Cognitive epidemiology
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Cognitive epidemiology is a field of research that examines the associations between intelligence test scores (IQ scores or extracted ''g''-factors) and health, more specifically morbidity (mental and physical) and mortality. Typically, test scores are obtained at an early age, and compared to later morbidity and mortality. In addition to exploring and establishing these associations, cognitive epidemiology seeks to understand causal relationships between intelligence and health outcomes. Researchers in the field argue that intelligence measured at an early age is an important predictor of later health and mortality 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. A study of one million Swedish men found "a strong link between cognitive ability and the risk of death." A similar study of 4,289 former US soldiers showed a similar relationship between IQ and
mortality Mortality is the state of being mortal, or susceptible to death; the opposite of immortality. Mortality may also refer to: * Fish mortality, a parameter used in fisheries population dynamics to account for the loss of fish in a fish stock throug ...
. The strong inverse correlation between intelligence and mortality has raised questions as to how better public education could delay mortality. There is a known positive correlation between socioeconomic position and health. A 2006 study found that controlling for IQ caused a marked reduction in this association. Research in
Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
has shown that a 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. Another Scottish study found that once individuals had reached old age (79 in this study), it was no longer childhood intelligence or current intelligence scores that best predicted mortality but the relative decline in cognitive abilities from age 11 to age 79. They also found that fluid abilities were better predictors of survival in old age than crystallized abilities. The relationship between childhood intelligence and mortality has even been found to hold for gifted children, those with an intelligence over 135. A 15-point increase in intelligence was associated with a decreased risk of mortality of 32%. This relationship was present until an intelligence score of 163 at which point there was no further advantage of a higher intelligence on mortality risk. A
meta-analysis A meta-analysis is a statistical analysis that combines the results of multiple scientific studies. Meta-analyses can be performed when there are multiple scientific studies addressing the same question, with each individual study reporting me ...
of the relationship between intelligence and mortality found that there was a 24% increase in mortality for a 1 SD (15 point) drop in IQ score. This meta-analysis also concluded that the association between intelligence and mortality was similar for men and women despite sex differences in disease prevalence and life expectancies. A whole population follow-up over 68 years showed that the association with overall mortality was also present for most major causes of death. The exceptions were cancers unrelated to smoking and suicide. There is also a strong inverse correlation between intelligence and adult morbidity. Long term sick leave in adulthood has been shown to be related to lower cognitive abilities, as has likelihood of receiving a disability pension.


Physical illness


Coronary heart disease

Among the findings of cognitive epidemiology is that men with a higher IQ have less risk of dying from
coronary heart disease Coronary artery disease (CAD), also called coronary heart disease (CHD), ischemic heart disease (IHD), myocardial ischemia, or simply heart disease, involves the reduction of blood flow to the heart muscle due to build-up of atherosclerotic pla ...
. The association is attenuated, but not removed, when controlling for
socio-economic Socioeconomics (also known as social economics) is the social science that studies how economic activity affects and is shaped by social processes. In general it analyzes how modern societies progress, stagnate, or regress because of their local ...
variables, such as educational attainment or income. This suggests that IQ may be an independent risk factor for mortality. One study found that low verbal, visuospatial and arithmetic scores were particularly good predictors of coronary heart disease.
Atherosclerosis Atherosclerosis is a pattern of the disease arteriosclerosis in which the wall of the artery develops abnormalities, called lesions. These lesions may lead to narrowing due to the buildup of atheroma, atheromatous plaque. At onset there are usu ...
or thickening of the artery walls due to fatty substances is a major factor in heart disease and some forms of stroke. It has also been linked to lower IQ.


Obesity

Lower intelligence in childhood and adolescence correlates with an increased risk of
obesity Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's we ...
. One study found that a 15-point increase in intelligence score was associated with a 24% decrease in risk of obesity at age 51. The direction of this relationship has been greatly debated with some arguing that obesity causes lower intelligence, however, recent studies have indicated that a lower intelligence increases the chances of obesity.


Blood pressure

Higher intelligence in childhood and adulthood has been linked to lower blood pressure and a lower risk of
hypertension Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high bl ...
.


Stroke

Strong evidence has been found in support of a link between intelligence and
stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...
, with those with lower intelligence being at greater risk of stroke. One study found visuospatial reasoning was the best predictor of stroke compared to other
cognitive tests Cognitive tests are assessments of the cognitive capabilities of humans and other animals. Tests administered to humans include various forms of IQ tests; those administered to animals include the mirror test (a test of visual self-awareness) and ...
. Further this study found that controlling for socioeconomic variables did little to attenuate the relationship between visuospatial reasoning and stroke.


