A new book from developmental psychologist Jerome Kagan is now available, published by The Dana Foundation, and available at Amazon - The Temperamental Thread: How Genes, Culture, Time and Luck Make Us Who We Are (although there is no Kindle version).
Kagan has probably contributed more to our understanding of temperament than any other psychologist. Here is the section devoted to his basic ideas on temperament from his Wikipedia entry:
I have a hard time with his rejection of attachment theory - the evidence is overwhelming that attachment is a crucial element of child development. Certainly, there is some mix between innate temperament and attachment experiences, which would explain why two children with exactly the same hellish upbringing will come through it in different ways.
According to Kagan, (conventionally):
"temperament refers to stable behavioral and emotional reactions that appear early and are influenced in part by genetic constitution."
Temperament is perhaps what Kagan is best known for. He began his remarkable work on temperament after his research in Guatemala. Kagan was primarily focused on children’s fear and apprehension . It was during this time that Kagan discovered children as having one of two types of temperament: inhibited and uninhibited. Inhibited temperament, also known as highly reactive, can best be described as a child being more reserved, guarded, and introverted whereas uninhibited, or low reactive, children tend to be more outgoing, extroverted, and are very comfortable in social situations . As a result of his ground breaking work on temperament, we know that these characteristics have the ability to influence later behavior depending on how they interact with the environment 
Kagan rejects "attachment theory", British psychiatrist John Bowlby's notion that the bond between caregiver and infant is crucially influential in later emotional and even intellectual growth. He has also criticized Judith Rich Harris's theory that peer groups matter more than parents in influencing the personality of children. He believes that both sides in the nature/nurture debates were too rigid, and that the development of personality is still not well understood.
For example: My sister and I grew up in the same family, and we were close in age. Yet when our father died, I self-destructed for a few years then got my shit together and got some help (therapy and Buddhism). On the other hand, she acted out a little as a teen but never to the extent that I did. She seemed to get her life together, married a good man, had a family, then in her late twenties became an addict and eventually died as an indirect result of her addictions. Attachment can't really explain that difference, but temperament can.
Still, attachment does explain why we both went through periods of self-destruction. We both had insecure avoidant attachments (we grow up to have dismissive-avoidant attachment in relationships) as a result of being raised by the same mother. This made it hard for us to allow others to get too close, since we feared the kind of smothering we received from our mother.
Anyway - he is brilliant, but we cannot disregard attachment theory, no matter how well temperament explains some parts of who we are.
In the following article, the Harvard Gazette looks at his work on temperament:
Psychology pioneer probes childhood trends influencing temperamentAsk babies who they are, and they’ll babble something that seems nonsensical. Turns out, they’re onto something.
Jerome Kagan, a developmental psychologist and the Daniel and Amy Starch Professor of Psychology Emeritus, has spent the past 30 years of his lengthy career studying the temperaments of those little people, which originate in a child’s unique biology, along with the experiences that shape their personalities. These discoveries are summarized in his new book, “The Temperamental Thread.”
Twenty percent of Kagan’s 4-month-old infant subjects were labeled high reactive, “a behavioral profile marked by vigorous motor activity and crying to unfamiliar experiences.” And 40 percent were labeled low reactive because they showed the opposite behaviors. Both temperaments are modest predictors of future personalities, depending on how children responded to their environments. (Another 40 percent belonged to neither group.)
“The high-reactive infants are biased to become children who are timid, shy, and cautious in unfamiliar situations. This is a personality trait known as inhibited,” said Kagan. “The low reactives are biased to develop into outgoing, spontaneous, fearless children — uninhibited.”
Kagan also explores links between temperament and gender, ethnicity, mental illness, and more. The difference between males and females is always newsworthy fodder, and, according to Kagan, “over the past 50 years, many scientists have discovered intriguing biological differences between males and females that imply different patterns of temperaments in girls and boys.”
“The most obvious are related to the molecules oxytocin and vasopressin, and the sex hormones. It appears that these molecules, in conjunction with others and experience, bias girls to care more about the quality of their social relationships and bias boys to care more about their potency and relative status with other males.”
Kagan said he’d always been curious about the mind and “the persistence of beliefs that are not in accord with experience,” and recalled arguing at a young age with his mother, who believed in inborn traits of personality.
“During the 1940s and ’50s, many citizens and social scientists believed that the main, if not the only, cause of the problems that plague our species were childhood experiences,” said Kagan. “This belief was an heir of Freudian ideas and the confidence of behaviorists, who were demonstrating the power of experience to shape animal behavior. It followed that anyone who discovered the specific experiences that led to a mental illness, crime, or school failure would be a hero doing God’s work. Who would not entertain the idea of becoming a child psychologist, given this Zeitgeist?”
Although retired, Kagan still enjoys collaborations with colleagues Nancy Snidman of Children’s Hospital and Assistant Professor of Psychiatry Carl Schwartz, and has begun to write “a set of essays on some contemporary but controversial issues that surround the meanings and measurements of the concepts of happiness, morality, brain bases for psychological states, and mental illnesses.”
But what about Kagan’s baby subjects? Where are they now? “Infant temperaments act to limit what children will become; they do not guarantee a particular personality,” he noted.
“A life itinerary is like the game of ‘Twenty Questions.’ Each new piece of information eliminates a large number of possibilities, but many still remain.”
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