An intelligence quotient or IQ is a score derived from one of several different standardized tests attempting to measure intelligence. IQ tests are used as predictors of educational achievement. People with low IQ scores are sometimes placed in special-needs education, and people with high IQ scores are sometimes placed in gifted programs or enrichment programs. While one need not consider those below a certain IQ to be "subhuman" anymore than one need consider those above it to be "superhuman", such attitudes do exist, bringing discussions on IQ far outside the purely scientific scope. IQ scores are also used by social scientists; in particular, they study the distribution of IQ scores in populations and the relationships between IQ score and other variables. IQ correlates with job performance and income, also with the social status of the parents. Recent work has demonstrated links between IQ and both morbidity and mortality. While IQ heritability has been investigated for nearly a century, controversy remains as to how much is heritable, and the mechanisms for heritability are still a matter of some debate. The same study says that IQ of the same person may vary with age, and the heritable component becomes more significant with age. The average IQ scores for many populations were rising at an average rate of three points per decade during the 20th century with most of the increase in the lower half of the IQ range: a phenomenon called the Flynn effect. It is disputed whether these changes in scores reflect real changes in intellectual abilities, or merely methodological problems with past testing.