‘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.’
Gottfredson (1997)


IQ is not simply a measure of intelligence, it is the most accurate (i.e. reliable and valid) of all psychological tests and assessments at predicting the performance of simple tasks, academic success, job performance, health, longevity, functional literacy, socioeconomic advancement, ‘social pathologies’ and is correlated with brain size. If we conduct various different cognitive tests that measure seemingly unrelated skills, the results will be positively correlated. The application of factor analysis shows us that there is one primary dominant factor: we call this intelligence (and it coincides with our ‘common sense’ interpretation of intelligence). See Spearman (1904), Carroll (1994), Neisser, et al. (1996), Gottfredson (1997), Jensen (1998), Mackintosh (1998), Deary (2001).

From Carroll (1993, p. 16):
aptitude is a cognitive ability that is possibly predictive of certain kinds of future learning success.
achievement is the extent that certain behaviors have been learned.
"...an ability is clearly a measurement of aptitude for some particular future learning success if, in a sample of individuals tested both in aptitude and in achievement in some specified learning or training activity at two points of times, once before training (time A) and once after training (time B):

  1. There is reliable variance in the measure of aptitude at time A.
  2. There is no reliable variance in achievement tested at time A, because no learning has occurred.
  3. As a consequence of condition 2, above, there is no significant correlation of aptitude and achievement at time A,
  4. No significant change in aptitude is observed from time A to time B.
  5. Significant change in achievement is observed from time A to time B, with reliable variance in achievement at time B.
  6. There is a significant correlation between aptitude measured at time A with achievement at time B (trivially, this will be the same as the correlation between aptitude and achievement both measured at time B)."

On that basis, my own definition:
intelligence is the ability of an individual to perform a novel cognitive task.

Heritability of Intelligence

Heritability is the proportion of total phenotype variance attributable to genetic variance.

G = variance of the genetically diverse population in a uniform environment
E = variance of genetically identical subjects in different environments
heritability = G/(G+E)

Sources of evidence:

AgeBroad heritabilityNarrow heritability
late adolescence0.75

Broad heritability is a measure of the heritability attributable to all kinds of genes, whilst narrow heritability is a measure of the action of additive genes only.


Academics who subscribe to the above notions listed here.

Intelligence around the World

RegionMean IQ
Northern North America97.0
North America98.0
Middle America87.0
Central America83.2
Northern South America88.0
Western South America86.8
Central South America85.0
Eastern South America87.0
Southern South America95.2
Northern Europe98.9
Western Europe99.2
Central Europe100.5
Eastern Europe96.0
Southwestern Europe98.2
Southern Europe100.4
Southeastern Europe93.0
Northern Africa81.2
Western Africa67.5
Central Africa66.8
Eastern Africa68.3
Southern Africa72.7
Southeastern Africa72.0
Middle East84.9
Northern Asia96.0
Central Asia88.5
Eastern Asia100.8
Southwestern Asia90.0
Southern Asia81.0
Southeastern Asia89.7

Africa (excluding Northern Africa): 68.6
Southern Asia: 81.0
Europe: 98.6
Eastern Asia (excluding mainland China): 105.2

The mean IQ of countries lies within the range 59 (Equatorial Guinea and Sao Tome/Principe) to 107 (Hong Kong). The mean IQ for the population of the entire world is 88.8. The median of the mean IQ of countries is 84. For countries with a mean IQ above 90 there exists a positive correlation between wealth and IQ. The six countries described as belonging to the ‘Axis of Evil’ and ‘Beyond the Axis of Evil’ (Cuba, Iran, Iraq, Libya, North Korea and Syria), with the exception of North Korea have mean IQs in the range of 84 and 87.

Are between-group IQ differences genetic?

Yes, Lynn (2006, p. 66–68) gives six reasons.

Are IQ tests culturally biased?

No. Herrnstein and Murray (1994, p. 280–286, 649–661), Jensen (1998, p. 360–369), Levin (2005, p. 62–73).

Is an individual's IQ score stable over time?

Deary, et al. (2000) found that the corrected correlation between test scores at age 11 and age 77 was 0.73.

Sex Differences

I (Sewell 2007) performed a meta-study of 26 articles that measure adult male and female intelligence which produced an average difference in g of a male advantage of about 2 IQ points.

Intelligence in Different Subjects

The Reference Frame: IQ in different fields

Flynn Effect

Herrnstein and Murray (1994, p. 307–309)
Jensen (1998, p. 318–333)
Levin (2005, p. 128–130)
Lynn (2006, p. 5–6)