One of the things that irks me is really smart people who still deny that the concept of IQ, the fact that it can be quite accurately tested, or it’s usefulness as a predictor of success.
As this article lays out pretty nicely, the basic moving parts of IQ and the testing of it have been decently understood for some time now, and anyone wanting to know what real scientists agree on can take a look at the following, definitive paper on the topic:
The interesting thing about this paper was that the paper represents a consensus on what science knew at the time (1997) about intelligence, signed by 52 experts in the field. And as the article above points out, the points of agreement haven’t changed since then among scientists, yet people still dismiss this knowledge as “myth”.
So here’s the content of the paper, and just as a point of interest, I think the most important section is the one on practical importance.
The Meaning and Measurement of Intelligence
Intelligence is a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience. It is not merely book learning, a narrow academic skill, or test-taking smarts. Rather, it reflects a broader and deeper capability for comprehending our surroundings — “catching on,” “making sense” of things, or “figuring out” what to do.
Intelligence, so defined, can be measured, and intelligence tests measure it well. They are among the most accurate (in technical terms, reliable and valid) of all psychological tests and assessments. They do not measure creativity, character, personality, or other important differences among individuals, nor are they intended to.
While there are different types of intelligence tests, they all measure the same intelligence. Some use words or numbers and require specific cultural knowledge (like vocabulary). Others do not, and instead use shapes or designs and require knowledge of only simple, universal concepts (many/few, open/closed, up/down).
The spread of people along the IQ continuum, from low to high, can be represented well by the BELL CURVE (in statistical jargon, the “normal CURVE”). Most people cluster around the average (IQ 100). Few are either very bright or very dull: About 3% of Americans score above IQ 130 (often considered the threshold for “giftedness”), with about the same percentage below IQ 70 (IQ 70-75 often being considered the threshold for mental retardation).
Intelligence tests are not culturally biased against American blacks or other native-born, English-speaking peoples in the U.S. Rather, IQ scores predict equally accurately for all such Americans, regardless of race and social class. Individuals who do not understand English well can be given either a nonverbal test or one in their native language. The brain processes underlying intelligence are still little understood. Current research looks, for example, at speed of neural transmission, glucose (energy) uptake, and electrical activity of the brain.
- Members of all racial-ethnic groups can be found at every IQ level. The BELL CURVES of different groups overlap considerably, but groups often differ in where their members tend to cluster along the IQ line. The BELL CURVES for some groups (Jews and East Asians) are centered somewhat higher than for whites in general. Other groups (blacks and Hispanics) are centered somewhat lower than non-Hispanic whites.
IQ is strongly related, probably more so than any other single measurable human trait, to many important educational, occupational, economic, and social outcomes. Its relation to the welfare and performance of individuals is very strong in some arenas in life (education, military training), moderate but robust in others (social competence), and modest but consistent in others (law-abidingness). Whatever IQ tests measure, it is of great practical and social importance.
A high IQ is an advantage in life because virtually all activities require some reasoning and decision-making. Conversely, a low IQ is often a disadvantage, especially in disorganized environments. Of course, a high IQ no more guarantees success than a low IQ guarantees failure in life. There are many exceptions, but the odds for success in our society greatly favor individuals with higher IQs.
The practical advantages of having a higher IQ increase as life settings become more complex (novel, ambiguous, changing, unpredictable, or multi-faceted). For example, a high IQ is generally necessary to perform well in highly complex or fluid jobs (the professions, management); it is a considerable advantage in moderately complex jobs (crafts, clerical and police work); but it provides less advantage in settings that require only routine decision making or simple problem solving (unskilled work).
Differences in intelligence certainly are not the only factor affecting performance in education, training, and highly complex jobs (no one claims they are), but intelligence is often the most important. When individuals have already been selected for high (or low) intelligence and so do not differ as much in IQ, as in graduate school (or special education), other influences on performance loom larger in comparison.
Certain personality traits, special talents, aptitudes, physical capabilities, experience, and the like are important (sometimes essential) for successful performance in many jobs, but they have narrower (or unknown) applicability or “transferability” across tasks and settings compared with general intelligence. Some scholars choose to refer to these other human traits as other “intelligences.”
