In the house of alleged Cleveland kidnapper Ariel Castro, multiple padlocks kept interior doors shut, windows were nailed shut, and guests weren't allowed upstairs or in the basement.

In retrospect, some question why visitors didn't notice that something was amiss over the 10 years that three women were allegedly held captive there before their recent escape. But psychologists say few people would: We are so focused on the task on hand that it's easy to miss aberrant details.

In fact, psychologists Christopher Chabris and Daniel Simons found that half of people watching a video failed to notice a man dressed in a gorilla suit walking through a basketball game when they were told to count the number of passes among team members. Still, we think we notice things.

NEWS: How Our Brains Warp the World

"We have a bias to believe that we will notice all the important things," said Chabris, a professor at Union College and co-author with Simons of "The Invisible Gorilla".

In another experiment, Chabris and Simons staged a fake fight along a path and had subjects run past while chasing someone.

“Many did not notice [the fight],” Chabris said. “The thing you think is salient -- people fighting -- should grab your attention, but if you’re chasing a suspect it’s shockingly easy to miss things that in retrospect seem obvious.”

So, padlocks on doors in a grimy house? It's entirely plausible that a neighbor going to borrow an egg, for instance, wouldn't see them.

"You're not going over there with the task of looking for something unusual," Chabris said.

Our brains form fast, general categorizations of our surroundings, said Barbara Tversky, a psychology professor at Teachers College, Columbia University.

Person in gorilla suitiStockphoto

"The world is terribly confusing; there's too much happening at the same time -- visually, auditorily, everything -- and the way we cope is by categorizing," she said. "We process the minimum we need in order to behave properly. Things that are aberrant blend in."

Plus, humans are able to focus inwardly while performing outward tasks, such as walking to the grocery store.

NEWS: Why Our Self-Perception Is So Off

"We can walk down the street and think about a grocery list," she said. "Our minds are interested in more interesting things than just walking around the world. We aren't aware of how much we're missing."

Our minds tend to play another trick with 20/20 hindsight, Chabris said. We may think certain signs were clues now, but cluttered, dirty houses, and even heavily locked doors, don't necessarily mean that someone is breaking the law.

"There's a line between being a kidnapper and being eccentric," Chabris said. "Whether those things are really signs that a heinous 10-year kidnapping is underway ... of all the people in the world with lots of locks on doors or grimy, unkempt interiors or even cardboard tents in their living rooms -- or all three -- how many are kidnappers or serial rapists?"

Of course, close friends and family might notice unusual behavior more than casual acquaintances.

"They might see changes in his behavior (adding more padlocks?) that others don't," Chabris said. "But the competing hypotheses -- 'he is eccentric' and 'he has people imprisoned behind there' -- are still much different in likelihood. So even someone who knows him well would be unlikely to entertain the latter. It's just so rare, and the alternative (being eccentric) so much more common."

Combined with the way our brains process memory, it's enough to make psychologists question the importance placed on eyewitness testimony.

"Add it all together and it is a problem," Chabris said. "As a cognitive psychologist who studies it, I wish there were a way to get more of that knowledge in the legal system."