Intelligence is a commonly discussed characteristic, and many may consider it a straight forward concept.  It has been described as “problem-solving skills. . . logical reasoning, and/or adaptation to the environment” (Seigel 1989), in other words, an innate ability to find solutions.  When it comes to biology, the intricate details of intelligence are still rather misty.  Research suggests that intelligence is related to a combination of density and connectedness of the neurons of the brain.  It is not a simple matter of size or number of synapses, but a complicated web of multiple variables (Higgens & George 2007).

Theoretically, intelligence is an intrinsic quality that does not change over time.  There is evidence that suggests IQ is fairly constant throughout life.  In fact, mental ability in old age has been shown to be strongly influenced by childhood intelligence (Deary et al. 2004).  However, intelligence has also been shown to have a positive relationship with education (Lynn & Vanhanon 2002).  Such evidence begs the question, if intelligence is intrinsic and constant over time, shouldn’t it remain constant regardless of education?  If education truly does result in an increase in intelligence does our definition and understanding of intelligence need to change?  Or is the problem related to the method used to measure intelligence?

Intelligence, has been correlated with numerous variables, from health to material wellbeing.  It has been suggested that there is evidence indicating that intelligence is correlated to longevity.  One study found that childhood psychometric intelligence was associated with improved survival past the age of 76.  It is difficult to conclude this to be a direct relationship.  Lower IQ is also associated with lower socioeconomic background (Hernstein & Murray 1994), lower socioeconomic background is similarly correlated with greater disease and health concerns.  So it would make sense that such individuals with lower IQ and socioeconomic background have shorter life spans.  To clearly understand the relationship between intelligence and longevity, one would need to control for socioeconomic status (Deary et al. 2004).

Numerous studies have been conducted investigating the relationship between intelligence and prosperity.  One study found a positive relationship between a nation’s average IQ and GDP (Lynn & Vanhanon 2002).  Some investigations have found a positive relationship between IQ and financial wellbeing (Wachtel 1976), or that there are few individuals with high IQ who live in poverty (Hernstein & Murray 1994).  Other studies have found IQ to be a poor predictor of and individual’s monetary situation.  One study found a strong relationship between IQ and annual income, but that relationship did not translate to wealth or net worth.  In fact, high IQ also had a correlation with behaviors that could lead to financial distress (Zagorsky 2007).

Research has also investigated the relationship between intelligence to certain mental illnesses.  High IQ is a risk factor for bipolar disorder.  However, IQ has been shown to have a protective effect against schizophrenia (Matheson & Langdon 2008)

One difficulty encountered in the endeavor to understand intelligence is finding an accurate method for measuring intelligence.  IQ tests, probably the most common measure used to quantify intelligence, provide a number that purportedly reflects intelligence.  However, some argue that tests are not a truly objective measure.   Some tests rely heavily on written language.  As a result, a test may not accurately reflect the intelligence of say an individual with a learning disability or who did not have educational opportunities.  A low score could reflect such an individual’s difficulty with written language, nor their innate ability to solve problems (Seigel 1989).  IQ measure could even be impacted by external factors of the day of examination such as health or stress level (Zagorsky 2007).

Test outcomes can be skewed in other ways as well.  Researchers Croizet and Dutrévis found that students from disadvantaged socioeconomic backgrounds performed poorer on standardized tests than other students, but only when they were told that such test would be used as a diagnostic for intellectual ability.  When students from disadvantaged backgrounds did not know that the tests would be used as a diagnostic measure for intellectual ability, their scores were as good as the scores of their higher socioeconomic level counterparts.  Evidently scores can be impacted by perceived intent (Croizet & Dutrévis 2004).  This suggests that tests of intelligence may not accurately reflect the problem solving capacity of individuals from disadvantaged backgrounds.

It seems that there are certainly some holes in our current understanding of intelligence, particularly with regards to its relationship to socioeconomic status and education.  If intelligence is truly an intrinsic characteristic, one would think that it would not be related to such external factors.  On the other hand, if intelligence truly does have a substantial relationship with things like education and socioeconomic status, one would conclude that it is not an intrinsic characteristic, but rather an acquirable trait.  In reality, intelligence is most likely a combination of both intrinsic capacities for problem solving and education.  With the recent increases in our understanding of the plasticity of the brain, it seems reasonable to conclude that intelligence is not a static quality.  It also indicates that interventions can be taken to increase the IQ level of individuals from less advantaged backgrounds.



