Intelligence: Knowns and Unknowns - Findings

Findings

"Intelligence: Knowns and Unknowns" stated that many different theories of intelligence have been proposed. Many questions were still unanswered. Most research had been done on psychometric testing which was also by far the most widely used in practical settings. Intelligence quotient (IQ) tests do correlate with one another and that the view that the general intelligence factor (g) is a statistical artifact is a minority one. IQ scores are fairly stable during development in the sense that while a child reasoning ability increases, the child relative ranking in comparison to that of other individuals of the same age is fairly stable during development. The report stated that IQ scores measure important skills as they correlate fairly well (0.5) with grades. This implied that the explained variance (given certain linear assumptions) is 25%. "Wherever it has been studied, children with high scores on tests of intelligence tend to learn more of what is taught in school than their lower-scoring peers. There may be styles of teaching and methods of instruction that will decrease or increase this correlation, but none that consistently eliminates it has yet been found." IQ scores also correlated with school achievement tests designed to measure knowledge of the curriculum. Other personal characteristics affecting this may be persistence, interest in school, and willingness to study which may be influenced by the degree of encouragement for academic achievement a child receives and more general cultural factors. Test scores were the best single predictor of an individual's years of education. They were somewhat more important than social class as measured by occupation/education of parents.

IQ scores were also correlated (0.3-0.5) with various measures of job performance such as supervisor ratings and work samples. The correlations were higher when the unreliability of such measures were controlled for. IQ scores were sometimes described as the "best available predictor" of job performance. Intelligence test scores did correlate significantly with social status and income later in life. They were somewhat less important for this than parental SES although the effects of parental SES and IQ were hard to separate. IQ rests had lower negative correlations with certain socially undesirable outcomes such as that children with high IQ were less likely to engage in juvenile crime. One example being a study finding a correlation of -0.19 (-0.17 with social class controlled for) between IQ scores and number of juvenile offenses in a large Danish sample. This implied that the explained variance (given certain linear assumptions) is less than 4% for these negative outcomes.

While both genetic and environmental variables were involved in the manifestation of intelligence, the role of genetics had been shown to increase in importance with age. In particular, the effect of the family environment shared by all children in a family, while important in early childhood, became quite small (zero in some studies) by late adolescence. Why this occurs is unclear. One possibility is that people with different genes tend to seek out different environments that reinforce the effects of those genes. Nonetheless, there were several important environmental factors which were known to affect IQ, such as having received very poor or interrupted schooling. However, regarding interventions such as the Head Start Program and similar programs lasting one or two years, while producing initial IQ gains, these had disappeared by the end of elementary school, although there may be other benefits such as more likely to finish high school. The more intensive Abecedarian Project had produced more long-lasting gains. The report stated that a number of biological factors, including malnutrition, exposure to toxic substances, and various prenatal and perinatal factors, resulted in lowered IQ under at least some conditions. The much-discussed "Flynn effect", which refers to the striking worldwide mean IQ increase over time, seemed too large to have simply reflected increased test sophistication. Possible explanations included improved nutrition and more complex environment. It was also unclear to what degree the IQ increase reflected real gain in intelligence.