Never underestimate a man who overestimates himself. It is the kind of line that gets shared as motivation, a wink at the underdog who believes his way into rooms he has no obvious business being in. Taken as a slogan it sounds like a celebration of pure bluster, which is easy to dismiss. But the aphorism is more interesting than that, because psychology has quite a lot to say about people who overestimate themselves, and the research lands in a genuinely double-sided place. There is a real and uncomfortable truth buried in the line, and there is also a trap in it, and the useful thing is to hold both at once.
Why you really should not underestimate him
The truth first, because it is the part people find hardest to accept. Overconfidence is not just tolerated socially; it is actively rewarded, often regardless of whether it is deserved. In a series of studies, Cameron Anderson, Don Moore and colleagues found that overconfident individuals attained higher social status in groups, because their confidence produced a behavioral signature, an air of calm certainty, that made them appear more competent to everyone around them, even when their actual ability was no greater.
This is why the aphorism has teeth. The man who overestimates himself frequently persuades others to overestimate him too. His certainty reads as competence, his willingness to speak first and doubt least gets taken as a sign that he knows something, and he ends up with the influence, the opportunities, and the benefit of the doubt that a more accurate but more hesitant person never receives. Underestimate him and you may find he has talked his way into the promotion, the deal, or the leadership of the very room where you assumed he would be exposed. Confidence is a social signal that other people read quickly and trust more than they should, which means self-belief, even unearned, is a genuine form of power. That is not an endorsement of bluffing; it is a description of how reliably it works on human audiences.
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Why you also should not overrate him
Now the other edge, which the motivational version conveniently leaves out. Confidence and competence are only loosely related, and the gap between them is widest exactly where you would least want it to be. In their well-known research, Justin Kruger and David Dunning showed that the least skilled people tend to overestimate themselves the most, because the same lack of knowledge that produces poor performance also robs them of the ability to see that it is poor, so those in the bottom quartile rated themselves as well above average.
This is the sharp caveat the slogan hides. The very fact that a man overestimates himself is, on average, weak evidence about his actual ability, and in some cases it is evidence pointing the wrong way, because deep incompetence often comes packaged with the serene confidence that nothing is wrong. So while you should not underestimate the social force of his self-belief, you should be very careful not to mistake that self-belief for proof that he is right. The two errors are different. Assuming a confident man will fail socially is naive, because confidence wins people over. Assuming a confident man must therefore be competent is equally naive, because confidence and competence come apart, and the loudest certainty sometimes sits on the thinnest foundation.
Holding both truths at once
The honest reading of the aphorism, then, is not the triumphant one printed on the poster. It is a warning to calibrate two separate judgments rather than collapsing them into one. Never underestimate what a man’s overconfidence will do to the people around him, because it will often carry him further than his ability alone would. But never confuse that momentum with a verdict on whether he is actually good, or right, or safe to follow, because the same trait that moves audiences is a notoriously poor guide to the truth. The people who get burned by overconfident men are usually the ones who could not tell these two things apart, either dismissing the bluster too early or swallowing it whole.
It is worth adding that a measured version of self-belief is genuinely valuable, and not everyone who backs himself is deluded. Confidence can be self-fulfilling, giving people the nerve to try things a more timid person never would, and some of that boldness pays off in ways caution never could. The distinction that matters is whether the confidence is doing the work of getting someone to act and learn, or merely the work of impressing a crowd while the substance stays hollow. So take the line seriously but not literally. Watch the overconfident man closely, respect what his certainty can accomplish, and reserve judgment on whether he has actually earned it until you have seen something beyond the certainty itself.