories as well. Upon returning to Harvard in the fall of 1973, I devoted much of the work of my own laboratory to the pursuit of this idea.

I summarized my revised views of development in 1984 in The Nature of the Child. This book (and its 1994 revision) contained three major themes. The first was that the major changes in behavior over the first few years of life depend on stages of brain maturation. This idea implied the second theme—namely, that the habits and emotions established during the first year might be so seriously altered as to have little influence on the psychological profiles of older adolescents. The third theme was that the human capacity to understand the distinction between right and wrong emerges during the second year. All three ideas, which were tentative twenty-eight years ago, are now firm facts thanks to the efforts of many investigators.

As I was searching for a writing project in the spring of 2011, the idea of revising The Nature of the Child pierced consciousness and T.J. Kelleher of Basic Books found this proposal attractive. Upon completing the early draft s of each chapter, I was surprised by the need to recast the arguments and to elaborate three questions that had been less clearly articulated in the 1984 book: What is the expected developmental course for the cognitive talents, motor skills, emotions, beliefs, and moral values that are inherent possibilities in all children? How does variation in experience affect the rates at which these properties develop and the forms they assume? And, finally, what factors determine the variation among children and adults within every community? The present book probes the concepts of morality and emotion more deeply than the original and addresses a concern that was less salient in 1984 but is now widespread: mental illness in children and adolescents. Because this book covers a larger territory than the earlier one, it needed a new title.

Chapter 1 considers the influences of culture and history. Each person’s experiences in a particular culture during a particular era select one profile from an envelope of possibilities that existed during the first hours after birth. Human behavior is controlled by features in the local setting and the person’s motives and beliefs. On the one hand, children must react to events that threaten their survival or mental serenity. They must do something if attacked and maintain relationships with those supporting them. On the other hand, many actions are provoked by ideas, especially representations of the properties one ought to attain—whether good grades, friends, love, money, a higher status, or greater power.

Events during a historical era within a culture often challenge existing values to produce a generation with different ethical premises. The generation of Americans who came to maturity after 1970 were more tolerant, more skeptical of authority, and less prudish about sexuality than their grandparents.

Chapters 2 and 3 document the biologically based progression of cognitive advances during the first three years. Among the most important advances are the nature of the infant’s representations of experience, the enhancement of working memory, and the emergence of the first forms of language, inference, a moral sense, and consciousness.

Developmental scientists are engaged in a lively debate over the similarity between the infant’s knowledge and what seem to be similar ideas in adolescents. Some psychologists claim that the infant’s understanding of the concepts of number and causality shares important features with thirteen-year-olds’ understanding of the corresponding conceptions. I consider the evidence and side with the skeptics.

Chapter 4 considers the complementary influences on development of a variety of factors, including parental practices, identifications with family members and social groups, birth order, size of community, and historical era. I award considerable power to the social class of the child’s family. Many psychologists regard a child’s class of rearing as a nuisance variable that must be controlled statistically in order to prove the critical influence of a particular experience, whether harsh punishment, abuse, bullying, or maternal illness. Unfortunately, statistically controlling for the consequences of class eliminates an important causal condition because the influence of the unpleasant experiences listed above is diluted in children from more advantaged families. The child’s social class represents a large collection of correlated experiences that cannot be removed from the total pattern without affecting the outcome.

The child’s identifications with parents, family pedigree, class, and ethnicity—based on shared features and vicarious emotions—have a profound effect on moods and expectations that can last a lifetime. Unfortunately, psychologists have not invented methods that measure these identifications accurately. Hence, I was forced to rely on memoirs and autobiographies to document the emotional consequences of being, for example, the grandchild of an eminent writer or Nazi official. This chapter also considers the popular concept of attachment and concludes that John Bowlby’s bold assertion that the quality of an infant’s attachment to a parent is a sensitive predictor of the person’s later adjustment has not been affirmed.

Chapter 5 deals with two critical puzzles: which properties of early childhood are preserved and whether the child’s developmental stage affects the degree of preservation. The evidence implies that most public behaviors show minimal long-term preservation until children reach six or seven years of age. This fact motivates a discussion of the stages of psychological development that are accompanied by new cognitive talents that, in turn, have implications for the preservation of habits and moods.

