If we consider that intelligence manifests itself in everyday life as the ability to deal with complexity, then it is easy to see why it has great functional or practical importance. Children, for example, are regularly exposed to complex tasks once they begin school. Schooling requires above all that students learn, solve problems and think abstractly. That IQ is quite a good predictor of differences in educational achievement is therefore not surprising. When scores on both IQ and standardized achievement tests in different subjects are averaged over several years, the two averages correlate as highly as different IQ tests from the same individual do. High-ability students also master material at many times the rate of their low-ability peers. Many investigations have helped quantify this discrepancy. For example, a 1969 study done for the U.S. Army by the Human Resources Research Office found that enlistees in the bottom fifth of the ability distribution required two to six times as many teaching trials and prompts as did their higher-ability peers to attain minimal proficiency in rifle assembly, monitoring signals, combat plotting and other basic military tasks. Similarly, in school settings the ratio of learning rates between "fast" and "slow" students is typically five to one.
The scholarly content of many IQ tests and their strong correlations with educational success can give the impression that g is only a narrow academic ability. But general mental ability also predicts job performance, and in more complex jobs it does so better than any other single personal trait, including education and experience. The army's Project A, a seven-year study conducted in the 1980s to improve the recruitment and training process, found that general mental ability correlated strongly with both technical proficiency and soldiering in the nine specialties studied, among them infantry, military police and medical specialist. Research in the civilian sector has revealed the same pattern. Furthermore, although the addition of personality traits such as conscientiousness can help hone the prediction of job performance, the inclusion of specific mental aptitudes such as verbal fluency or mathematical skill rarely does. The predictive value of mental tests in the work arena stems almost entirely from their measurement of g, and that value rises with the complexity and prestige level of the job.
Half a century of military and civilian research has converged to draw a portrait of occupational opportunity along the IQ continuum. Individuals in the top 5 percent of the adult IQ distribution (above IQ 125) can essentially train themselves, and few occupations are beyond their reach mentally. Persons of average IQ (between 90 and 110) are not competitive for most professional and executive-level work but are easily trained for the bulk of jobs in the American economy. In contrast, adults in the bottom 5 percent of the IQ distribution (below 75) are very difficult to train and are not competitive for any occupation on the basis of ability. Serious problems in training low-IQ military recruits during World War II led Congress to ban enlistment from the lowest 10 percent (below 80) of the population, and no civilian occupation in modern economies routinely recruits its workers from that range. Current military enlistment standards exclude any individual whose IQ is below about 85.
The importance of g in job performance, as in schooling, is related to complexity. Occupations differ considerably in the complexity of their demands, and as that complexity rises, higher g levels become a bigger asset and lower g levels a bigger handicap. Similarly, everyday tasks and environments also differ significantly in their cognitive complexity. The degree to which a person's g level will come to bear on daily life depends on how much novelty and ambiguity that person's everyday tasks and surroundings present and how much continual learning, judgment and decision making they require. As gamblers, employers and bankers know, even marginal differences in rates of return will yield big gains--or losses--over time. Hence, even small differences in g among people can exert large, cumulative influences across social and economic life.
