Only 20 percent managed to break out of the illusory confinement and continue their lines in the white space surrounding the dots.
In the 1970s, however, very few were even aware of its existence, even though it had been around for almost a century.
The second group was told that the solution required the lines to be drawn outside the imaginary box bordering the dot array.
In other words, the “trick” was revealed in advance.
Indeed, the concept enjoyed such strong popularity and intuitive appeal that no one bothered to check the facts.
No one, that is, before two different research teams—Clarke Burnham with Kenneth Davis, and Joseph Alba with Robert Weisberg—ran another experiment using the same puzzle but a different research procedure.
The idea went viral (via 1970s-era media and word of mouth, of course).