In the classic review of intelligence research (Intelligence: Knowns and Unknowns), Neisser et al (1996) write the following
Stability. Intelligence test scores are fairly stable during development. When Jones and Bayley (1941) tested a sample of children annually throughout childhood and adolescence, for example, scores obtained at age 18 were correlated r = .77 with scores that had been obtained at age 6 and r = .89 with scores from age 12. When scores were averaged across several successive tests to remove short-term fluctuations, the correlations were even higher. The mean for ages 17 and 18 was correlated r = .86 with the mean for ages 5, 6, and 7, and r = .96 with the mean for ages 11, 12, and 13. (For comparable findings in a more recent study, see Moffitt, Caspi, Harkness, & Silva, 1993.) Nevertheless, IQ scores do change over time. In the same study (Jones & Bayley, 1941), the average change between age 12 and age 17 was 7.1 IQ points; some individuals changed as much as 18 points.
Typically, IQ scores are standardised for age and thus remove age related differences.
My question is
What is the relationship between age and raw intelligence test score at the group-level from around age 5 and into adultyhood?
From what I've read, I understand that at the group-level intelligence increases monotonically with age during childhood and that it levels-off at some point (perhaps around 18?). Also, below a certain age (e.g., in infancy), completely different tests are required to measured intelligence. I also understand that individuals may experience various trajectories (e.g., some might plateau earlier, etc.), but I'm interested in the group-level. Thus, an ideal answer would probably provide a sense of the functional form of the increase in intelligence over childhood and adolescence.
My first thought for an answer was to look at the test manual for a standardised child intelligence test, such as the WISC, but I don't currently have easy access to that.
- Jones, H. E., and Bayley, N. (1941). The Berkeley Growth Study. Child Development, 12, 167-173.
- Moffitt, T. E., Gabrielli, W. F., Mednick, S. A., & Schulsinger, E (1981). Socioeconomic status, IQ, and delinquency. Journal of Abnormal Psychology, 90, 152-156.
- Neisser, U., Boodoo, G., Bouchard, T. J., Boykin, A. W., Brody, N., Ceci, S. J., Halpern, D. F., Loehlin, J. C., Perloff, R., Sternberg, R. J., and Urbina, S. (1996). Intelligence: Knowns and unknowns. American Psychologist, 51:77-101. FREE PDF