Croizet, J., & Dutrévis, M. (2004). Socioeconomic status and intelligence: Why test scores do not equal merit. Journal of Poverty, 8, 91-107. doi:10.1300/J134v08n03_05

Deary, I.J., Whiteman, M.C., Starr, J.M., Whalley, L.J., & Fox, H.C. (2004). The impact of childhood intelligence on later life: following up the Scottish mental surveys of 1932 and 1947. Journal of Personality and Social Psychology, 86, 130–147. doi: 10.1037/0022-3514.86.1.130

Hernstein, R., & Murray, C. (1994). The Bell curve: Intelligence and class structure in American life. New York, NY: Free Press.

Higgens, E.S., & George M.S. (2007). The neuroscience of clinical psychology. Philadelphia, PA: Lippincott Williams & Wilkins.

Lynn, R., & Vanhanen, T. (2002). IQ and the wealth of nations. Westport, CT: Praeger Publishers.

Matheson, S., & Langdon, R. (2008). Schizotypal traits impact upon executive working memory and aspects of IQ. Psychiatry Research, 159, 207-214. doi:

Sewell, W. H., & Shah, V. P. (1967). Socioeconomic status, intelligence, and the attainment of higher education. Sociology of Education, 40, 1-23. doi:10.2307/2112184

Siegel, L. S. (1989). IQ is irrelevant to the definition of learning disabilities. Journal of Learning Disabilities, 22, 469-78, 486.

Wachtel, P. (1976). The effect on earnings of school and college investment expenditures. The Review of Economics and Statictics, 58, 326-331.

Zagorsky, J. L. (2007). Do you have to be smart to be rich? the impact of IQ on wealth, income and financial distress. Intelligence, 35, 489-501. doi:

6 thoughts on “Intelligence

  1. Fascinating that high IQ is protective for schizophrenia but a risk factor for bipolar disorder. Some studies have shown that people with higher IQs have higher rates of mental illness in general, and it’s incredible how many of the most famous thinkers/artists in history were mentally ill. Creativity, intelligence, and mental illness seem to stick together. Obviously it’s all speculative based on their art/writing, but still interesting. Socrates had auditory hallucinations. Kierkegaard was either depressed or epileptic or both. Nietzche was diagnosed with tertiary syphilis when he was alive, and posthumously with bipolar disorder with psychotic features and frontotemporal dementia. Interesting that we will never know what he would have been like mentally without the syphilis. Kurt Vonnegut, Sylvia Plath, David Foster Wallace, the list goes on.

    On that note, this article on how half of all US presidents had a mental illness

  2. Interesting topic Maria! As you were speaking of social factors, I was wondering about our current prison system and some of the research that has shown correlations between intelligence and imprisonment. I found this one study particularly interesting, as it defined the lack of empathy as a function of intelligence. In their words: “the ability to understand another person’s emotions may be a primary function of intelligence”, which may correlate with higher recidivism amongst those with lower “intelligence”, though how this is defined is still up for debate. Not sure I totally buy it, but an interesting perspective to consider, particularly if we then start to look at earlier interventions.

  3. This book really is interesting on artificial intelligence.
    What really defines intelligence? AI these days can calculate most everything and there are algorithms that can emulate human thought. However, this AI cannot yet think as critically as humans and do not take emotional intelligence into account. Although AI is very promising, no single inventor has been able to mimic the complexity of the brain. However, these machines have the world’s knowledge at their disposal and can retrieve it. Humans are limited in their ability to recall and retain, but they can think critically and perform much better on certain measures on intelligence than humans do. So do we have a test yet that measures human intelligence, not just intelligence in general?I’m not convinced the IQ test can do this.

  4. It’s so interesting that intelligence is alluded to be “fixed”, yet there is correlation with GDP and SES. It appears we may have to look into society as a whole and consider SES as the greater underlying factor rather than the idea of intelligence. The study by Croizet and Dutrévis (2004) points out that intelligence can also be affected just by the perceived intent and differences in SES. As you mentioned, we must control for SES in order to accurately measure intelligence and the role it plays in mental health.

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