Wael Taji writes: “The connection between intelligence and behavioral problems, such as Conduct Disorder (CD) or Antisocial Personality Disorder (APD), was well-known around the time of the Port Arthur Massacre. A review by biostatistician and UCSD professor Sonia Jain cites contemporaneous studies to suggest that low IQ scores in childhood should be considered a risk factor for APD and CD. In 2010, several psychologists published results from a longitudinal study containing data on over a million Swedish men, who were tracked from conscription for a little over 20 years. They found that IQ scores tested during conscription were a significant and robust predictor, not only for APD or CD, but for all categories of mental disorders. Conscripts with low IQ were substantially more likely to be diagnosed with one or more mental disorders, to suffer from mood and personality disorders, and to be hospitalized for mental illness. Those in the lowest band—like Bryant—were most at risk of severe psychological disorders.”
Professor Linda Gottfredson wrote:
1. IQ (as long as it's a good measure of g) predicts a broad range of life outcomes better than does SES [socio-economic status], from GPA to longevity. Corollary: You can wash out IQ's apparent predictive superiority only if you load your SES battery with additional surrogates for parents' or own g.
2. The phenotypic correlations between IQ and measures of social class (education, occupational prestige, income) are from a half to two-thirds genetic in origin.
3. SES cannot explain the big IQ differences among siblings growing up in the same household: They differ two-thirds as much in IQ, on the average (11-12 points), as do any two random strangers (~17 points). This is a glaring fact that SES enthusiasts have studiously ignored.
4. Adult functional literacy (e.g., see the fed's NALS survey) predicts life outcomes in exactly the same pattern as does IQ, though they won't tell you that. Functional literacy is measured by having subjects carry out everyday life tasks, such as using a menu to figure out the price for something. Persons scoring at levels 1-2 (out of 5) have been described as not having the ability to use their rights or meet their responsibilities in the modern world (40% of whites, 80% of blacks). Pick out a few NALS tasks at various levels and ask your critic what % of adults s/he thinks can perform them. They will be shocked and so will you when you see the data–go to my 1997 "Why g matters" article for NALS, or my 2002 "highly general and highly practical" chapter for health literacy items–e.g., on diabetes.
5. IQ predicts on-the-job performance better overall than any other single predictor (SES isn't even in the running), it predicts better when performance is objectively rather than subjectively measured, and when the tasks/occupations are more complex in what they require workers to do. At the same cognitive complexity level, IQ predicts job performance equally well in manual and non-manual jobs (e.g., trades vs. clerical. The exact same complexity pattern is found with functional literacy–the hardest items are the most complex (require more inference, are abstract rather than concrete, contain more distracting irrelevant information, etc.)
6. A large followup of Australian veterans found that IQ was the best predictor of death by age 40 (had 50+ predictors). Vehicle fatalities were the biggest cause (as is typical), and, compared to men with IQs of 100+, men of IQ85-100 had twice the rate and men IQ 80-85 had three times the rate. (Remember, SES could not explain this.) The US (and apparently Australia) forbid induction of persons below IQ 80 because they are not sufficiently trainable–found out the hard way.
7. Finally, if you succeed in describing g as a general learning and reasoning ability (one that gives high g people an increasing edge when tasks are more complex), then it is easy to show g's life and death relevance when you describe how health self-care and accident prevention are highly dependent on learning and reasoning. Consider what it takes to be an effective diabetic–lots and lots of judgment on a daily basis, or you're likely to lose your sight, your limbs, etc.
Of all human traits, variation in general intelligence (g) is the functionally most important in modern life. The first question that behavior genetics tackled was ‘‘how heritable are within-group differences in intelligence?’’—the answer: ‘‘very.’’
Gottfredson said: "Keep in mind that false belief in infinite human malleability led to some of the worst horrors of the 20th century. I also think it is patronizing and usually self-serving when elites contend that the American public cannot be trusted with certain facts."
If all 13‐year‐olds took the same 15‐minute test (WASI), I could give you each child’s odds for all these adult outcomes without knowing anything else about them.
– Drops out of high school,
– Holds mostly unskilled jobs, skilled jobs vs. professional jobs
– Performs those jobs well
– Lives in poverty AND
– Can find a particular intersection on a map, or grams of carbohydrate per serving on a food label
– Adheres to a medical treatment regimen for diabetes or other chronic illness
– Dies prematurely
The first step in assessing the real-life importance of g/IQ is to determine whether scores on highly g-loaded tests (tests that measure g well) predict differences in valued life outcomes. Correlations do not prove causation, but they are a first step in doing so. The most studied outcomes are performance in school (such as school marks and achievement test scores), performance on the job (mostly supervisor ratings), socioeconomic advancement (level of education, occupation, and income), and social pathology (adult criminality, poverty, unemployment, dependence on welfare, children outside of marriage). The relations of intelligence to health, health behavior, resilience in the face of extreme adversity, longevity (length of life), and functional literacy (the ability to do routine reading, writing, and arithmetic tasks in modern societies) have also begun to draw much attention. Thousands of studies have looked at the impact of mental abilities on school and job performance, and large national longitudinal studies in both Europe and the United States have shown that IQ is related to various forms of socioeconomic success and failure. Here are their most general findings about g’s association with life outcomes.
Correlations with IQ are pervasive. IQ predicts all the foregoing outcomes to some degree. Subjective well-being (happiness) is the rare exception: it is regularly found not to correlate meaningfully with IQ level. In general, g relates more to instrumental behavior than emotional reactions.