Human morality has always been a source of deep curiosity. I continue to believe, as I did in 1984, that acquisition of the concepts right, wrong, good, and bad affect many aspects of the child’s behavior. Chapter 6 contains an analysis of the varied meanings of morality as well as a description of the phases that precede the establishment of a more permanent moral position during adolescence. I remain skeptical of the Darwinian notion that human morality is a derivative of the sociability of monkeys and apes. Indeed, I argue that the defining feature of human altruism is a person’s intention, whereas the comparable feature in animals is the consequence of an agent’s action on another animal. Humans help others because they want to regard themselves as good persons and wish to avoid the unpleasant feeling of guilt that can occur when one is indifferent to the suffering of another. Despite the use of the same word by students of animals and humans, the two meanings of altruism are seriously discrepant.

The puzzle surrounding the relation between bodily feelings and human emotional states continues to evade satisfying solutions. Chapter 7 begins with an analysis of the many definitions of emotion and claims that the popular emotional words in all languages are interpretations of bodily feelings. These words can be used in the absence of a feeling—and when a feeling is present, words often fail to specify the quality and origin of the feeling and the target of any given behavior.

The extraordinary increase in the diagnoses of mental illnesses in children and adults over the past three decades demanded a chapter on this troublesome fact. Chapter 8 questions the usefulness of contemporary illness categories because they are indifferent to the causes of the symptom; it also discusses why a belief in the effectiveness of a therapy, whether a drug or psychotherapy, is the most essential reason for remission or cure.

The final chapter describes four reasons for the slower progress of the social sciences compared with the biological sciences. One barrier is the habit of relying on single causes and single outcome measures rather than on patterns of causes and outcomes. Too many psychologists studying humans rely solely on questionnaires as evidence. This information does not capture the complexity of the feelings, intentions, and thoughts that the informant’s answers purport to describe. Even when single behaviors are measured, the accompanying concepts do not have an unambiguous meaning because most behaviors are the result of more than one condition. To understand the theoretical significance or meaning of a particular behavior, we need additional information, including measurements of the brain and/or bodily activity accompanying a behavior.

Many psychologists are exploiting technical advances in brain measurement such as the electroencephalogram, the magnetic resonance scanner, and the magnetoencephalogram. The results of studies using these machines have taught us the homely truth that two sources of evidence are always better than one. When replies to questionnaires are combined with measures of the brain, investigators have a richer understanding of the meaning of both the verbal statements and the biology.

The lack of attention to processes that account for the striking psychological differences between members of divergent social classes is a second barrier to progress in the social sciences. Many reports document disparities in academic achievement, mental illness, and criminal behavior, but few psychologists try to discover the patina of events that create the private perceptions of one’s place in society.

A preference for initiating research guided by an intuitively attractive hypothesis, rather than a puzzling phenomenon, is a third obstruction to progress. Natural scientists usually try to understand the causes of a robust fact. Why do cats beget cats? Why does the moon’s position in the sky change throughout each month? Why do only some people come down with a fever during an epidemic? Why does milk sour?

By contrast, social scientists more often begin with a big word, such as regulation, anxiety, or stress, that originates in an intuition and does not specify the agent, setting, or source of evidence for the concept. The temptation to fall in love with an abstract idea and search for proof of its existence in evidence produced by a single procedure administered in a single setting is a major reason for the slow progress of the social sciences. Many significant discoveries in biology were accidental observations that had not been predicted from existing theory. These include the power of experience to silence a gene, the effect of chronic stress on the integrity of the immune system, the neural bases for knowing one’s location in an environment, and the role of the amygdala in states of fear.

The inability to explain how a psychological phenomenon emerges from a brain state is a fourth barrier. The reasons for this state of affairs include a reluctance to acknowledge that a brain state can be the foundation for more than one psychological outcome; the problems trailing heavy reliance on magnetic scanners, which provide too crude an index of psychological processes; and neuroscientists’ failure to invent a vocabulary that describes brain profiles.

Despite these problems, there are good reasons to celebrate the substantial progress of the last thirty years. Few psychologists today argue, as many did earlier, that children learn to speak or develop a moral sense through conditioning mechanisms alone. We now recognize the contribution of temperament to personality development and the term emotion, which had been viewed as too fuzzy to study, now has a journal with that term as its title. The wall that existed between thought and feeling has been breached.

