Figure 1. The psychometric model has a hierarchical structure, with specific tests giving information about specific cognitive abilities such as verbal and spatial abilities, processing speed and memory ability. These correlated abilities in turn give information about general cognitive ability.

Figure 2. Tyron's classic study shows successful selective breeding on errors in maze learning in rats. The maze was a long and difficult T maze with 17 blind alleys. Errors refer to the total number of blind alley entrances in 19 trials. The original population (P) of 142 animals shows a continuous distribution. P animals with the fewest errors were bred together and those with the most errors were also bred together. Their progeny (F₁) were assessed and then similarly bred. For each group, from 66 to 115 progeny were assessed across the eight generations. (A copy of original data from Tyron RC (1946) Individual differences. In: Moss FA (ed.) Comparative Psychology, pp. 331–365. New York: Prentice-Hall In.)