Long ago, on the forecourt of the Temple of Apollo in Delphi, the Seven Sages were said to have carved two words into a stone: Know Thyself. The words, Socrates tells us, were to be construed as both a greeting and an admonition to worshipers from the Greek god of music, poetry and healing: Know thyself—know your truthful soul, your imperfect and human self—before entering this hallowed ground.

And this ancient and concise nugget is no less wise and important today, says organizational psychologist Tasha Eurich. “There is strong scientific evidence that people who know themselves and how others see them are happier. They make smarter decisions. They have better personal and professional relationships. They raise more mature children…They’re more effective leaders with more enthusiastic employees. They even lead more profitable companies.” Or so argues Eurich in her new book Insight: Why We’re Not as Self-Aware as We Think, and How Seeing Ourselves Clearly Helps Us Succeed at Work and in Life, which is due out in May.

The lengthy subtitle—and the ample list of benefits self-awareness supposedly brings with it—might suggest a facile self-help book, a Stuart Smalley mirror exercise in auto appraisal. But Eurich has clearly thought and dug deeply into the subject. The result is both a sprawling exploration of the psychic frailty that leads to self-delusion and self-aggrandizement, and—importantly—a compassionate, helpful guide for avoiding that path (or reversing it). She’s also pretty good at grounding all of this in academic research and in real-world examples from everywhere from the business realm to the battlefield.

While the notion of self-awareness as a route to self-improvement can be traced as far back as 600 B.C., she says, it has been only in the past four decades or so that psychologists and others have truly studied it—and tried to understand it on a scientific basis. And that careful study, Eurich says, has yielded some surprises.

One is that there is no apparent relationship between having what she calls internal self-awareness (a clear “understanding of your values, passions, aspirations, ideal environment, patterns, reactions, and impact on others”) and external self-awareness (“knowing how other people see you.”) It’s a lot more common than we might think for individuals to genuinely know themselves and have no clue as to how their colleagues see them—and vice-versa.

A second surprise is that we seem to get less self-aware as we get older. “Experienced leaders are more likely to overestimate their abilities,” writes Eurich, “Similarly, older managers tend to misjudge their performance relative to their boss’s ratings of them far more than their younger peers do.”

The good news is that we can learn to identify our blind spots and teach ourselves how to be more self-aware. The bad news is, the most effective tools may come with a bit of pain. Eurich describes a feedback process she calls the “Dinner of Truth.” (And, yes, the name is appropriately ominous.) The gist is that you invite someone to dinner and ask him or her “to share the one thing that annoys them most about you.”

Why over dinner? “Eating is intimate,” she explains. And once you’ve sat down at the table, it’s hard to run away and hide after 60 seconds.