In a course on witness interviewing, the instructor opened with a demonstration. He drew a black vertical line in the middle of a dry erase board. Under it, he began to draw a horizontal line. Pausing, he asked the class to let him know when the horizontal line was equal in length to the vertical line. He continued drawing and the class watched, mentally measuring the lines.
One person said the lines were equal. Then a few more. Then a majority of the class agreed the lines were the same length.
The instructor measured the lines with a ruler. The horizontal line was now actually much longer! He measured again, correcting the lines to the same length. The illusion remained. The vertical line appeared longer, even though we “knew” the lengths were identical.
Why the line demonstration? He took a room full of people who knew it all, who’d conducted thousands of interviews, whose business it was to judge perceptions…and showed us just how susceptible we were to illusion.
We’d walked into the classroom with our individual buckets, full and bursting with knowledge and experience. With a single drawing exercise, he’d shown us a bigger bucket. He’d opened our minds to learn.