The Verifiability Approach: Counting the Details That Can Be Checked

Galit Nahari, Aldert Vrij and Ronald Fisher identified a dilemma that a liar cannot escape, and then built a technique that forces them to sit in it.

To be believed, an account must contain detail. Sparse accounts are unconvincing, and every interviewee knows this — richness of detail is one of the cues both laypeople and professionals rely on.

But detail is dangerous to a liar in proportion to how checkable it is. Naming a person who can confirm your account, citing a location covered by cameras, referring to a documented transaction, or specifying a time that a phone record would corroborate all create the possibility of being disproved.

So the liar has to supply detail that cannot be checked. And that is a measurable property of a text.

The verifiability approach counts details in two categories. Verifiable: named or identifiable witnesses, specific documented locations and times, activities recorded electronically, transactions, communications that leave a trace. Unverifiable: internal states, thoughts, feelings, activities performed alone, interactions with unnamed people, sensory impressions.

Truth-tellers, in the studies, produce more verifiable detail. Liars produce comparable or greater total detail with a much lower proportion of it verifiable.

The technique then adds a step that makes it substantially stronger and is the interesting part. The interviewee is told, explicitly, that the interviewer is interested in detail that can be checked, and that accounts containing checkable information are more likely to be believed.

This information helps a truth-teller and traps a liar. The honest interviewee simply complies, adding names and places. The liar is now in the dilemma consciously: supply checkable detail and risk refutation, or fail to comply with a stated criterion and appear evasive. Nahari's 2014 study found that this instruction increased the discriminative power of the measure — and that when liars attempted the countermeasure of adding verifiable detail, the added detail tended to be of a kind that did not survive checking.

What makes this approach different in kind from most of Part 4 is that it does not end in a credibility judgment. It ends in a list. The output of a verifiability-focused interview is a set of specific claims that someone can go and check, which converts a psychological assessment into an investigative task.

Effect sizes vary across studies and contexts, and replication has been mixed in some applied settings — this is a useful tool rather than a solved problem. But the underlying discipline transfers to any situation where you are assessing an account, and it requires no training: ask what in this story could be independently confirmed, and then notice how much of a rich, fluent, convincing narrative turns out to consist of things only the teller could know.