Cancer

Studies exploring the link between
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
and intelligence have come to varying conclusions. A few studies, which were mostly small have found an increased risk of death from cancer in those with lower intelligence. Other studies have found an increased risk of skin cancer with higher intelligence. However, on the whole most studies have found no consistent link between cancer and intelligence.


Psychiatric


Bipolar disorder and intelligence

Bipolar disorder Bipolar disorder, previously known as manic depression, is a mental disorder characterized by periods of depression and periods of abnormally elevated mood that last from days to weeks each. If the elevated mood is severe or associated with ...
is a
mood disorder A mood disorder, also known as an affective disorder, is any of a group of conditions of mental and behavioral disorder where a disturbance in the person's mood is the main underlying feature. The classification is in the ''Diagnostic and Stat ...
characterized by periods of elevated mood known as
mania Mania, also known as manic syndrome, is a mental and behavioral disorder defined as a state of abnormally elevated arousal, affect, and energy level, or "a state of heightened overall activation with enhanced affective expression together wit ...
or
hypomania Hypomania (literally "under mania" or "less than mania") is a mental and behavioural disorder, characterised essentially by an apparently non-contextual elevation of mood ( euphoria) that contributes to persistently disinhibited behaviour. Th ...
and periods of depression. Anecdotal and biographical evidence popularized the idea that those with bipolar disorder are tormented geniuses that are uniquely equipped with high levels of
creativity Creativity is a phenomenon whereby something new and valuable is formed. The created item may be intangible (such as an idea, a scientific theory, a musical composition, or a joke) or a physical object (such as an invention, a printed literary w ...
and superior intelligence. Bipolar disorder is relatively rare, affecting only 2.5% of the population, as it is also the case with especially high intelligence. The uncommon nature of the disorder and rarity of high IQ pose unique challenges in sourcing large enough samples that are required to conduct a rigorous analysis of the association between intelligence and bipolar disorder. Nevertheless, there has been much progress starting from the mid-90s, with several studies beginning to shed a light on this elusive relationship. One such study examined individual compulsory school grades of Swedish pupils between the ages of 15 and 16 to find that individuals with excellent school performance had a nearly four times increased rate to develop a variation of bipolar disorder later in life than those with average grades. The same study also found that students with lowest grades were at a moderately increased risk of developing bipolar disorder with nearly a twofold increase when compared to average-grade students. A New Zealand study of 1,037 males and females from the 1972–1973
birth cohort Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...
of Dunedin suggests that lower childhood IQs were associated with an increased risk of developing
schizophrenia spectrum disorders A spectrum disorder is a mental disorder that includes a range of linked conditions, sometimes also extending to include singular symptoms and trait theory, traits. The different elements of a spectrum either have a similar appearance or are though ...
,
major depression Major depressive disorder (MDD), also known as clinical depression, is a mental disorder characterized by at least two weeks of pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. Introdu ...
, and
generalized anxiety disorder Generalized anxiety disorder (GAD) is a mental and behavioral disorder, specifically an anxiety disorder characterized by excessive, uncontrollable and often irrational worry about events or activities. Worry often interferes with daily function ...
in adulthood; whereas higher childhood IQ predicted an increased likelihood of mania. This study only included eight cases of mania and thus should only be used to support already existing trends. In the largest study yet published analyzing the relationship between bipolar disorder and intelligence,
Edinburgh University The University of Edinburgh ( sco, University o Edinburgh, gd, Oilthigh Dhùn Èideann; abbreviated as ''Edin.'' in post-nominals) is a public research university based in Edinburgh, Scotland. Granted a royal charter by King James VI in 1582 ...
researchers looked at the link between intelligence and bipolar disorder in a sample of over one million men enlisted in the Swedish army during a 22-year follow-up period. Regression results showed that the risk of hospitalization for bipolar disorder with
comorbidity In medicine, comorbidity - from Latin morbus ("sickness"), co ("together"), -ity (as if - several sicknesses together) - is the presence of one or more additional conditions often wikt:co-occur#Verb, co-occurring (that is, wikt:concomitant#Adjecti ...
to other mental health illnesses decreased in a linear pattern with an increase in IQ. However, when researchers restricted the analysis to men without any psychiatric comorbidity, the relationship between bipolar disorder and intelligence followed a J-curve. These findings suggest that men of extremely high intelligence are at a higher risk of experiencing bipolar in its purest form, and demands future investigation of the correlation between extreme brightness and pure bipolar. Additional support of a potential association between high intelligence and bipolar disorder comes from biographical and anecdotal evidence, and primarily focus on the relationship between
creativity and bipolar disorder Links between creativity and mental health have been extensively discussed and studied by psychologists and other researchers for centuries. Parallels can be drawn to connect creativity to major mental disorders including bipolar disorder, schiz ...
. Doctor Kay Redfield Jamison has been a prolific writer on the subject publishing several articles and an extensive book analyzing the relationship between the artistic temperament and mood disorders. Although a link between bipolar disorder and creativity has been established, there is no confirming evidence suggesting any significant relationship between creativity and intelligence. Additionally, even though some of these studies suggest a potential benefit to bipolar disorder in regards to intelligence, there is significant amount of controversy as to the individual and societal cost of this presumed intellectual advantage. Bipolar disorder is characterized by periods of immense pain and suffering, self-destructive behaviors, and has one of the highest mortality rates of all mental illnesses.


