I often hear a fellow senior citizen declare, “I wish I’d spent more time with my family.” In fact he easily could have, but preferred golf, or working overtime, or whatnot. In any case, his behavior belied his self-report. Such self-reporting is dismally unreliable, and no one understands this better than Jerome Kagan, the esteemed psychologist whose new book distills the accumulated wisdom of his astonishingly productive six-decade career—a period during which, Kagan is sad to report, “psychology slipped from a position of prominence...to a subordinate rank in the academy.” In 340 thought-provoking pages deeply grounded in his encyclopedic knowledge of social-science research, including his own exemplary studies of temperament, Kagan explains the methodological stumbling blocks, unwarranted assumptions, systemic constraints, and sometimes startling scientific naïveté that have left us—after more than a century of bustling and expensive research—with so feeble an understanding of ourselves.

Take those self-reports, for example. Innumerable published studies are based on responses to questionnaires or interviewers asking things like, “Were you bullied as a child? What effect did that have upon you? Do you feel uneasy with strangers?” The problem with such studies, as Kagan points out, is that “the correlation between what people say and what they actually do ranges from negligible to modest.” (To take one example, the group of adults who say they used sunscreen at a swimming pool on a given day shows a low correlation—0.2, to be precise—with the number of people shown by skin swabs actually to have sunscreen on them.) Kagan wields this objection in a persuasive dismissal of currently popular “happiness” research, showing that it is a mistake to presume that something called “lifetime well-being” exists just because people responding to a questionnaire say they have it. “Psychologists who study human psychological states,” he advises, “have an exaggerated respect for words.”

A deeper and subtler source of error in psychological studies, Kagan explains, is reliance on observation of a single variable (such as a genetic marker, pulse-rate change, or startle reflex) rather than an entire pattern (gene plus self-report plus observed behavior plus biological measure plus stressful experience). Such a complex pattern is much more likely than a single variable to yield a generalizable principle with predictive power. In his chapter “Who Is Mentally Ill?” Kagan laments that psychiatric diagnosis relies so heavily on patients’ subjective accounts of their own symptoms, augmented only modestly by the clinician’s direct observation during interview and, less frequently, by information from family. Unfortunately, the present state of the art provides little in the way of useful biological measures to confirm clinical diagnoses. Unlike our medical and surgical colleagues, we psychiatrists seldom get answers from imaging technology or laboratory reports. Kagan suggests that diagnostic categories should “combine symptoms with their origins.” No doubt that will happen once those origins are elucidated, but today the causes of schizophrenia, obsessional disorder, bipolar disorder, and most other psychiatric afflictions remain unknown.

Kagan’s discussion of the state of current clinical practice in psychology and especially psychiatry makes a number of telling points: the Diagnostic and Statistical Manual (DSM-IV) is not securely based on science; too much medication is prescribed to too many patients, particularly children; pharmaceutical company money corrupts research, practice, and even teaching; children with temper tantrums are mistakenly believed to have bipolar disorder; many psychiatrists practice too hurriedly to genuinely understand their patients.

All true. Yet even someone who concurs with these criticisms might be excused for taking Kagan’s view of the clinic as that of an outsider looking in. I suspect he has never had principal responsibility for the treatment of a psychotic patient. Those of us who have are apt to hold the pharmacopoeia in higher regard than he does. His complaint that the categories in DSM-IV don’t represent hard science is accurate enough. But whether compulsive gambling, for instance, is a psychiatric disorder is not finally a question for science. It may be a semantic, moral, metaphysical, even epistemological question—and certainly it is a practical one, since the answer determines whether remedial efforts will be paid for by health-care insurance. The system demands an answer, up or down, and that is not a scientific question but a bogsat (Bunch of Guys Sitting at a Table) of precisely the type Kagan bemoans.

Psychology’s Ghosts could benefit from some professional editing. Kagan’s prose is often turgid (“The primary themes would be the significance of the context in which individuals from different social classes and cultures try to cope with each day’s responsibilities through their private interpretations of experiences in their different contexts”). His thoughts about the definition, causes, and treatment of psychiatric disorders can be hard to follow, as when he attempts to analogize psychological and dental diagnoses, arguing that the fact that “neither dentists nor patients regard tooth decay as an illness that requires an abnormal biological vulnerability” allows us to “separate the concept of depression from the concept of illness,” since “most humans are capable of becoming...despondent...after losing a cherished goal.”

More seriously, while Kagan repeatedly warns us of cardinal pitfalls in social-science methodology, he frequently ignores his own admonitions; one is startled to see scrupulous regard for method give way to vapid pontificating. Kagan informs us that “after 1950 everyone became obsessed with aggrandizing their own egos.” Everyone? He asserts that “many contemporary North Americans and Europeans feel they are lonely,” that this loneliness “can lead to...compromises in the immune and cardiovascular systems,” and that “the primary causes of the...compromised biology are the absence of an ethically praiseworthy goal and reluctance to commit to another because of the possibility of betrayal.” How can one possibly know this? Kagan may be guessing right, but is this an example of how he hopes to strengthen the discipline of psychology?

Jerome Kagan’s own studies of human temperament from infancy to adulthood constitute a masterpiece of research in psychology, and his new book is crammed with fascinating information gleaned from a lifetime’s acquaintance with the lab and the literature. The author is immensely informed, and fills his pages with references to myriad facets of Western culture. Slogging through Psychology’s Ghosts can be hard work in places, but in the end the slog is well worth the effort.