Intelligence is difficult to define because it is a complex construct. There are many different theories and definitions that are complementary and contradictory. In general, intelligence can be described as “the ability to learn from experience, to solve problems and to adapt to new situations” (D.G. Myers 2014). Different people’s brains work at different levels of efficiency and thus people are considered to have different levels of intelligence (cf. I.J. Deary et al. 2010). Intelligence test scores are expressed and calculated as intelligence quotient (IQ) (cf. David G. Myers 2014). IQ is used in science primarily to measure intelligence and its influence on other factors.
The IQ is associated with a number of socio-economic factors such as income and educational attainment but also with diseases and life span: ”There was a 60% greater risk in the hazard of being admitted for schizophrenia, a 50% greater risk for mood disorders, and a 75% greater risk of alcohol-related disorders*”( I.J. Deary et al 2010) if the IQ is one standard deviation (=15 points) lower.
A large number of studies investigate the relationship between intelligence and Alzheimer’s disease. In the “nun-study” from Snowdon et al. 1996., he examined the linguistic ability – which is strongly associated with cognition – by testing the grammatical complexity and creativity in writing samples in 101 nuns at the age of 22. These results were used to examine the relationship to Alzheimer’s Disease (AD) of the nuns at age 80. Low complexity in writing samples was associated with lower mental abilities in later life. A similar study by Whalley et al. 2000 was able to find an association between lower IQ measurement scores and higher risk of AD in later life. There was no evidence for a relationship between IQ and AD at a young age though. Larsson et al. 2017 looked at over 54.000 participants to determine the influence of lifestyle/dietary, cardiometabolic, socioeconomic and inflammatory factors on Alzheimer’s disease. The results suggest that a higher level of intelligence and education lead to a lower risk of Alzheimer’s disease.
There is a significant amount of research exploring the relation of the presence of high levels of intelligence and the predicted lower risk of mental illness (cf. Yeo et al. 2011, Karpinski et al. 2016, Wraw et al. 2016). One possible explanation for the relationship between Intelligence/IQ and mental disease is the more efficient use of processing pathways in the brain, based on the greater amount of cognitive capacity that comes from either IQ or education.
Thus, individuals with more efficient processing can sustain for example more Alzheimer’s pathology until the first symptoms appear, compared to individuals with lower cognitive capacity. (cf. Larsson et al. 2017, Schmand et al. 1997)
* using a general score derived from four diverse mental tests (cf. I.J. Deary et al. 2010)