Identical Twins by Julie Crvens/Pinterest
Source: Identical Twins by Julie Crvens/Pinterest

Introduction—Matters of Definition

Besides my title, consider these kindred expressions: “That’s the pot calling the kettle black”; “I know you are, but what am I?”; “Look who’s talking!” or You should talk!”—or even, by extension, “It takes a thief to catch a thief.”

Let’s start by looking at the meanings attributed to this enduring proverb, first noted as far back as 1665 (

The saying “It takes one to know one” has been defined in several complementary ways. But what unites all these definitions is that, since its beginnings, the expression has been used in a disparaging, derogatory way—as in, “You think I’m dishonest?! Well, it takes one to know one.”

The phrase, employed as a stinging retort, is really a counter-accusation. Almost always it’s implemented to insult the (presumed) insulter. And what it clearly implies is that the verbally attacking individual is only able to recognize—and be annoyed by—the quality being criticized because they themselves embody this fault.

The Proverb as Illuminating the Phenomenon of Projection

Doubtless, of all proverbs this has to be one of the most cynical. And more than anything else, what it speaks to is the psychoanalytic theory of projection. That is, a pivotal—and almost universal—defense is to deny in ourselves qualities we perceive negatively, while transferring to others these disliked or rejected aspects of the self. That way, personally unacceptable attributes, impulses, or tendencies remain unconscious, helping us avoid disturbing feelings of anxiety or shame.

So whenever someone tells us disapprovingly that we possess some unfavorable quality, we’re compelled to react instantly by “assigning” them this same label. And doing so shields us from having to look inwards, confront ourselves and admit the possible veracity of such criticism, and so escape any self-disapproval.

What’s especially fascinating about this saying (and which commentators don’t seem to notice) is that it cuts two ways. It assumes the accuser is projecting their own shortcomings onto the other, as well as the accused projecting back on the accuser what perhaps both—through projection—are choosing to deny.

We all have what’s been deemed a “shadow side” (e.g., see my “Just How Dark Is Your Dark Side?”) and, frankly, it takes a fair amount of positive self-regard to concede this to ourselves and others. Lacking such broad self-acceptance, we’re apt to defensively, and vigorously, hurl back this psychological “hot potato” as soon as it's landed on us. It’s roughly equivalent to: “You think you’re okay but I’m not? Well, I’m okay—it’s you who's not!” Which makes this all sound rather like “child’s play”—as in, Pee-Wee Herman’s smirkingly infantile: “I know you are but what am I?”

An Attempt to Verify the Truth of “It Takes One . . . “

So, might there be any research studies seeking to test the validity of the proverbial claim: “It takes one to know one”?

The only relevant experiment I could find is one that focuses on lying and marshals evidence to support the hypothesis that liars are more skilled at detecting falsehoods than are more honest individuals. Called “Detecting Deception: The Scope and Limits,” by Kamila E. Sip & others (Trends in Cognitive Sciences, 12, 2008, 48-53), this investigation is outlined in a Scientific American piece by Travis Riddle. Entitled “Liars: It Takes One to Know One” (July 24, 2012), it scrutinizes the above research, carried out at the University of London and the University College London.

This social-psychological study cleverly manipulated variables in a game the researchers designated “The Deceptive Interaction Task.” Without going into the intricate details of the experiment, its conclusions offer support for the proverb in that its results suggest that the best human lie detectors are themselves good liars (!). As regards the real-life implications of this study, Riddle remarks:

This is an important demonstration of a phenomenon with which our culture is justifiably fascinated. Lying, whether from a politician, an athlete, a poker player, or a frog [strategically simulating the croak of a much larger frog!] is an important determinant of who wins and loses. Elections, court cases, card games . . . all rely on lying and lie detection abilities.

The Psychological and Ethical Dilemmas in “It Takes One . . . ”

This essentially sarcastic adage implies a certain nihilistic solipsism. For the metaphysical concept of solipsism postulates that it’s impossible to really know anything beyond one’s consciousness: that what we sense about others is only a product of our imagination and has no existence outside it. Therefore, any presumed knowledge external to one’s own mind must at best be regarded as tentative—or hypothetical.

