In this research, Kaufman (2000) sets out to investigate the changes in intellectual ability for a wide range of individuals, aged between 16 – 89 years, brought about by ageing. For the two case studies in this paper, educational attainment is the cohort variable that is controlled. The study found that, in both longitudinal and cross sectional studies, there was a significant decline in performance and verbal IQs. The paper shows that, differences in values for the Fluid IQ between the WAIS-III and KAIT studies are caused by different methods used for measurement rather than generational differences. Lastly, for Horn’s theory, the WAIS-III IQ figures are less relevant compared to the four factor index figures.
According to the case studies in this paper, the Verbal IQ in both cases remains relatively the same throughout most of the age group with a relatively unsubstantial increase after the peak years of 45 – 54. This implies that verbal capabilities of the individual are maintained once they are attained, throughout the individual’s life. The significance of maintained Verbal IQs and significant steady increase thereafter until the 60s, after which it declines, shows that verbal capabilities are more of a crystallized intelligence factor. The general decline thereafter is brought about by the decline in Performance IQ combined with the decline in the Processing Speed Index. This is supported by the study sampled by Kaufman (2000), showing similar decline in the figures for both the Performance IQ and the WMI figures at around the same time for individuals in the age bracket above 60 years.
In relation to Fluid Intelligence factors such as the Perception Organization Index, there was a steady decline in the Perception Organization Index, Processing speed Index and the performance IQ. These factors peaked between the ages of 16 – 25 and steadily declined afterwards. As such fluid intelligence factors are more susceptible to age at an early age. They are termed as being vulnerable. The studies, both cross-sectional and longitudinal support this statement from the figures obtained from the subtests measuring this factors. They show that after the ages of 35 – 44, the mean Verbal and Performance IQs, even without an educational adjustment, vary greatly by up to more than 12 point. In the 45 – 64 age groups the values vary by up to 20 points (Kaufman 2000). Both studies came to the same conclusion despite the different methods of instrumentation they used to achieve the results, proving that Horns’ fluid-crystallization theory and the WAIS-III theories are both true.
In short, Kaufman (2002) is implying that there is a significant level of truth that the ageing does indeed affect the cognitive functions of human beings. The elements of the case study discussed above from the article support this fact. There are however some methodological issues that arise. The results based on the studies excluded patients who were above 45 years old and had known cases of mental illness and other illnesses related to the mind. Experts have argued that this could give results that understate the level of declining cognitive functions due to age.
Another key area of concern that needs to be addressed is the relationship between motor function and the level of intelligence in an individual. Further research on the issue could explain the relevance between a decline in Verbal IQ and Performance IQ during old age and as individuals gradually age.
Kaufman, S., A. (2001). WAIS-III IQs, Horn’s Theory and Generational Changes from Young Adulthood to Old Age. Intelligence (29). 131-167.