Together, the notions of normal distribution and correlation allow us to consider how our abilities vary in relation to each other and in relation to the abilities of others in the population, and hence how well we can use scores on one variable to predict scores on another.

Binet and his associate Th éodore Simon used a range of tasks in their first intelligence tests. These included around 30 items of increasing difficulty, from simple items that even intellectually disabled children were able to complete. More complex tasks included pointing to body parts and defining words such as ‘house ’or ‘wheel ’, and tasks that were harder still, such as repeating back strings of digits and constructing sentences involving several specified words.Interestingly, vocabulary and digit recall tasks are still used in our most advanced intelligence tests today.
Binet was also the first psychologist to specify that such tests must be:

1. administered and scored in a careful and standardized manner if comparisons between children ’s performance are to be valid and reliable;
2. presented in the same order to all children and in order of increasing difficulty so that each child can pass as many tests as possible; and
3. administered in a one-to-one setting and only where the examiner has first established a friendly rapport with the child.Psychologists still adhere to these very important principles of testing today.

The higher the IQ score, the more intelligent the person. The average IQ is 90 –110, with university students usually having an IQ of at least 115.

Spearman ’s finding of a general feature that underlies performance in many areas was so radical that it became the hallmark of his work. Spearman likened g to mental energy –a limited resource available to all intellectual tasks. So the idea was that individuals differ in general intelligence because they have different amounts of this mental energy.

All of the answers given above refer to characteristics associated with mental retardation.

IQ test scores represent phenotypic measures of intelligence. Phenotypic refers to the expression of our genes in behavioural traits that we can measure. Genotypic refers to the DNA we inherited from our parents. Environmental, modular and general are not standard terms used to refer to the genetics of IQ.

Such a difference has to be the result of an environmental change, because gene frequencies in populations could not change so quickly. If there can be shifts of one standard deviation in IQ between generations that are environmental in origin, despite the high heritability of IQ differences, why could there not be similar environmentally mediated differences between populations within the current generation?

Anderson suggests that the first route to knowledge is through thought (central processes) and is related to differences in IQ. Thoughtful problem-solving can be done either by verbalizing a problem (using language-like propositions to think) or by visualizing it (using visuo-spatial representations to think). For this to happen, we need two different kinds of knowledge acquisition routines, each generated by one of two specific processors.
It is proposed that these processors are the source of individual differences in specific abilities, which, in turn, are constrained by the speed of a basic processing mechanism. So, at a slow processing speed, only the simplest kinds of thoughts of either kind can occur. (It is argued that the speed of the basic processing mechanism can be measured using tasks such as inspection time and reaction time.) It is suggested by Anderson that this constraint is the basis of individual differences in general intelligence and the reason for manifest specific abilities being correlated (giving rise to the g factor).

Many researchers and other professionals have challenged the claims of a race-IQ link by emphasizing the environmental influences that can lead to observed differences, which include the disadvantaged status and treatment of minority groups, socio-economic differences and biased IQ tests.

Wechsler tests are the most widely used individual tests of intelligence.