In 1869 British scientist Sir Francis Galton compared people based on their awards and accomplishments for research that led him to research individual reaction time and specificity of the senses. These tests have since been proven to correlate with academic success. In 1904 Alfred Binet and Theodore Simon developed the first modern IQ test on behalf of the French government, which had asked for a test to determine if a child had a mental disability. The partners found that while sensory tests explained some things, tests that involved memory, problem solving, reasoning and vocabulary were more accurate at predicting academic success than Galton's test. Some of the tests, such as cognitive and memorization tests used by Binet and Simon, are still used in the 21st century.
Originally, IQ tests were used to test whether children were of lower intelligence than their peers or if they were just being lazy in school. Children would take the test and the scores would be compared to others of the same age group. If the child scored lower than their peers, they were considered not as intelligent and were placed in a special education program.
In the 21st century the tests are used to measure true mental potential, unbiased by culture, of adults rather than children. Tests are measured against other tests from the whole population and age range, instead of just being measured against a certain age. IQ tests still measure the mean, or average, using a standard deviation chart. A standard deviation chart has a bell curve, which means most people, the average, fall under one tightly packed space in the bell curve. The rest of the curve is dotted with outliers, those few who are more or less intelligent than the general population.
Intelligence Interval Cognitive Designation from IQtest.com:
40 - 54 Severely challenged
55 - 69 Challenged
70 - 84 Below average
85 - 114 Average
115 - 129 Above average
130 - 144 Gifted
145 - 159 Genius
160 - 175 Extraordinary genius
About 68 percent of test takers score in the average range. Only 2.3 percent of test takers score in the gifted and challenged range, and less than 1 percent fall into the genius and severely challenged range.