Most important, many members of the younger cohort are willing to learn the complex technologies needed to measure brain activity, genes, and molecular concentrations in order to evaluate the biological influences on development. The scientific study of children is less than 150 years old. Physics has a 400-year history if we assign its birth to Galileo’s discoveries in the early seventeenth century. The first cohorts of natural scientists who followed Galileo had no idea that leptons, quarks, and bosons were the foundations of matter. I hope that readers will regard my interpretation of the hard-won victories of talented investigators as reasonable, readable, and an incentive for reflection. I have tried to be honest, occasionally harshly honest, in my interpretations of the available evidence—though such evidence may be uneven, scattered, and insufficiently firm to insulate many tentative conclusions from further questioning. Every author imagines an audience sitting on a shoulder peeking at the prose being cobbled together. As I composed this manuscript, my imaginary readers were those who had not made up their minds about the defining features of human nature and the forces that transform infants into children, children into adolescents, and adolescents into adults. I hope such readers exist.

I thank Marshall Haith, David Kupfer, Jay Schulkin, and Robert Levine for their critical comments on separate chapters. I am also deeply indebted to Thomas Kelleher for his masterful editing of a draft that I thought was fine. I thank Christine Arden for her excellent copyediting and Moira Dillon for preparing figures.


Setting the Stage

There are two answers to the question “What does it mean to be human?” On the one hand, most members of the species Homo sapiens inherit a brain and body that award them the potential to acquire a set of psychological properties that distinguish them from every other animal. On the other hand, most of these properties, especially varied talents, memories, beliefs, moral standards, and emotions, began as vessels that have to be filled. The potential to learn a language is present in the newborn, but that inherent capacity can result in the acquisition of any of six thousand different languages. The child’s cultural setting and historical era “fill” these vessels. The beliefs held by medieval French adults concerning family relationships, sex, and life after death were not shared by thirteenth-century Chinese adults, are not held by contemporary Parisians, and would not be present in the minds of infants from all three settings. A satisfying understanding of how children acquire the habits, skills, emotions, values, and ideas that define their culture has so far evaded us. This book weaves a developmental thread into the larger tapestry we lovingly call human nature.

The mystery of the infant’s mind makes it easy to construe early behaviors as confirming whatever assumptions an observer would like to believe. Imagine three women watching a sixteen-week-old infant’s facial expressions, directions of gaze, limb movements, and vocalizations. After about twenty minutes of simply watching, one woman, standing behind the infant, moves a mobile constructed from attractively colored toys back and forth in front of the infant’s face while all three observers note the vigor of the infant’s arm and leg movements and any fretting or crying. When the infant has calmed down, the second woman places a drop of sugar water on the infant’s tongue, waits a few minutes, and then places a drop of lemon juice on the tongue while all three observers note any change in facial expression. The third woman then looks down at the infant and—in sequence—smiles, frowns, says a few words, and finally gently caresses the infant’s forehead. When the three women share what they believe the infant felt, perceived, or thought during the past forty minutes, they discover that they arrived at dissimilar impressions.

The ambiguity of infant behaviors, which we might compare to the facial expression on the Mona Lisa, frustrates those who study development. Are infants born without ideas, or do they possess certain core understandings of the world? Do they have a self? Are they biased toward selfish or caring behavior? Are they consciously aware of events around them? Although the answers to these questions are being debated, one truth seems relatively certain. All children, excluding the small number with serious compromises in brain function, have the potential to acquire a large number of talents, beliefs, habits, values, and emotions. The family and the local culture select for elaboration those properties that are likely to protect the child from harm and allow the adult to enjoy respect and acceptance from a majority in the society.

The practices of Mayan Indian mothers in small, isolated villages in northwest Guatemala provide an example of cultural selection. Mayan parents believe that infants during the first year are vulnerable to being harmed by the stares of strangers. The gaze of a man wet with perspiration from a day’s work is exceptionally dangerous. Mothers protect their infants by wrapping them tightly and placing them in a hammock in a dark region at the back of the hut where, except for times when they are nursing, the infants remain for most of the first year. As a result, these one-year-olds are pale, listless, and display a motor and psychological profile that lags behind most of their counterparts elsewhere in the world. After their first birthday, however, mothers no longer consider them vulnerable and they are allowed to leave the hut to play with objects and other children. By their third birthday they have developed the basic motor skills and psychological talents that are inherent in the biology of all three-year-olds.

