Back in my palm-deciphering period, I got unmistakable indications that something was amiss with paranormal methods for characterising people. Curious about my palmistry successes, I put elements of the system to the test, sometimes reading someone’s heart line as if it were the headline — that sort of thing.

None of my alterations of tightly specified practices made any difference to my level of success. For instance, whether I followed or violated the proper procedure for uncovering “the presence of a secret area of self-doubt” within my subjects, they typically responded with the same guilty nod.

On one particular evening, I was feeling out of place at a house party where I knew almost no one. Because interacting socially with strangers is one of my secret areas of self-doubt, I began doing palmistry as a way to fit in. I even read the home owner’s palm twice, once at the beginning of the night and once when he returned a couple of hours and several drinks later, wanting to know more. In the middle of the first reading, I’d bent back his thumb and said, “You know, I can tell that you are quite a stubborn man.” During the second reading, when bending back his thumb, I said, “You know, I can tell that you are quite a flexible man.” After each of the opposing depictions, he thought for a second and admitted that I was absolutely right about who he really was.

What was going on? How could my readings be viewed as accurate no matter what (within reason) I claimed to see? Critics of the paranormal offer a standard explanation: palmists or astrologers or phrenologists (head bump readers) describe characteristics so widespread — stubbornness and flexibility, for example — that almost everyone can identify with them. This point is surely true, but it doesn’t resolve the whole mystery. If it’s so easy for people to spot their own tendencies for both stubbornness and flexibility, shouldn’t these opposites cancel themselves out upon quick reflection? When I labelled the home owner at that party a stubborn man, why didn’t he counter me then and there with the natural self-awareness of his flexibility? Why did he see the truth solely in the trait I suggested, when I suggested it?

The answer has to do with a common operating tendency that can alter a person’s decisions dramatically. Suppose at a party I bent back your thumb slightly and, on the basis of its resistance and curvature, proclaimed you “quite a stubborn individual, someone who resists being pressured in a direction you don’t want to go.” I will have focused you on the trait of stubbornness, sending you down a single psychological chute constructed unfairly to confirm my judgement.

Here’s how it would work: to test if I were right, you’d automatically begin searching your memory for times when you’d acted stubbornly — only for those times — and you’d almost certainly come upon a ready instance, as mulishness under pressure is a frequent personal failing. If you extended this biased search further, you’d hit on other, similar occurrences. With a blink of self-recognition, you’d likely up at me and admit that I was on target.

Now imagine instead that I’d labeled you “quite a flexible individual, someone who, after getting new information, is willing to take it into account and adjust your position.” I’d have focused you oppositely this time, sending you down a different chute: one rigged to ensure that you’d find occasions in your past when you embraced change. As a result, you’d be likely to look up from that equally biased memory search and declare me absolutely right about your fundamental flexibility.

There’s a very human reason for why you’d be prone to fall for my trick. Its obtuse scientific name is “positive test strategy.” But it comes down to this: in deciding whether a possibility is correct, people typically look for hits rather than misses, for confirmations of the idea rather than for disconfirmations. It is easier to register the presence of something than its absence. The great mystery novelist Sir Arthur Conan Doyle understood this tendency in crafting the anything but ordinary thinking style of Sherlock Holmes. The brilliant Holmes was as unrelenting in his attention to what didn’t occur as to what did. Recall that in one of Doyle’s most popular mystery stories, 'Silver Blaze.' Holmes realizes that a theft under investigation is an inside job (and could not have been committed by the stranger police had under arrest) because during the crime a guard dog didn’t bark. His less intellectually disciplined counterparts, content to rely mainly on the presence rather than the absence of confirming evidence, never match his powers of deduction.

Regrettably you, I, and most everyone else fall into the sub-Holmesian category in this regard. In a song by Jimmy Buffett, a former lover has to be informed — five separate times! — that the lack of something can convey the telling presence of something: “If the phone doesn’t ring, it’s me.”