Schizophrenia and cognition

Schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social withdra ...
is chronic and
disabling Disability is the experience of any condition that makes it more difficult for a person to do certain activities or have equitable access within a given society. Disabilities may be cognitive, developmental, intellectual, mental, physical, ...
mental illness A mental disorder, also referred to as a mental illness or psychiatric disorder, is a behavioral or mental pattern that causes significant distress or impairment of personal functioning. Such features may be persistent, relapsing and remitti ...
that is characterized by abnormal behavior,
psychotic episodes Psychosis is a condition of the mind that results in difficulties determining what is real and what is not real. Symptoms may include delusions and hallucinations, among other features. Additional symptoms are incoherent speech and behavior ...
and inability to distinguish between reality and
fantasy Fantasy is a genre of speculative fiction involving Magic (supernatural), magical elements, typically set in a fictional universe and sometimes inspired by mythology and folklore. Its roots are in oral traditions, which then became fantasy ...
. Even though schizophrenia can severely impair those with the disorder, there has been a great interest in the relationship of this disorder and intelligence. Interest in the association of intelligence and schizophrenia has been widespread partly stems from the perceived connection between schizophrenia and creativity, and
posthumous Posthumous may refer to: * Posthumous award - an award, prize or medal granted after the recipient's death * Posthumous publication – material published after the author's death * Posthumous (album), ''Posthumous'' (album), by Warne Marsh, 1987 * ...
research of famous intellectuals that have been insinuated to have had the illness. Hollywood played a pivotal role popularizing the myth of the schizophrenic genius with the movie A Beautiful Mind that depicted the life story of
Nobel Laureate The Nobel Prizes ( sv, Nobelpriset, no, Nobelprisen) are awarded annually by the Royal Swedish Academy of Sciences, the Swedish Academy, the Karolinska Institutet, and the Norwegian Nobel Committee to individuals and organizations who make out ...
, John Nash and his struggle with the illness. Although stories of extremely bright individuals with schizophrenia such as that of John Nash do exist, they are the outliers and not the norm. Studies analyzing the association between schizophrenia and intelligence overwhelmingly suggest that schizophrenia is linked to lower intelligence and decreased cognitive functioning. Since the manifestation of schizophrenia is partly characterized by cognitive and motor declines, current research focuses on understanding premorbid IQ patterns of schizophrenia patients. In the most comprehensive
meta-analysis A meta-analysis is a statistical analysis that combines the results of multiple scientific studies. Meta-analyses can be performed when there are multiple scientific studies addressing the same question, with each individual study reporting me ...
published since the groundbreaking study by Aylward et al. in 1984, researchers at
Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...
found a medium-sized deficit in global cognition prior to the onset of schizophrenia. The mean premorbid IQ estimate for schizophrenia samples was 94.7 or 0.35
standard deviations In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values. A low standard deviation indicates that the values tend to be close to the mean (also called the expected value) of the set, while ...
below the mean, and thus at the lower end of the average IQ range. Additionally, all studies containing reliable premorbid and post-onset IQ estimates of schizophrenia patients found significant decline in IQ scores when comparing premorbid IQ to post-onset IQ. However, while the decline in IQ over the course of the onset of schizophrenia is consistent with theory, some alternative explanations for this decline suggested by the researchers include the clinical state of the patients and/or side effects of antipsychotic medications. A recent study published in March 2015 edition of the
American Journal of Psychiatry ''The American Journal of Psychiatry'' is a monthly peer-reviewed medical journal covering all aspects of psychiatry, and is the official journal of the American Psychiatric Association. The first volume was issued in 1844, at which time it was k ...
suggests that not only there is no correlation between high IQ and schizophrenia, but rather that a high IQ may be protective against the illness. Researchers from the
Virginia Commonwealth University Virginia Commonwealth University (VCU) is a public research university in Richmond, Virginia. VCU was founded in 1838 as the medical department of Hampden–Sydney College, becoming the Medical College of Virginia in 1854. In 1968, the Virgini ...
analyzed IQ data from over 1.2 million Swedish males born between 1951 and 1975 at ages 18 to 20 years old to investigate future risk of schizophrenia as a function of IQ scores. The researchers created stratified models using pairs of relatives to adjust for family clusters and later applied
regression models Regression or regressions may refer to: Science * Marine regression, coastal advance due to falling sea level, the opposite of marine transgression * Regression (medicine), a characteristic of diseases to express lighter symptoms or less extent ( ...
to examine the interaction between IQ and
genetic predisposition A genetic predisposition is a genetic characteristic which influences the possible phenotypic development of an individual organism within a species or population under the influence of environmental conditions. In medicine, genetic susceptibilit ...
to schizophrenia. Results from the study suggest that subjects with low IQ were more sensitive to the effect of genetic liability to schizophrenia than those with high IQ and that the relationship between IQ and schizophrenia is not a consequence of shared genetic or familial-environmental
risk factors In epidemiology, a risk factor or determinant is a variable associated with an increased risk of disease or infection. Due to a lack of harmonization across disciplines, determinant, in its more widely accepted scientific meaning, is often use ...
, but may instead be causal.