Correlations with IQ vary systematically by type of outcome. IQ’s predictive value ranges widely, depending on the outcome in question. For example, when averaged over several years, performance on standardized tests of academic achievement correlates about as highly with IQ as two IQ tests do with each other (over .8 on a scale of -1.0 to 1.0). In contrast, correlations with IQ are closer to .6-.7 for school marks, years of education completed, and longevity. They are about .5 with prestige level of occupation, .3 to .4 with income (the correlations rising with age), and .2 with law-abidingness.
Correlations with IQ are higher when tasks are more complex. To illustrate, when jobs are ranked in overall complexity of work, the correlations between IQ and job performance rise from .2 for simple, unskilled jobs, to .5 in middle-level jobs (skilled trades, most clerical work), to .8 in the most complex (doctors, engineers, top executives). Stated another way, it matters little how intelligent workers are in low-level jobs, but it matters a great deal in high-level jobs, regardless of whether the job seems academic or not.
IQ/g is best single predictor, mental or non-mental. IQ/g usually predicts major life outcomes better than does any other single predictor in broad samples of individuals. For example, whether IQ predicts strongly (educational performance) or weakly (law-abidingness), it predicts better than does social class background…
Social privilege theory also predicts that the impact of environmental conditions will accumulate with age, but longitudinal studies show that IQ actually becomes more heritable over the life span (from 40% before entering elementary school to 80% by mid-adulthood). Perhaps most surprising of all, differences in family advantage have no lasting effect on IQ by adolescence, at least in the U.S. and Europe, so family members are no more alike in IQ by adulthood than their genetic relatedness would predict…To take one example, the post-World War II communist government of Warsaw, Poland, assigned families of all social classes to the same housing, schools, and health services, but this social leveling failed to narrow intelligence differences in the next generation…
The pattern is that, when two groups differ in average IQ, the proportions of their populations found at each point on the IQ distribution differ most at the extremes, or tails, of the IQ distribution. This is seen most clearly by looking at the ratios in the bottom three rows of Figure 3. Take, for example, blacks and whites above IQ 100. Blacks become progressively rarer, relative to whites, at higher IQ levels: 1:3 above IQ 100, 1:7 above IQ 110, and only 1:30 above IQ 125…
IQ 75 signals the ability level below which individuals are not likely to master the elementary school curriculum or function independently in adulthood in modern societies. They are likely to be eligible for special educational services in school and for Supplemental Security Income (SSI) from the U.S. government, which is financial support provided to mentally and physically disabled adults. Of course, many do marry, hold a job, raise children, and otherwise function adequately as adults. However, their independence is precarious because they have difficulty getting and keeping jobs that pay a living wage. They are difficult to train except for the simplest tasks, so they are fortunate in industrialized nations to get any paying job at all. While only 1 out of 50 Asian-Americans faces such risk, Figure 3 shows that 1 out of 6 black- Americans does.
IQ 85 is a second important minimum threshold because the U.S. military sets its minimum enlistment standards at about this level. Although the military is often viewed as the employer of last resort, this minimum standard rules out almost half of blacks (44%) and a third of Hispanics (34%), but far fewer whites (13%) and Asians (8%). The U.S. military has twice experimented with recruiting men of IQ 80-85 (the first time on purpose and the second time by accident), but both times it found that such men could not master soldiering well enough to justify their costs. Individuals in this IQ range are not considered mentally retarded and they therefore receive no special educational or social services, but their poor learning and reasoning abilities mean that they are not competitive for many jobs, if any, in the civilian economy. They live at the edge of unemployability in modern nations, and the jobs they do get are typically the least prestigious and lowest paying: for example, janitor, food service worker, hospital orderly, or parts assembler in a factory.
IQ 85 is also close to the upper boundary for Level 1 functional literacy, the lowest of five levels in the U.S. government’s 1992 National Adult Literacy Survey (NALS). Adults at this literacy level are typically able to carry out only very simple tasks, such as locating the expiration date on a driver’s license or totaling a bank deposit slip, but they typically cannot perform more difficult tasks, such as locating two particular pieces of information in a sports article (Level 2), writing a brief letter explaining an error in a credit card bill (Level 3), determining correct change using information in a menu (Level 4), or determining shipping and total costs on an order form for items in a catalog (Level 5). Most routine communications with businesses and social service agencies, including job applications, are thus beyond the capabilities of persons with only Level 1 literacy. Their problem is not that they cannot read the words, but that they are not able to understand or use the ideas that the words convey…
IQ 105 can be viewed as the minimum threshold for achieving moderately high levels of success. It has been estimated to be the point at which individuals have a 50-50 chance of doing well enough in secondary school to be admitted to a four-year university in the United States. People above this level are highly competitive for middle-level jobs (clerical, crafts and repair, sales, police and firefighting), and they are good contenders for the lower tiers of managerial and professional work (supervisory, technical, accounting, nursing, teaching). Figure 3 shows that Asian-Americans are 6-7 times more likely than blacks to exceed the IQ 105 threshold. The percentages are 53%, 40%, 27%, and 8%, respectively, for Asians, whites, Hispanics, and blacks.
IQ 115 marks the ability threshold for being competitive as a candidate for graduate or professional school in the U.S. and thus for high levels of socioeconomic success. Partly because of their higher educational promise, individuals above this IQ level have the best prospects for gaining the most coveted occupational positions in a society. This is the IQ range in which individuals can be self-instructing and are, in fact, expected to instruct, advise, and supervise others in their community and work environments. This is therefore the IQ range from which cultural leaders tend to emerge and be recruited. The percentages exceeding this threshold are, respectively, 40% (Asians), 28% (whites), 10% (Hispanics), and 4% (blacks).