Because the solution is, in hindsight, deceptively simple, clients tended to admit they should have thought of it themselves.
Guilford was one of the first academic researchers who dared to conduct a study of creativity.
In the 1970s, however, very few were even aware of its existence, even though it had been around for almost a century.
If you have tried solving this puzzle, you can confirm that your first attempts usually involve sketching lines inside the imaginary square.
The correct solution, however, requires you to draw lines that extend beyond the area defined by the dots.
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).