In various studies performed at the University of Chicago, L. L. Thurstone developed the primary mental abilities theory based for the most part on the results of two large-scale studies that he conducted. In his theory there are six or seven primary mental abilities that determines a person’s intelligence. Each individual has his or her own pattern of strengths and weaknesses that can be measured. He theorized that there was no central, encompassing general intelligence quotient as Spearman had promoted, but consisted of these interrelated, but independent nonetheless, abilities.

In 1934 Thurstone and his wife, Mrs. Thurstone, developed a battery of 56 tests that were administered to 240 volunteers, most of whom were students at the University of Chicago. Through factor analysis and a method called the centroid method, he separated the data into thirteen factors. He compared the correlation of the factors and determined that there were seven primary factors (see below).

Verbal comprehension (or Verbal Ability): Found in such things as verbal reasoning, reasoning by analogy, and reading comprehension. It is "characterized primarily by its reference to ideas and the meanings of words."

Word fluency: Facility with words in special contexts, such as anagrams, rhyming, etc.

Number ability: Arithmetic computation

Spatial ability: The ability to mentally manipulate and visualize geometric relations; facility in spatial and visual imagery.

Associative memory: The ability to make random paired associations that require rote memory; memorizing skills.

General reasoning ability (or Induction): Facility in finding rules or principles in test items, such as in a number series.

Perceptual speed: Facility in finding or in recognizing particular items in a perceptual field.

In this series of tests he had not found a general intelligence factor encompassing all the lesser factors, but others claimed that his experiment was biased. The participants were mostly highly intelligent college students within a limited age range. Other psychologists reanalyzed his data and claimed that there was an underlying g factor. In 1941 he would perform another study to confirm his findings.

In this study a series of sixty tests were given to 710 eighth-grade students in 11 one-hour sections. This study was more diverse, being administered to students with a wide range of different intelligence levels. In this experiment they found the same seven factors that were discovered in the previous study. However, upon further analysis, several of these factors were found to correlate with each other. For example, the spatial factor was found to have some correlation with the verbal comprehension factor and the induction factor. This additional correlation was labeled as a "second-order general factor".

This view of intelligence became widely popular over the next decades as the Thurstone duo came up with more tests for various grade levels. Later researchers also developed the idea of multiple second-order factors (see Raymond B. Cattell [link to Cattell page]). More recent researchers like Howard Gardner have developed more elaborate theories, but Thurstone still remains an early step in the development of multiple intelligences. It also must be mentioned that these seven primary mental abilities may not be the only ones.