Post-traumatic stress disorder and traumatic exposure

The ''Archive of General Psychiatry'' published a
longitudinal study A longitudinal study (or longitudinal survey, or panel study) is a research design that involves repeated observations of the same variables (e.g., people) over short or long periods of time (i.e., uses longitudinal data). It is often a type of obs ...
of a randomly selected sample of 713 study participants (336 boys and 377 girls), from urban and suburban settings. Of that group, nearly 76 percent had had at least one traumatic event. Those participants were assessed at age 6 years and followed up to age 17 years. In that group of children, those with an IQ above 115 were significantly less likely to have Post-Traumatic Stress Disorder as a result of the trauma, less likely to display behavioral problems, and less likely to experience a trauma. The low incidence of Post-Traumatic Stress Disorder among children with higher IQs was true even if the child grew up in an urban environment (where trauma averaged three times the rate of the suburb), or had behavioral problems.


Other disorders

Post-traumatic stress disorder Post-traumatic stress disorder (PTSD) is a mental and behavioral disorder that can develop because of exposure to a traumatic event, such as sexual assault, warfare, traffic collisions, child abuse, domestic violence, or other threats on ...
, severe depression, and
schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social withdra ...
are less prevalent in higher IQ bands. Some studies have found that higher IQ persons show a higher prevalence of
Obsessive Compulsive Disorder Obsession may refer to: Psychology * Celebrity worship syndrome, obsessive addictive disorder to a celebrity's personal and professional life * Fixation (psychology), a persistent attachment to an object or idea * Idée fixe (psychology), a preo ...
, but a 2017 meta study found the opposite, that people with OCD had slightly lower average IQs.


