Health and intelligence

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Health can affect intelligence in various ways.

Socioeconomic status

There is a well-known association between lower socioeconomic status and poorer health. Socioeconomic status is usually measured by factors such as social class, education, income, and residential area. However, after controlling for IQ this association was markedly reduced.[1]

Similarly, many of the differences in overall health between different races that are usually blamed on socioeconomic differences and discrimination may actually to a large degree be due to race and intelligence differences.

Overall mortality and morbidity

A strong inverse correlation between early life intelligence and mortality has been shown across different populations, in different countries, and in different epochs.[2][3][4]

"First, ...intelligence is associated with more education, and thereafter with more professional occupations

that might place the person in healthier environments. ...Second, people with higher intelligence might engage in more healthy behaviours. ...Third, mental test scores from early life might act as a record of insults to the brain that have occurred before that date. ...Fourth, mental test scores obtained in youth might be an indicator of a well-put-together system. It is hypothesized that a well-wired body is more able to respond effectively to environmental insults..."[5]

A study in Scotland found that 15-point lower IQ meant people had a fifth less chance of seeing their 76th birthday, while those with a 30-point disadvantage were 37% less likely than those with a higher IQ to live that long.[6]

Effects of IQ on health related behaviors

People with higher IQ have been found to smoke less, to drink alcohol less heavily, to eat better diets, and to be more physically active.[7][8]

One study found an "inverse linear association" between childhood intelligence and hospital admissions for injuries in adulthood. The association between childhood IQ and the risk of later injury remained after accounting for factors such as the child's socioeconomic background.[9]

A British study found that higher childhood IQ correlated with one's chance of becoming a vegetarian in adulthood.[10]

Dementia

Mental ability scores were significantly lower in children who eventually developed late-onset dementia when compared with other children tested.[11] Blacks in the US and the UK have a higher prevalence of dementia.[12][13]

A decrease in IQ has also been shown as an early predictor of late-onset Alzheimer's Disease and other forms of dementia. A 2004 study found that tests of cognitive ability provided useful predictive information up to a decade before the onset of dementia.[14] However, when diagnosing individuals with a higher level of cognitive ability,patients should not be diagnosed from the standard norm but from an adjusted high-IQ norm that measured changes against the individual's higher ability level.[15]

Psychiatric disorders

Lower IQ in childhood is associated with higher risk of many psychiatric disorders.[16][17][18][19]

Stress, such as caused by exposure to violence and psychological trauma, during childhood has been associated with lower IQ. It is unclear if it is the stress that causes the lower IQ or if having lower IQ and lower IQ parents causes increased risk of exposure to violence and psychological trauma. High IQ has also been found to protect post-traumatic stress disorder and behavorial problems after a trauma.[20][21][22][23]

Major depression, affecting about 16% of the population on at least one occasion in their lives and the leading cause of disability in North America, may give symptoms similar to dementia. Patients treated for depression score higher on IQ tests than before treatment.[24][25]

Myopia and hyperopia

A 2008 literature review writes that studies in several nations have found a relationship between myopia and higher IQ and between myopia and school achievement. Several, but not all, studies have found hyperopia to be associated with lower IQ and school achievements. A common explanation for myopia is near-work. Regarding the relationship to IQ, several explanations have been proposed. One is that the myopic child is better adapted at reading, and reads and studies more, which increases intelligence. The reverse explanation is that the intelligent and studious child reads more which causes myopia. Another is that the myopic child have an advantage at IQ testing which is near work because of less eye strain. Still another explanation is that pleiotropic gene(s) affect the size of both brain and eyes simultaneously. According to the two most recent studies in the review, IQ may be associated with myopia in schoolchildren, independent of books read per week.[21] Epidemiological ethnic studies show large differences in the prevalence of myopia, for example often very high prevalence among East Asian children, but the results have been argued to be inconsistent and largely reflect environmental factors.[26]

Other associations

Long working hours (55 vs. 40) was associated with reduced scores on cognitive tests in a 5-year study on midlife British civil servants.[27]

Higher intelligence was associated with higher quality sperm in one study.[28]

Health related factors that may lower IQ

A large number of factors have been argued, with varying empirical support, to lower intelligence. Such factors may be particularly or only important during pregnancy and childhood when the brain is growing and the blood-brain barrier is less effective. Such impairment during development may sometimes be permanent and irreversible, sometimes be partially or wholly compensated for by later growth. Such factors include:

