With the evidence we have today, the myth that we only use 10% of our brains sounds ridiculous. But back in the day, why did so many people believe it?
Nobody knows exactly how this myth started, but some psychologists believe Karl Lashley had something to do with it. In the 1920s and 30s, Lashley did many experiments on rats; he would make them learn a task with their brains intact, then he would remove large portions of their brains and make them redo the task (Washington). His results showed that the even without most of their brain, the rats could still accomplish the task. Therefore, Lashley concluded that most of the brain is unused and unnecessary. Another psychologist that contributed to this myth is William James. In the 1900s, James was discussing the brain and said, "The average person rarely achieves but a small portion of his or her potential" (How Stuff Works). From these men, the idea that people only use 10% of their brain was spread across the world.
In more recent years, psychics and major national companies have publicly announced their agreement with the myth that humans only use 10% of their brains. Psychics Caroline Myss, Uri Geller, and Michael Clark have all stated that everyone has the potential of psychic powers, but people don't know how to tap into it since they only use 10% of their brains (Snopes). In 1998, a U.S. Satellite Broadcasting ad showed a picture of a brain and the words, "You only use 11 percent of its potential" (Snopes). Because the ad was on television and from a seemingly reliable source, many people believed the myth.
However, it has been proven that we use more than 10% of our brains. Since there is no way to test the ability of psychic powers, psychologists consider this to be an extraordinary claim. In addition, with the availability of different types of brain imaging machines, it can be physically proven that humans use their whole brains (Snopes). Brain scans have shown that regardless of what people are doing, their brains are always active. An inactive part of the brain would only occur if it was brain damaged (How Things Work). These are solid ways to prove that humans use more than 10% of their brains.
http://health.howstuffworks.com/human-body/systems/nervous-system/10-brain-myths10.htm (How Stuff Works)