MORE than 10 years ago, someone in a gorilla suit wandered through a group of people tossing basketballs to each other.
It was caught on film and shown to a group of Harvard University students. When asked about the gorilla, half of them said they didn't see any gorilla.
The reason they didn't see it was because before they watched the video they were asked to count how many times the people wearing white shirts were about to pass the ball.
Now known as "The Invisible Gorilla" experiment, the study highlighted the fact that people miss a lot of what goes on around them.
Not surprisingly, the Invisible Gorilla became an internet star, but it also became a landmark study in psychology and even made an appearance on CSI.
Now it has a sequel - The Monkey Business Illusion.
SPOILER ALERT: Watch the video before reading on
It's basically the same experiment, except this time it takes into account that fact that some viewers will know what they're signing up for.
It didn't help them, because as the gorilla saunters onto the screen, the curtain changes colour and one of the players leaves - and again, most viewers completely missed it.
Of those who knew a gorilla was coming, only 17 per cent saw at least one other change.
Of those that didn't know a gorilla was coming, 29 per cent saw at least one of the other changes.
One of the men behind the experiments, Professor Dan Simons of the University of Illinois, said he posed the new scenario because "it would be fun to see if I could get away with other changes".
"If anything, there is a trend for people who know about the gorilla to be more likely to miss the other event," he said.
"What's interesting is that most people firmly believe that as long as we're looking at the world, we'll notice anything important that happens.
"In part, we have that mistaken intuition because we're only aware of the unexpected events that we do notice and not the ones we never saw."
Prof Simons said the second Invisible Gorilla exercise showed that people can't train themselves to be more aware of what's happening in their field of vision, but they could at least learn their limitations.
"If you know that you have these limits on attention and awareness, you can take steps to avoid the consequences the might have for you," he said.
"For example, you can avoid talking on the phone while driving because having a cell phone conversation further reduces your ability to notice unexpected events.
"Or you might watch your kids more carefully at the swimming pool because you'll recognise that, despite their best efforts, lifeguards won't automatically notice if a child is in trouble."
Prof Simons said strong reactions to the video ranged from disbelief to anger and accusations that the team switched the tape when replaying it to show the gorilla.
"It's hard to rationalise missing the gorilla except by coming to grips with the limitations of awareness," he said.
Together with his colleague, Christopher Chabris, Prof Simons has released a book about their 10 years of research.
Called The Invisible Gorilla, it details the experiment and how it might explain such things as why companies spend billions on product they knows will fail and why award-winning movies are full of editing mistakes.
Prof Simons says the book is "more than a catalogue of human failings".
"We also explain why people succumb to these everyday illusions and what we can do to inoculate ourselves against their effects."
And for the record, those that assumed it was a man in the gorilla suit were also wrong.
In the original video, it was a woman.
"Almost everyone assumes that it's a man in the suit," Prof Simons said.
"I'm not sure why. Many professional mascots are actually women as well, but people seem to assume that mascots must be male. Odd."
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