  • Micronutrient deficiency, in particular iodine and iron deficiency.
  • Protein-calorie malnutrition.
  • Essential fatty acids malnutrition.
  • Lack of breastfeeding.
  • Intrauterine growth retardation.
  • Untreated endocrine disorders in mother or child.
  • Infectious diseases including parasites such as malaria, HIV, intestinal worms, and meningitis.
  • Environmental toxins such as lead, mercury, arsenic, and organochlorides.
  • Humans are being exposed a wide range of new chemicals, such as medicines, food additives, and industrial chemicals, with poorly understood long-term effects on intelligence, especially during childhood.
  • Recreational drugs such as tobacco, alcohol, and cannabis.
  • Birth complications such as asphyxia
  • Head injuries
  • Diseases related to aging such as stroke, cancer, and Alzheimer's disease.
  • Other diseases such as endocrine disorders or depression.

Improvements of such factors is one of the proposed explanations for the Flynn effect and may play some part regarding countries and intelligence differences. However, developed countries have usually developed policies regarding many of these factors such as laws requiring fortification of foods with micronutrients, health controls of pregnant mothers and infants, eradication of malaria and other infectious diseases, and banning of environmental toxins. This makes such factors relatively less important as an explanation for race and intelligence differences in the same country.

References

  1. Batty GD, Der G, Macintyre S, Deary IJ (2006) Does IQ explain socioeconomic inequalities in health? Evidence from a population based cohort study in the west of Scotland. BMJ 332 (7541):580-4. DOI:10.1136/bmj.38723.660637.AE PMID: 16452104
  2. Deary I (2008) Why do intelligent people live longer? Nature 456 (7219):175-6. DOI:10.1038/456175a PMID: 19005537
  3. Calvin, C. M., Deary, I. J., Fenton, C., Roberts, B. A., Der, G., Leckenby, N., & Batty, G. D. (2011). Intelligence in youth and all-cause-mortality: systematic review with meta-analysis. International Journal of Epidemiology, 40(3), 626-644. DOI:1093/ije/dyr079 PMID: 21037248
  4. Traynor Kirsten. Old and Wise. December 13, 2010. Scientific American http://www.scientificamerican.com/article.cfm?id=old-and-wise
  5. Hauser RM, Palloni A (2011)Adolescent IQ and survival in the Wisconsin longitudinal study. J Gerontol B Psychol Sci Soc Sci 66 Suppl 1 ():i91-101. DOI:10.1093/geronb/gbr037 PMID: 21743056
  6. Whalley LJ, Deary IJ (2001) Longitudinal cohort study of childhood IQ and survival up to age 76. BMJ 322 (7290):819. PMID: 11290633
  7. Taylor MD, Hart CL, Smith GD, Starr JM, Hole DJ, Whalley LJ et al. (2005) Childhood IQ and social factors on smoking behaviour, lung function and smoking-related outcomes in adulthood: linking the Scottish Mental Survey 1932 and the Midspan studies. Br J Health Psychol 10 (Pt 3):399-410. DOI:10.1348/135910705X25075 PMID: 16238855
  8. People with higher IQs live longer. 2:22PM GMT 16 Mar 2009. The Daily Telegraph. http://www.telegraph.co.uk/science/5000772/People-with-higher-IQs-live-longer.html
  9. Lawlor DA, Clark H, Leon DA (2007) Associations between childhood intelligence and hospital admissions for unintentional injuries in adulthood: the Aberdeen Children of the 1950s cohort study. Am J Public Health 97 (2):291-7. DOI:10.2105/AJPH.2005.080168 PMID: 17194859
  10. Gale CR, Deary IJ, Schoon I, Batty GD (2007) IQ in childhood and vegetarianism in adulthood: 1970 British cohort study. BMJ 334 (7587):245. DOI:10.1136/bmj.39030.675069.55 PMID: 17175567
  11. Khandaker GM, Barnett JH, White IR, Jones PB (2011) A quantitative meta-analysis of population-based studies of premorbid intelligence and schizophrenia. Schizophr Res 132 (2-3):220-7. DOI:10.1016/j.schres.2011.06.017 PMID: 21764562
  12. Adelman S, Blanchard M, Livingston G (2009) A systematic review of the prevalence and covariates of dementia or relative cognitive impairment in the older African-Caribbean population in Britain. Int J Geriatr Psychiatry 24 (7):657-65. DOI:10.1002/gps.2186 PMID: 19235788
