Zener Cards are a popular tool for testing ESP abilities. A Zener Card pack consists of five different cards, duplicated five times for a total of 25 cards. The five cards are shown below: Circle, Cross, Wavy Lines, Square and Star.
Zener Cards were developed in the 1930s by psychologist Karl Zener, and used extensively by parapsychologist J. B. Rhine (1895-1980).
To conduct a basic Zener Card test, the experimenter shuffles the deck, chooses and discreetly observes a card at random, while the subject attempts to identify the card using only their mind.
In theory there should be a 20% chance of correctly identifying each card. A higher result than this, replicated over many tests, would indicate the presence of some form of ESP.
In reality, the test can be influenced by numerous factors. Most obvious problems such as cheating can be easily eliminated with strict controls and separation of the experimenter and subject. Other factors are less obvious, for example, subjects are more likely to choose the star card than other cards.
J. B. Rhine interpreted the results of his own testing as providing support for the existence of ESP. However other researchers have thus far been unable to replicate his results.