In 1905, psychologists  Alfred Binet and Théodore Simon designed a test for children  who were struggling in school in France. Designed to determine which children required individualized attention, their method formed  the basis of the IQ test. Beginning in the late 19th century, researchers hypothesized that cognitive abilities like verbal reasoning, working memory, and visual-spatial skills reflected an underlying  general intelligence, or g factor. Simon and Binet designed a battery of  tests to measure each of these abilities and combine the results  into a single score. Questions were adjusted  for each age group, and a child’s score reflected how they  performed relative to others their age. Dividing someone’s score by their age  and multiplying the result by 100 yielded the intelligence quotient, or IQ. Today, a score of 100 represents  the average of a sample population, with 68% of the population  scoring within 15 points of 100. Simon and Binet thought the skills  their test assessed would reflect general intelligence. But both then and now, there’s no single agreed upon  definition of general intelligence. And that left the door open  for people to use the test in service of their own preconceived  assumptions about intelligence. What started as a way to identify  those who needed academic help quickly became used to sort  people in other ways, often in service of deeply flawed  ideologies. One of the first large-scale  implementations occurred in the United States during WWI,  when the military used an IQ test to sort recruits and screen  them for officer training. At that time, many people  believed in eugenics, the idea that desirable  and undesirable genetic traits could and should be controlled  in humans through selective breeding. There were many problems  with this line of thinking, among them the idea that intelligence was not only fixed and inherited, but also linked to a person’s race. Under the influence of eugenics, scientists used the results  of the military initiative to make erroneous claims  that certain racial groups were intellectually superior to others. Without taking into account  that many of the recruits tested were new immigrants to the United States who lacked formal education  or English language exposure, they created an erroneous  intelligence hierarchy of ethnic groups. The intersection of eugenics and IQ testing influenced not only science, but policy as well. In 1924, the state of Virginia  created policy allowing for the forced sterilization  of people with low IQ scores— a decision the United States  Supreme Court upheld. In Nazi Germany, the government  authorized the murder of children based on low IQ. Following the Holocaust  and the Civil Rights Movement, the discriminatory uses of IQ tests were challenged on both  moral and scientific grounds. Scientists began to gather evidence  of environmental impacts on IQ. For example, as IQ tests were periodically recalibrated over the 20th century, new generations scored consistently higher on old tests than each previous generation. This phenomenon,  known as the Flynn Effect, happened much too fast to be caused  by inherited evolutionary traits. Instead, the cause was likely  environmental— improved education,  better healthcare, and better nutrition. In the mid-twentieth century, psychologists also attempted  to use IQ tests to evaluate things other than  general intelligence, particularly schizophrenia, depression, and other psychiatric conditions. These diagnoses relied in part on  the clinical judgment of the evaluators, and used a subset of the tests  used to determine IQ— a practice later research found does  not yield clinically useful information. Today, IQ tests employ many similar design elements and types of questions as the early tests, though we have better techniques for identifying potential bias in the test. They’re no longer used to diagnose psychiatric conditions. But a similarly problematic practice  using subtest scores is still sometimes used to diagnose learning disabilities, against the advice of many experts. Psychologists around the world  still use IQ tests to identify intellectual disability, and the results can be used  to determine appropriate educational support, job training, and assisted living. IQ test results have been used  to justify horrific policies and scientifically baseless ideologies. That doesn’t mean the test itself  is worthless— in fact, it does a good job of measuring the reasoning and problem-solving skills it sets out to. But that isn’t the same thing  as measuring a person’s potential. Though there are many complicated political, historical, scientific, and cultural issues wrapped up  in IQ testing, more and more researchers  agree on this point, and reject the notion that individuals can be categorized by a single numerical score.