An intelligence quotient or IQ is a score derived from one of several standardized tests designed to assess intelligence. The abbreviation “IQ” comes from the German term Intelligenz-Quotient, originally coined by psychologist William Stern. When current IQ tests are developed, the median raw score of the norming sample is defined as IQ 100 and scores each standard deviation (SD) up or down are defined as 15 IQ points greater or less, although this was not always so historically. By this definition, approximately 95 percent of the population scores an IQ between 70 and 130, which is within two standard deviations of the median.
IQ scores have been shown to be associated with such factors as morbidity and mortality, parental social status, and, to a substantial degree, biological parental IQ. While the heritability of IQ has been investigated for nearly a century, there is still debate about the significance of heritability estimates and the mechanisms of inheritance.
IQ scores are used as predictors of educational achievement, special needs, job performance and income. They are also used to study IQ distributions in populations and the correlations between IQ and other variables. Raw scores on IQ tests for many populations have been rising at an average rate that scales to three IQ points per decade since the early 20th century, a phenomenon called the Flynn effect. Investigation of differing patterns of increases in IQ battery subtest scores informs current research on human intelligence.