The first group was given the same instructions as the participants in Guilford’s experiment.
The second group was told that the solution required the lines to be drawn outside the imaginary box bordering the dot array.
Most people assume that 60 percent to 90 percent of the group given the clue would solve the puzzle easily. What’s more, in statistical terms, this 5 percent improvement over the subjects of Guilford’s original study is insignificant.
In other words, the difference could easily be due to what statisticians call sampling error.
In the 1970s, however, very few were even aware of its existence, even though it had been around for almost a century.
At the first stages, all the participants in Guilford’s original study censored their own thinking by limiting the possible solutions to those within the imaginary square (even those who eventually solved the puzzle).
Even though they weren’t instructed to restrain themselves from considering such a solution, they were unable to “see” the white space beyond the square’s boundaries.
Only 20 percent managed to break out of the illusory confinement and continue their lines in the white space surrounding the dots.
The symmetry, the beautiful simplicity of the solution, and the fact that 80 percent of the participants were effectively blinded by the boundaries of the square led Guilford and the readers of his books to leap to the sweeping conclusion that creativity requires you to go outside the box.
The idea went viral (via 1970s-era media and word of mouth, of course).