Cards bearing one of five patterns (circle, square, star, wavy lines, triangle) on one face (unmarked on the other), originating from Karl Zener's work at the parapsychology laboratory of Duke University investigating extrasensory perception (ESP) and transmission.

The premise behind the use of these cards is that the sender looks at the marked face of the card and mentally projects the image at the receiver, who makes a note of what image he or she thinks they're picking up at any given moment. Since you have a 20% chance of guessing by random the pattern on any given card, the average "hit rate" (successful reception of image) also hovers about that level when the sender, receiver or both are garden variety non-psychic mundanes such as you or I.

But those who believe themselves possessive of strange mental acuity will claim that they can regularly defy such statistics and will insist you try them out with their own set of cards an in the same room as them, two circumstances which are intolerable to the rare scientific-minded parapsychologist. Ideally the experiment should be performed with the sender and reciever in different rooms or even in different buildings.

As with any random guessing game, if the test is run on the same person enough times eventually they will end up with a particular sequence with a much higher hit rate than their usual performance. Unless it is maintained in subsequent experiments, this is only to be considered numbers being wacky, and does not indicate a momentary spike in psychic abilities in either the sender or receiver.

One other thing used by psychic claimants to refute results on this test to the contrary is to suggest that their psychic abilities are more predictive than penetrative - "Look here, I guessed the next card here instead of the card it actually was! And here I did it three times in a row!" When you double your odds of guessing the right card from 20% to 40%, anyone can pull off the appearance of psychicity 8)

Zener cards are perhaps best well-known to the general public as seen being (mis-)used by Dr. Peter Venkman in the beginning of the movie Ghostbusters, as he uses electroshock aversion therapy to attempt to provoke latent ESP abilities from a twitchy bighair and to provoke a date from a hottie by pretending that she's guessing the cards correctly:

    Corridor outside Paranormal Studies Laboratory
    We hear DR. PETER VENKMAN giving the ESP test. On door: "Dr. Egon Spengler Dr. Raymond Stantz Dr. Peter Venkman Venkman burn in hell Maid please make up this room as soon as possible"

    Inside lab
    PETER gives ESP test to MALE STUDENT and JENNIFER. MALE STUDENT is hooked up to electrodes.

    PETER
    All right, I'm going to turn over the next card. I want you to concentrate. I want you to tell me what it is.
    card is a star

    MALE STUDENT
    Square?

    PETER
    showing him card and shocking him
    Good guess, but wrong.
    holds a circle card up in front of JENNIFER
    Clear your head. All right, tell me what you think it is.

    JENNIFER
    Is it a star?

    PETER
    It is a star! Very good. That's great.
    holds up a square card for MALE STUDENT
    All right. Think hard. What is it?

    MALE STUDENT
    Circle?

    PETER
    showing him square and shocking him
    Ooh, close, but definitely wrong.
    MALE STUDENT's gum shoots out of his mouth. He puts it back in and keeps chewing. PETER holds a plus sign card up for JENNIFER.
    Okay. All right. Ready? What is it?
    no answer
    Come on.

    JENNIFER
    Figure eight.

    PETER
    pretending to be astonished
    Incredible. That's five for five. You can't see these, can you?

    JENNIFER
    No, no.

    PETER
    You're not cheating me, are you?

    JENNIFER
    No, I swear, they're just coming to me.

    PETER
    to MALE STUDENT
    Okay. Nervous?

    MALE STUDENT
    Yes... I don't like this.

    PETER
    Don't worry, you only have seventy-five more to go.
    holds up a card with three wavy lines
    Okay, what's this one?

    MALE STUDENT
    A couple of wavy lines.

    PETER
    who wants to zap him just for fun
    Sorry! This isn't your lucky day!

    MALE STUDENT
    I know. I -
    PETER reaches for the little lever. JENNIFER seems amused, so PETER winks to her. MALE STUDENT stumbles over some words before PETER zaps him.
    Hey! I'm getting a little tired of this!

    PETER
    You volunteered, didn't you? We're paying you, aren't we?

    MALE STUDENT
    Yeah, but I didn't know you were giving me electric shocks! What are you trying to prove here anyway?

    PETER
    I'm studying the effect of negative reinforcement on ESP ability.

    MALE STUDENT
    The effect?! I'll tell you what the effect is! It's pissing me off!

    PETER
    Well, then maybe my theory is correct!

    MALE STUDENT
    ripping electrodes off hands
    You can keep the five bucks, I've had it!
    runs out of room and slams door

    PETER
    I will mister!
    kindly, to JENNIFER
    You may as well get used to that, that's the kind of resentment that your ability is going to provoke in some people.

    JENNIFER
    Do you think I have it, Dr. Venkman?

    PETER
    You're no fluke, Jennifer.

Zener cards are one of the more across-the-board and easily-identified examples of paranormal paraphernalia, and as such tend to pop up in a number of places independant of the plot, merely to establish the mood:
    Outside the Prisoner's cottage, the birds are singing. Inside, on the Prisoner's kitchen table, is a large heap of cards marked with the traditional ESP symbols: the circle, the cross, the square, the star, and the three wavy lines. Next to the heap is a small pile of cards that are face-down.

    Prisoner:

      Now.

      (After a second, a young woman -- Alison, Number 24 -- replies in a confident voice.)

    Alison:
      ... A circle.
    Prisoner:
      Right.

      (He throws the card he is holding onto the heap. It is indeed marked with a circle. He picks up the next card from the pile and looks first at it, then at Alison, who we see is sitting in a meditative pose facing away from the Prisoner on the edge of the coffee table in the living room. She has her index fingers steepled against her forehead.)

    Prisoner:
      Now.
    Alison:
      A... star.
    Prisoner:
      Right.

      (He picks up the next card. It has a square on it. Alison turns to look at him.)

    Alison:Prisoner:
      What for?
    Alison:Prisoner:
      Now.

      (She resumes her pose.)

    Alison:
      ... A square, no, a cross... no, a square. Definitely.
    Prisoner:
      Are you sure it's a square?
    Alison:
      Yes!

      (The Prisoner puts the card down, impressed.)

    Alison:
      I don't know what I'd have done without your help; nobody else believed in me.
    Prisoner:
      They have no imagination.

      (He picks up the next card. It's marked with a cross like an X.)

    Prisoner:Alison:
      There's still a month.

      (He looks at the card, then at her.)

    Prisoner:
      NOW!

      (Again she resumes her pose.)

    Alison:
      ... A cross.
    Prisoner:
      Right... Now?
    Alison:
      ... Three wavy lines.
    Prisoner:
      That's the lot.

    - The Prisoner, The Schizoid Man.

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