  13. Sheffield KM, Peek MK (2011) Changes in the prevalence of cognitive impairment among older Americans, 1993-2004: overall trends and differences by race/ethnicity. Am J Epidemiol 174 (3):274-83. DOI:10.1093/aje/kwr074 PMID: 21622948
  14. Cervilla J, Prince M, Joels S, Lovestone S, Mann A (2004) Premorbid cognitive testing predicts the onset of dementia and Alzheimer's disease better than and independently of APOE genotype. J Neurol Neurosurg Psychiatry 75 (8):1100-6. DOI:10.1136/jnnp.2003.028076 PMID: 15258208
  15. Rentz DM, Huh TJ, Faust RR, Budson AE, Scinto LF, Sperling RA et al. (2004) Use of IQ-adjusted norms to predict progressive cognitive decline in highly intelligent older individuals.] Neuropsychology 18 (1):38-49. DOI:10.1037/0894-4105.18.1.38 PMID: [http://pubmed.gov/14744186 14744186
  16. Woodberry KA, Giuliano AJ, Seidman LJ (2008) Premorbid IQ in schizophrenia: a meta-analytic review. Am J Psychiatry 165 (5):579-87. DOI:10.1176/appi.ajp.2008.07081242 PMID: 18413704
  17. Koenen KC, Moffitt TE, Roberts AL, Martin LT, Kubzansky L, Harrington H et al. (2009) Childhood IQ and adult mental disorders: a test of the cognitive reserve hypothesis. Am J Psychiatry 166 (1):50-7. DOI:10.1176/appi.ajp.2008.08030343 PMID: 19047325
  18. Gale CR, Batty GD, Tynelius P, Deary IJ, Rasmussen F (2010) Intelligence in early adulthood and subsequent hospitalization for mental disorders.] Epidemiology 21 (1):70-7. DOI:10.1097/EDE.0b013e3181c17da8 PMID: 19907333
  19. Urfer-Parnas A, Lykke Mortensen E, Saebye D, Parnas J (2010) Pre-morbid IQ in mental disorders: a Danish draft-board study of 7486 psychiatric patients.] Psychol Med 40 (4):547-56. DOI:10.1017/S0033291709990754 PMID: [http://pubmed.gov/19656427 19656427
  20. Blair C (2006) How similar are fluid cognition and general intelligence? A developmental neuroscience perspective on fluid cognition as an aspect of human cognitive ability. Behav Brain Sci 29 (2):109-25; discussion 125-60. DOI:10.1017/S0140525X06009034 PMID: 16606477
  21. 21.0 21.1 Breslau N, Lucia VC, Alvarado GF (2006) Intelligence and other predisposing factors in exposure to trauma and posttraumatic stress disorder: a follow-up study at age 17 years. Arch Gen Psychiatry 63 (11):1238-45. DOI:10.1001/archpsyc.63.11.1238 PMID: 17088504 Cite error: Invalid <ref> tag; name "" defined multiple times with different content
  22. Delaney-Black V, Covington C, Ondersma SJ, Nordstrom-Klee B, Templin T, Ager J et al. (2002) Violence exposure, trauma, and IQ and/or reading deficits among urban children. Arch Pediatr Adolesc Med 156 (3):280-5. PMID: 11876674
  23. Saltzman KM, Weems CF, Carrion VG (2006) IQ and posttraumatic stress symptoms in children exposed to interpersonal violence. Child Psychiatry Hum Dev 36 (3):261-72. DOI:10.1007/s10578-005-0002-5 PMID: 16362242
  24. Sackeim HA, Freeman J, McElhiney M, Coleman E, Prudic J, Devanand DP (1992) Effects of major depression on estimates of intelligence. J Clin Exp Neuropsychol 14 (2):268-88. DOI:10.1080/01688639208402828 PMID: 1572949
  25. Mandelli L, Serretti A, Colombo C, Florita M, Santoro A, Rossini D et al. (2006) Improvement of cognitive functioning in mood disorder patients with depressive symptomatic recovery during treatment: an exploratory analysis. Psychiatry Clin Neurosci 60 (5):598-604. DOI:10.1111/j.1440-1819.2006.01564.x PMID: 16958944
  26. Pan CW, Ramamurthy D, Saw SM (2012) Worldwide prevalence and risk factors for myopia. Ophthalmic Physiol Opt 32 (1):3-16. DOI:10.1111/j.1475-1313.2011.00884.x PMID: 22150586
  27. Virtanen M, Singh-Manoux A, Ferrie JE, Gimeno D, Marmot MG, Elovainio M et al. (2009)Long working hours and cognitive function: the Whitehall II Study. Am J Epidemiol 169 (5):596-605. DOI:10.1093/aje/kwn382 PMID: 19126590
  28. [http://bodyodd.msnbc.msn.com/archive/2008/12/16/1718831.aspx Brainy guy, better sperm? Take that, tough guy!
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