Zener Cards

Karl Zener was a perceptual psychologist, and the deck he designed for J. B. Rhine at Duke in the early 1930s solves a measurement problem rather than expressing anything. Twenty five cards carry five symbols, five of each: a circle, a cross, three wavy lines, a square and a five pointed star.

Every constraint on that design is statistical or perceptual. The symbols must be quickly distinguishable from one another at a glance and unambiguous when named aloud, so that a wrong call is a wrong call and not a misunderstanding. They must be simple enough to be visualised. They must be free of emotional loading, so that no card is more attractive to guess than another. And they must be equally represented, so that the expected number of hits by chance in a run of twenty five is exactly five, which makes the arithmetic of any excess trivial to compute.

The procedures were formalised alongside the deck and given abbreviations that still appear in the literature: calling the cards as they are turned, calling the whole pack down before any are turned, calling a pack in a distant room, and versions where a sender looks at each card. The card as an object is a laboratory instrument. Its later career as a divinatory or novelty deck came afterwards and has nothing to do with what it was made for.

The criticisms began almost at once and most of them were about the cards rather than about the statistics. Early decks were printed on stock thin enough that the symbol could sometimes be made out from the back under good light, and the pressure of the printing could emboss the reverse. Warped or marked edges let a card be identified once seen. A subject facing an experimenter who knew the card could pick up unintended cues from face or voice. Shuffling by hand does not produce the randomness the arithmetic assumes. And a run that is stopped when the results look good, or reported only when they do, will show an effect whatever is happening.

Rhine and his colleagues answered a good part of this: opaque backs, screens between subject and experimenter, separate rooms, mechanical shuffling, predetermined run lengths, independent recording. The pattern that emerged is the one usually cited against the programme, which is that reported effect sizes tended to fall as the controls tightened, and that independent laboratories did not reproduce the striking early results.

Parapsychology largely moved on from card guessing to other designs in the decades that followed. The deck survived as a cultural object instead, familiar from films, television and games to a great many people who could not name the man who made it or say what the fifth symbol is.