Sometimes what feels like a gut feeling is actually a mental glitch. The reason: We were built for life on the African savanna, not for the urbanized, industrialized, mechanized world most of us inhabit. "Our brains run on what amounts to a 100,000-year-old operating system," says Dean Buonomano, PhD, a professor of neurobiology and psychology at UCLA and the author of Brain Bugs: How the Brain's Flaws Shape Our Lives.

The result? Our intellectual equipment has its flaws. For instance, we're more afraid of being killed by strangers (rough odds: one in 100,000) than by cars (one in 10,000) because our instinctive fears haven't caught up with the dangers of the 21st century. Another example: If you could have $100 right now or $120 a month from now, which would you choose? Most people select the immediate cash because our ancient impulses steer us toward short-term gratification over long-term benefit.

An additional blind spot arises from the nature of our memory, which can cause us to confuse related concepts. Try this exercise, adapted from Brain Bugs:

Answer the first two questions below aloud, and then blurt out the first thing that pops into your mind in response to sentence 3:
  1. What continent is Kenya in?
  2. What are the two opposing colors in the game of chess?
  3. Name any animal.
About 50 percent of people answer sentence 3 with an animal from Africa, and roughly 20 percent say "zebra." But when asked to name an animal out of the blue, less than 1 percent of people answer "zebra." The skewed responses are due to an unconscious phenomenon known as priming, in which thoughts of one concept spread to related concepts, making them more likely to be recalled.

When an unexpected answer or impulse seems to burble up inside, it could be a great hunch—or it could be one of these mental glitches. By knowing the brain bugs most likely to trip you up, you'll be better equipped to tune them out.

Annie Murphy Paul's latest book is Origins: How the Nine Months Before Birth Shape the Rest of Our Lives (Free Press).

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From the August 2011 issue of O, the Oprah Magazine.