Source and Stability of Within-Group Differences
Individuals differ in intelligence due to differences in both their environments and genetic heritage. Heritability estimates range from 0.4 to 0.8 (on a scale from 0 to 1), most thereby indicating that genetics plays a bigger role than does environment in creating IQ differences among individuals. (Heritability is the squared correlation of phenotype with genotype.) If all environments were to become equal for everyone, heritability would rise to 100% because all remaining differences in IQ would necessarily be genetic in origin.
Members of the same family also tend to differ substantially in intelligence (by an average of about 12 IQ points) for both genetic and environmental reasons. They differ genetically because biological brothers and sisters share exactly half their genes with each parent and, on the average, only half with each other. They also differ in IQ because they experience different environments within the same family.
That IQ may be highly heritable does not mean that it is not affected by the environment. Individuals are not born with fixed, unchangeable levels of intelligence (no one claims they are). IQs do gradually stabilize during childhood, however, and generally change little thereafter.
Although the environment is important in creating IQ differences, we do not know yet how to manipulate it to raise low IQs permanently. Whether recent attempts show promise is still a matter of considerable scientific debate. Genetically caused differences are not necessarily irremediable (consider diabetes, poor vision, and phenal ketonuria), nor are environmentally caused ones necessarily remediable (consider injuries, poisons, severe neglect, and some diseases). Both may be preventable to some extent.
The following professors — all experts in intelligence and allied fields — have signed this statement:
- Richard D. Arvey, University of Minnesota
- Thomas J. Bouchard, Jr., University of Minnesota
- John B. Carroll, Un. of North Carolina at Chapel Hill
- Raymond B. Cattell, University of Hawaii
- David B. Cohen, University of Texas at Austin
- Rene V. Dawis, University of Minnesota
- Douglas K. Detterman, Case Western Reserve Un.
- Marvin Dunnette, University of Minnesota
- Hans Eysenck, University of London
- Jack Feldman, Georgia Institute of Technology
- Edwin A. Fleishman, George Mason University
- Grover C. Gilmore, Case Western Reserve University
- Robert A. Gordon, Johns Hopkins University
- Linda S. Gottfredson, University of Delaware
- Robert L. Greene, Case Western Reserve University
- Richard J.Haier, University of Callifornia at Irvine
- Garrett Hardin, University of California at Berkeley
- Robert Hogan, University of Tulsa
- Joseph M. Horn, University of Texas at Austin
- Lloyd G. Humphreys, University of Illinois at Urbana-Champaign
- John E. Hunter, Michigan State University
- Seymour W. Itzkoff, Smith College
- Douglas N. Jackson, Un. of Western Ontario
- James J. Jenkins, University of South Florida
- Arthur R. Jensen, University of California at Berkeley
- Alan S. Kaufman, University of Alabama
- Nadeen L. Kaufman, California School of Professional Psychology at San Diego
- Timothy Z. Keith, Alfred University
- Nadine Lambert, University of California at Berkeley
- John C. Loehlin, University of Texas at Austin
- David Lubinski, Iowa State University
- David T. Lykken, University of Minnesota
- Richard Lynn, University of Ulster at Coleraine
- Paul E. Meehl, University of Minnesota
- R. Travis Osborne, University of Georgia
- Robert Perloff, University of Pittsburgh
- Robert Plomin, Institute of Psychiatry, London
- Cecil R. Reynolds, Texas A & M University
- David C. Rowe, University of Arizona
- J. Philippe Rushton, Un. of Western Ontario
- Vincent Sarich, University of California at Berkeley
- Sandra Scarr, University of Virginia
- Frank L. Schmidt, University of Iowa
- Lyle F. Schoenfeldt, Texas A & M University
- James C. Sharf, George Washington University
- Herman Spitz, former director E.R. Johnstone Training and Research Center, Bordentown, N.J.
- Julian C. Stanley, Johns Hopkins University
- Del Thiessen, University of Texas at Austin
- Lee A. Thompson, Case Western Reserve University
- Robert M. Thorndike, Western Washington Un.
- Philip Anthony Vernon, Un. of Western Ontario
- Lee Willerman, University of Texas at Austin