three–seven effect n.
A phenomenon, based on population stereotypes, that is often used by stage conjurors pretending to project their thoughts telepathically to their audiences. The performer says: ‘I’m going to think of a two-digit number between one and fifty. I’ll make both digits odd but not the same, so fifteen would be OK, but eleven wouldn’t because the two odd digits are the same … [pause]. Now how many of you got thirty-seven?’ It usually turns out that more than one-third of the audience have thought of the number 37, partly because 3 and 7 are population stereotypes and partly because, surprisingly, the performer’s instructions leave only seven possible two-digit numbers, apart from 15, which is effectively eliminated by being used as an example. It is often used in phoney demonstrations of telepathy.