Twins by D. C. Atty/Flickr
Source: Twins by D. C. Atty/Flickr

The tragic implication of this belief is that if you can only comprehend the thoughts and behaviors of another by somehow becoming a literal “mirror” of them (think identical—really, really identical—twins), you’re prohibited from vicariously entering anyone else’s world. And that implies that we’re all isolated, living on our own private island and hopelessly separated from others. Consider this famous line from Joseph Conrad’s Heart of Darkness: “We live, as we dream—alone.”

Moreover, unless we totally dismiss all the evidence supporting the existence of human empathy (for starters, reflect on mirror neurons), then we hardly have to be that similar to another to appreciate, or be able to intuit, what they’re thinking and feeling.

As a psychologist, I recall in particular a time when, attending closely to a client’s rendering of an acutely painful experience, I noticed some unusual sensations rumbling around inside me. When I then asked the client whether he could feel in his body what he was presenting verbally—describing in detail what I myself had been experiencing—he looked at me incredulously, as though through some dark magic I’d managed to get inside him. With much skepticism (he was a fairly conservative M.D.!), he asked how in the world I could so precisely identify his sensations. And I replied that in simply picturing myself going through what he’d vividly characterized, those were just the sensations I myself felt.

The main point here is that if our empathy is reasonably developed, we don’t need to be the same, or even that similar, to the person empathized with. Through our evolved “fellow feeling,” we can identify their emotions, actions, or motives without being identical to them.

An experiment delineated in the journal Emotion (3, 2003,194-200) confirmed the hypothesis of authors D. R. Carney and J. A. Harrigan that [and this is simply the paper’s title] “It Takes One to Know One: Interpersonal Sensitivity Is Related to Accurate Assessments of Others’ Interpersonal Sensitivity.” And such sensitivity is generally understood as empathy, for both descriptors depict “the ability to accurately assess others’ abilities, states, and traits from nonverbal cues.”

Which gives a new, and much less cynical, definition to the expression: “It takes one to know one.” Obviously, if the other person is particularly insensitive, this ability will be less, since it’s harder to imagine what’s going on inside someone’s head when the way that person thinks is substantially differently from how the other individual does.

Finally, it’s worth looking at a piece by Michael Hurd, Ph.D., published in Capitalism Magazine (Sept. 20, 2004). It’s entitled “It Takes One to Know One—Or Does It?” and it criticizes the usefulness of this saying in its unfortunately combining a true concept with a false one. To Hurd, the problem is that the true part of the statement inclines us to accept the whole statement, which he (and I, too) regard as exaggerated or distorted.

Here the true cynical part is the projection aspect of the saying. This all-too-common propensity to deflect one’s faults onto another is anything but admirable. And Hurd testifies that his own work as therapist is replete with such examples—as in cheating husbands’ guilt-reducing inclination to see their wives as also being deceitful.

Still, the false part of this rather derisive, rationalizing aphorism is that merely because someone recognizes a negative quality in another doesn’t mean they themselves possess it. Honest people, for instance, are quite capable of identifying another’s dishonesty. So it’s critical that this expression not be taken at its word. Hurd is, however, exceptionally harsh in his conclusion:

The old saying, “It takes one to know one” is half true, half false—putting it into the category of a vicious falsehood, a package deal worse than a garden variety falsehood. Half a truth is actually worse than no truth at all.

Although I wouldn’t go anywhere as far as Hurd in his blanket condemnation of this expression, I’d argue that most proverbs—even including the single most “distinguished” one, the Golden Rule—require some qualification if the profound truth they exemplify is to be made less vulnerable to criticism. But then an aphorism with modifications thereby ceases to be an aphorism (!).

So I myself would conclude that the proverb is well worth taking seriously . . . but with a few grains of salt.

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To check out other posts I’ve done for Psychology Today online—on a broad variety of psychological topics—click here.

© 2017 Leon F. Seltzer, Ph.D. All Rights Reserved.

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