A tribe residing in an isolated region of New Guinea holds the rarer belief that all male infants are born sterile. Because these boys must eventually become fathers, the society invented a ritual to guarantee their future fertility. When a group of boys is within a few years of puberty, the older men of the village take them to a secluded spot, arrange the boys in a circle, and march around them playing flutes. From that day forward until late adolescence, but before they marry, the boys perform fellatio on the unmarried older adolescent boys in order to acquire the seed they need to become fathers. Once these boys become older adolescents, the fellatio ceases; when they marry and sire children, they affirm the truth of the culture’s premise.1

Nineteenth-century Americans were convinced that infants were born with an instinct for freedom and individualism that, acting together, facilitated their society’s ascent toward a state of perfection. A faith in the inevitability of a progression from less to more mature is present in the writings of the three major Western theorists of development: Freud, Erikson, and Piaget all posited a sequence of developmental stages through which children ascended to more satisfying, creative, or rational states. Adolescents do reason and regulate emotion more effectively than infants, but they are also more often angry, suspicious, deceitful, depressed, and anxious. Psychological development should be seen as a sequence of additions, losses, and transformations in which new traits emerge, no-longer-useful ones are discarded, and some remnants of earlier phases are retained as elements in new patterns. A stairway to paradise is a poor metaphor for development.

Far Eastern cultures, by contrast, regarded nature and society as following cycles in which benevolent eras alternated with intervals of adversity. The cyclical advances and retreats of glaciers throughout Earth’s long history provide an example from geology. The creative Greek and Roman societies were replaced by the Dark Ages, which, in turn, were followed by the advances of the medieval era.

Reflection on the societies that gave rise to important inventions during different historical periods supports a cyclical view. China and the Mediterranean basin fostered the most significant inventions during the 5,000 years before the modern era. Europe became dominant over the next 1,800 years, as did the United States during the past two centuries. It is obvious that the features of a cultural setting that make creative ideas and products more probable cycle over time and place. Application of a cyclical conception to human development implies that each stage is marked by specific talents, pleasures, and understandings and that no stage is inherently superior to another.

Although major transformations of beliefs and practices usually occur over long periods of time, important changes can occur in one or two generations. For example, Carl Degler notes that between 1760 and 1820 a majority of Americans adopted three new attitudes: (1) young adults were allowed to use sexual attraction as a basis for choosing a marital partner, (2) wives possessed moral authority in the home, and (3) rearing children was the mother’s major responsibility.2

The Mayan town of San Pedro, located on the side of a volcano bordering Lake Atitlan in northwest Guatemala, witnessed an equally rapid change in values: today, most youths complete at least twelve years of education, and men whose fathers, grandfathers, and great-grandfathers had been farmers are now plumbers, accountants, dentists, or lawyers using the Internet.3 I studied the nearby smaller, poorer village of San Marcos in 1972, when it had no foreigners, no running water or electricity, and minimal communication with the outside world. When my wife and I returned in 2008, we were surprised to learn that the village now had a small hotel, an Internet café, a yoga center, and an agency selling tickets to tourists who wanted the delight of a moonlight cruise on the lake.

China experienced three radical transformations during the twentieth century. In 1901, as an autonomous, mainly rural society controlled by an empress, the country suffered the humiliation of defeat and subjugation by European powers who occupied many of its large cities. Fifty years later, Mao Zedong transformed what was then a hierarchical society of peasants exploited by a small group of landlords into a despotic communist state. The third change, accomplished in only forty years, introduced a capitalist economy that celebrated a combination of individualism, materialism, corruption, and cynicism that was a novelty in this ancient society. It took less than half a century to transform China from an economically impoverished society into one of the economic powerhouses of the modern world.


On Sale
Jun 4, 2013
Page Count
352 pages
Basic Books

Jerome Kagan

About the Author

Jerome Kagan is emeritus professor of psychology at Harvard University and one of the pioneers of the field of developmental psychology. The author of numerous books including The Nature of the Child and Galen’s Prophecy, he received the William James Award from American Psychological Association. In 2009 he was featured in a New York Times Magazine cover story about anxiety.

Learn more about this author