There are two kinds of people in the world: people who think there are two kinds of people in the world and people who don’t. Katharine Briggs and Isabel Myers were the first kind, and the test they invented based on that belief, the Myers-Briggs Type Indicator, or MBTI, is the most popular personality test in the world. More than two million people take it every year. It is used in twenty-six countries to assess employees, students, soldiers, and potential marriage partners. It is used by Fortune 500 companies and universities, in self-improvement seminars and wellness retreats. There are more than two thousand personality tests on the market, many of them blatant knockoffs of the MBTI, but Myers-Briggs is No. 1. Merve Emre’s “The Personality Brokers” (Doubleday) is the story of how the MBTI fell to earth.

It was a long descent. Briggs and Myers were a mother-and-daughter team. To call them “mildly eccentric” would be indulging in a gender stereotype, but it seems fair to say that they were a little O.C.D. They devoted their lives to their system, and they kept the faith for a very long time. If they had not, there would be no MBTI today.

The mother, Katharine Cook Briggs, was born in 1875. When she died, in 1968, the test she inspired was all but forgotten. The daughter, Isabel Briggs Myers, was born in 1897. She codified her mother’s method of categorizing personalities, copyrighted it (in 1943), and spent the rest of her life trying to find a permanent home for the product. She died in 1980, just as the test’s popularity was taking off.

Since Katharine began studying personality differences when Isabel was four, this means that the two women persisted for almost eighty years before the MBTI became the commercial bonanza it is today. According to Emre, personality testing has become a two-billion-dollar industry. But Briggs and Myers were not in the personality game for the money. They truly believed that they had discovered a way to make work more efficient and human beings less unhappy.

Emre’s book follows closely the account of the development of the MBTI given in Annie Murphy Paul’s “The Cult of Personality Testing,” published in 2004 (a work that Emre surprisingly does not acknowledge). Both books describe Briggs and Myers as intellectually driven women in an era when career opportunities for intellectually driven women were slim. Neither one had any training in psychology or in psychiatry—or, for that matter, in testing—and neither ever worked in a laboratory or an academic institution. A third woman, Mary McCaulley, who came upon the test in 1968, the year Katharine died, was a professor of psychology at the University of Florida. She teamed up with Isabel, and was indispensable in turning the MBTI into a professional operation. But, essentially, the MBTI was home-cooked.

It arose out of one of the most mundane domestic circumstances—the fact that the members of a family often differ in tiny but stubbornly irreducible ways. One spouse is a planner and the other is spur of the moment. One child has a million friends and another child is a loner. In the home, differences like these are magnified, because people are less self-conscious and because enforced intimacy generates friction. But at work, too, people have noticeably divergent ways of operating.

Sibling and spousal differences are the kind of thing that might attract the interest of a housewife deprived of other ways to exercise her brain, and that’s what happened with Katharine Briggs. The key to the MBTI’s success is her insight that you can waste a lot of energy and bring on a lot of psychic pain if you think of these differences as incompatibilities that have to be ironed out. The differences are innate, and each type of personality is as “normal” as the others. There is no better way to be—logical or emotional, spontaneous or organized, party bro or brooder. These are not imperfections to be corrected. They are hardwired dispositions to be recognized and accommodated.

In the workplace, this means assigning tasks to people based on their personality types, which is one of the things that the MBTI is supposed to help companies do. (Emre says that the office-furniture designer Herman Miller uses a modified version of the MBTI to create chairs and desks for different personalities.) In life, it means recognizing that we are naturally more likely to get along with some people than with others, and that when people aren’t communicating it can simply be because they are broadcasting on different frequencies. We need to get used to it.

The MBTI folks therefore do not refer to their device—a ninety-three-item, a-or-b format questionnaire that subjects are not supposed to take a lot of time filling out—as a “test.” The MBTI is not something you can pass or fail. The MBTI is an “indicator,” and what it is meant to indicate is the type of personality you have been born with.

The theory behind the MBTI, actually, is not that there are two kinds of people in the world. It’s that there are sixteen kinds of people in the world, but that each personality type reduces to a set of elements taken from four either/or binaries. Everyone is either extroverted or introverted, sensing (meaning relying on sense data) or intuitive, thinking or feeling, judging or perceiving.

Your “score” on the test is the combination of the four characteristics indicated by your answers to the ninety-three questions, which ask such things as “In reading for pleasure, do you (a) Enjoy odd or original ways of saying things; or (b) Like writers to say exactly what they mean?” (It’s unclear why these are mutually exclusive alternatives.) Using an initial for each characteristic, with N standing for intuition, you can be scored an ESTJ, an INFP, or one of the remaining fourteen four-letter combinations. Emre says that using this initial shorthand is called “speaking type.”

The MBTI is different from other tests with high name recognition, like the Minnesota Multiphasic Personality Inventory (MMPI)—first published in 1943, the year Myers copyrighted the MBTI—because the MMPI is used in the diagnosis of psychiatric disorders, and the Myers-Briggs test is a human-resources technology, a nonjudgmental way of people-sorting. It is not designed to pick out the neurotics.

But the MBTI is also promoted as a means of self-discovery, and that is undoubtedly why it is so widely used today. The company that took ownership of the test in 1975 and now administers it, CPP, Inc., advertises learning your personality type as a potentially life-changing experience. Emre reports people telling her that they felt liberated after finding out their type, that it helped them cope with their work or with their marriage, that it empowered them to be themselves. (Emre participated in a training program for MBTI certification and does not report feeling liberated after taking the test, although that may be because she tried to sabotage her session by parodying back at her trainers the kind of language they were using. She was denied access to the Myers Briggs archives—not an ideal outcome for someone writing a book on the subject.)

Emre is an English professor at Oxford, and she began her research with an English professor’s skepticism about quantitative social science and “technologies of the self.” She brings in Bentham and Foucault to suggest the MBTI’s connection to regimes of surveillance and control. She refers to Stanley Milgram (of the “obedience to authority” experiment) and Philip Zimbardo (of the Stanford prison experiment), to remind us that scientistic nomenclature can shape behavior in troubling ways. She refers to William Whyte’s “The Organization Man,” published in 1956, as a way of associating the MBTI with a postwar culture of conformity. (Like a lot of people, she mistakenly calls Whyte a sociologist. Whyte was a magazine writer; he worked at Fortune. He was no more a social scientist than Briggs and Myers were.)