FORTUNE — When Frank Parsons opened the world’s first career guidance center in Boston in 1908, he began by asking prospective clients 116 penetrating questions about their ambitions, strengths, and weaknesses (and how often they bathed). But then he did something more unusual: He measured their skulls.
Parsons was a committed believer in phrenology. If you had a large forehead, he might recommend you become a lawyer or engineer. But if your skull was more developed behind the ears, you were of the “animal type” and best suited to manual work.
Career advice has, thankfully, come a long way since then. But now, instead of measuring the outside of people’s heads, it has become common to measure the inside using psychometric tests. Personality testing has grown into a major industry and is standard procedure in leadership and management courses, as part of job-interview processes, and, increasingly, in career counselling. But should we really trust such tests to deliver scientific, objective truth?
MORE: The epic inside story of fraud at a generic Lipitor giant
I have some bad news for you: Even the most sophisticated tests have considerable flaws. Take the Myers-Briggs Type Indicator (MBTI), the world’s most popular psychometric test, which is based on Jung’s theory of personality types. Over two million are administered every year. The MBTI places you in one of 16 personality types, based on dichotomous categories such as whether you are an introvert or an extrovert, or have a disposition towards being logical or emotional (what it calls “thinking” and ”feeling”).
The interesting — and somewhat alarming — fact about the MBTI is that, despite its popularity, it has been subject to sustained criticism by professional psychologists for over three decades. One problem is that it displays what statisticians call low “test-retest reliability.” So if you retake the test after only a five-week gap, there’s around a 50% chance that you will fall into a different personality category compared to the first time you took the test.
A second criticism is that the MBTI mistakenly assumes that personality falls into mutually exclusive categories. You are either an extrovert or an introvert, but never a mix of the two. Yet most people fall somewhere in the middle. If the MBTI also measured height, you would be classified as either tall or short, even though the majority of people are within a band of medium height.
The consequence is that the scores of two people labelled “introvert” and “extrovert” may be almost exactly the same, but they could be placed into different categories since they fall on either side of an imaginary dividing line.
One other thing, and this matters especially for anybody who thinks personality tests can guide them to a perfect career. According to official Myers-Briggs documents published by its exclusive European distributor, the test can “give you an insight into what kinds of work you might enjoy and be successful doing.” So if you are, like me, classified as INTJ (your dominant traits are being introverted, intuitive, and having a preference for thinking and judging), the best-fit occupations include management consultant, IT professional, and engineer.
Would a change to one of these careers make me more fulfilled? Unlikely, according to psychologist David Pittenger, because there is “no evidence to show a positive relation between MBTI type and success within an occupation … nor is there any data to suggest that specific types are more satisfied within specific occupations than are other types.” Pittenger advises “extreme caution in [the MBTI test’s] application as a counselling tool.” Then why is the MBTI so popular? Its success, he argues, is primarily due to “the beguiling nature of the horoscope-like summaries of personality and steady marketing.”
When I cite the avalanche of critical studies to career counsellors, coaches, and trainers who administer Myers-Briggs tests, they often point out that the test is not designed to match people to ideal careers. Yet many of them ignore the evidence and keep on handing them out, typically because they are still believers in it as a guide to personality types, but sometimes — I suspect — because it gives their advice a veneer of legitimacy.
Personality tests have their uses, even if they do not reveal any scientific truth about us. If we are in a state of confusion, they can be a great emotional comfort. They also raise interesting hypotheses that aid self-reflection: Until I took the MBTI, I had certainly never considered that IT could offer me a bright future (by the way, I apparently have the wrong personality type to be a writer).
MORE: The great expatriate hiring boom
Yet MBTI is not a magic pill that offers a secret path to a dream job. Wise career counsellors should treat such tests with caution, using them as only one of many ways of exploring who you are. Some even take the sensible step of avoiding them altogether, recognizing that human personality does not neatly fall into 16 or any other definitive number of categories: We are far more complex creatures than psychometric tests can ever reveal.
If we shouldn’t rely on personality tests, how can we find a fulfilling career? Let’s not go back to skull measuring. Instead, start with some useful advice that Aristotle offered over 2,000 years ago: “Where the needs of the world and your talents cross, there lies your vocation.”
Roman Krznaric is the author of How to Find Fulfilling Work, published by Picador, and is a faculty member of The School of Life.