What is Intelligence?

Intelligence is the ability to think clearly, reason logically or analytically, obtain and process new knowledge, solve problems, understand complex concepts, and communicate knowledge to others. It is not fully known what causes variations in intelligence (at the lowest level) although very recent research has indicated that it is related to the neuron firing speed in the brain (which differs from person to person.) It is established in the scientific community that intelligence is caused both by genetic factors and environmental factors, with between 40% and 80% of all intelligence variation being dependent on genes. 1

Intelligence or cognitive processing ability is fixed by the age of six in almost every case (exceptions are extraordinarily rare.) It is well-known in psychology that to have any effect on the intelligence of a person intervention must occur very early in life. Unfortunately, despite this, little success has ever been achieved in permanently raising intelligence via such intervention. The greatest effects on intelligence have been observed in neo-natal and early childhood nutrition and medical care. Although some studies or programs have purported to achieve lasting effects, the science of these efforts has been questionable, or the results irreproducible.

The I.Q. Connection

Many think of intelligence when they hear the term I.Q.. This is both exactly correct and wildly inaccurate. I.Q., or "intelligence quotient" was originally defined as a person's mental age divided by their physical age. If a 10 year-old could perform the mental tasks of a 16 year-old, they had an I.Q. of 160. This number was determined by the intelligence tests of the day (the early 1920's) and was held to accurately represent intelligence itself. However, it is extremely important to note that I.Q. only means the score one can achieve on an I.Q. test. It can only measure actual intelligence if the test contains the correct style of questions and is administered properly. I.Q. is only a valid measure of intelligence if the test it comes from is also valid.

The belief in the ability of a test to measure general intelligence is based on the work of British psychologist Charles Spearman. In 1904 Spearman noticed that many tests of mental ability were all correlated with one another -- a person who did well on one test was likely to do well on all others also. 2 Spearman took his data and applied a statistical method called factor analysis. He concluded that all of the correlation could be accurately explained by one factor, which he called the general intelligence factor, or g.

The Great g  Debate

From Spearman's time until the late 1970's, debate has raged about what g  is, what it means, and whether it actually exists or if it is merely a statistical artifact. Though these arguments have been laid to rest in the professional psychometric community, they remain abroad in the popular consciousness. In this respect, the prevailing wisdom is far behind the state of scientific knowledge. This is primarily due to the efforts of a liberal establishment that dislikes the implications of a g  that exists and has real meaning.

The argument against g   is that it is simply a creation of statistics obtained by manipulating the factor analysis in a certain manner. For decades, psychometricians studied, searched, and analyzed the data with the hope of obtaining "multiple intelligence factors" that explained intelligence better than Spearman's g. The search, in short, was fruitless. No method of factor analysis can produce any group of variables that explain test correlations better than g. In short, g does exist and is here to stay. In his book The Mismeasure of Man, Stephen Jay Gould uses this argument as his means of rejecting the notion of g. It should be noted that this argument no longer has any scientific standing among professionals in the field, based on overwhelming evidence against it.

Effects of Intelligence

Intelligence has a large statistical impact on society and everyday life. Many societal phenomenons, including poverty, education, wage earnings, crime, and others, correlate strongly with intelligence -- more than any other factor. While these things are typically studied in terms of class background or educational level, it is clearly shown by studies that intelligence explains these social phenomena (in general) far better than any other factor. In fact, some previously obvious correlations disappear when intelligence is taken into account. A complete study of these effects is the goal of The Bell Curve, by Richard Herrnstein and Charles Murray. (Contrary to popular opinion, the book is not about race.) Due to the huge effects intelligence has on societal trends, the study of it is of major importance, and the ability to genuinely improve intelligence should be highly sought after by scientists and laypeople alike.


1. Richard Herrnstein, Charles Murray. The Bell Curve. New York City: Simon & Schuster, 1994.

2. "Intelligence." Microsoft Encarta Online. encarta.msn.com. Microsoft Corp. Wed. Nov 12, 2002.

Other information from personal knowledge and consultation with teachers and professors in the field.