from Nick Stockton at wired.com:
You have finally finished writing your article. You've sweat over your choice of words and agonized about the best way to arrange them to effectively get your point across. You comb for errors, and by the time you publish you are absolutely certain that not a single typo survived. But, the first thing your readers notice isn't your carefully crafted message, it's the misspelled word in the fourth sentence.
The reason typos get through isn't because we're stupid or careless, it's because what we're doing is actually very smart, explains psychologist Tom Stafford, who studies typos of the University of Sheffield in the UK. "When you're writing, you're trying to convey meaning. It's a very high level task," he said.
As with all high level tasks, your brain generalizes simple, component parts (like turning letters into words and words into sentences) so it can focus on more complex tasks (like combining sentences into complex ideas). "We don't catch every detail, we're not like computers or NSA databases," said Stafford. "Rather, we take in sensory information and combine it with what we expect, and we extract meaning." When we're reading other peoples' work, this helps us arrive at meaning faster by using less brain power. When we're proof reading our own work, we know the meaning we want to convey. Because we expect that meaning to be there, it's easier for us to miss when parts (or all) of it are absent. The reason we don't see our own typos is because what we see on the screen is competing with the version that exists in our heads.
Generalization is the hallmark of all higher-level brain functions. It's similar to how our brains build maps of familiar places, compiling the sights, smells, and feel of a route. That mental map frees your brain up to think about other things. Sometimes this works against you, like when you accidentally drive to work on your way to a barbecue, because the route to your friend's house includes a section of your daily commute. We can become blind to details because our brain is operating on instinct. By the time you proof read your own work, your brain already knows the destination.
This explains why your readers are more likely to pick up on your errors. Even if you are using words and concepts that they are also familiar with, their brains are on this journey for the first time, so they are paying more attention to the details along the way and not anticipating the final destination.
The entire post can be read here.