Substance abuse

Substance abuse Substance abuse, also known as drug abuse, is the use of a drug in amounts or by methods which are harmful to the individual or others. It is a form of substance-related disorder. Differing definitions of drug abuse are used in public health, ...
is a patterned use of drug consumption in which a person uses substances in amounts or with methods that are harmful to themselves or to others. Substance abuse is commonly associated with a range of
maladaptive behavior Adaptive behavior is behavior that enables a person (usually used in the context of children) to cope in their environment with greatest success and least conflict with others. This is a term used in the areas of psychology and special education. ...
s that are both detrimental to the individual and to society. Given the terrible consequences that can transpire from abusing substances, recreational experimentation and/or recurrent use of drugs are traditionally thought to be most prevalent among marginalized strands of society. Nevertheless, the very opposite is true; research both in national and individual levels have found that the relationship between IQ and substance abuse indicates positive correlations between superior intelligence, higher alcohol consumption and drug consumption. The relationship between childhood IQ scores and
illegal drugs The prohibition of drugs through sumptuary legislation or religious law is a common means of attempting to prevent the recreational use of certain intoxicating substances. While some drugs are illegal to possess, many governments regulate the ...
use by adolescence and middle age has been found. High IQ scores at age 10 are positively associated with intake of
cannabis ''Cannabis'' () is a genus of flowering plants in the family Cannabaceae. The number of species within the genus is disputed. Three species may be recognized: ''Cannabis sativa'', '' C. indica'', and '' C. ruderalis''. Alternatively ...
,
cocaine Cocaine (from , from , ultimately from Quechuan languages, Quechua: ''kúka'') is a central nervous system (CNS) stimulant mainly recreational drug use, used recreationally for its euphoria, euphoric effects. It is primarily obtained from t ...
(only after 30 years of age), ecstasy,
amphetamine Amphetamine (contracted from alpha- methylphenethylamine) is a strong central nervous system (CNS) stimulant that is used in the treatment of attention deficit hyperactivity disorder (ADHD), narcolepsy, and obesity. It is also commonly used ...
and
polydrug Polysubstance use or poly drug use or refers to the use of combined psychoactive substances. Polysubstance use may be used for entheogenic, recreational, or off-label indications, with both legal and illegal substances. In many cases one d ...
and also highlight a stronger association between high IQ and drug use for women than men. Additionally, these findings are independent of
socio-economic status Socioeconomic status (SES) is an economic and sociological combined total measure of a person's work experience and of an individual's or family's economic access to resources and social position in relation to others. When analyzing a family's ...
or
psychological distress Mental distress or psychological distress encompasses the symptoms and experiences of a person's internal life that are commonly held to be troubling, confusing or out of the ordinary. Mental distress can potentially lead to a change of behavior, a ...
during formative years. A high IQ at age 11 was predictive of increased
alcohol dependency Alcohol dependence is a previous (DSM-IV and ICD-10) psychiatric diagnosis in which an individual is physically or psychologically dependent upon alcohol (also chemically known as ethanol). In 2013, it was reclassified as alcohol use disorder ...
later in life and a one standard deviation increase in IQ scores (15-points) was associated with a higher risk of illegal drug use. The counterintuitive nature of the correlation between high IQ and substance abuse has sparked a fervent debate in the scientific community with some researchers attributing these findings to IQ being an inadequate proxy of intelligence, while others fault employed
research methodologies In its most common sense, methodology is the study of research methods. However, the term can also refer to the methods themselves or to the philosophical discussion of associated background assumptions. A method is a structured procedure for bri ...
and unrepresentative data. However, with the increased number of studies publishing similar results, overwhelming consensus is that the association between high IQ and substance abuse is real, statistically significant and independent of other variables. There are several competing theories trying to make sense of this apparent paradox. Doctor James White postulates that people with higher IQs are more critical of information and thus less likely to accept facts at face value. While marketing campaigns against drugs may deter individuals with lower IQs from using drugs with disjoint arguments or overexaggeration of negative consequences, people with a higher IQ will seek to verify the validity of such claims in their immediate environment. White also eludes to an often-overlooked problem of people with higher IQ, the lack of adequate challenges and intellectual stimulation. White posits that high IQ individuals that are not sufficiently engaged in their lives may choose to forgo good judgment for the sake of stimulation. The most prominent theory attempting to explain the positive relationship between IQ and substance abuse; however, is the Savanna–IQ interaction hypothesis by
social psychologist Social psychology is the scientific study of how thoughts, feelings, and behaviors are influenced by the real or imagined presence of other people or by social norms. Social psychologists typically explain human behavior as a result of the re ...
Satoshi Kanazawa Satoshi Kanazawa (born 16 November 1962) is an American-born British evolutionary psychologist and writer. He is currently Reader in Management at the London School of Economics. Kanazawa's comments and research on race and intelligence, healt ...
. The theory is founded on the assumption that intelligence is a
domain-specific Domain specificity is a theoretical position in cognitive science (especially modern cognitive development) that argues that many aspects of cognition are supported by specialized, presumably evolutionarily specified, learning devices. The posit ...
adaptation that has
evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
as humans moved away from the birthplace of human race, the
savanna A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to ...
. Therefore, theory follows that as humans explored beyond the savannas, intelligence rather than
instinct Instinct is the inherent inclination of a living organism towards a particular complex behaviour, containing both innate (inborn) and learned elements. The simplest example of an instinctive behaviour is a fixed action pattern (FAP), in which a v ...
dictated
survival Survival, or the act of surviving, is the propensity of something to continue existing, particularly when this is done despite conditions that might kill or destroy it. The concept can be applied to humans and other living things (or, hypotheti ...
.
Natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
privileged those who possessed high IQ while simultaneously favoring those with an appetite for evolutionary novel behaviors and experiences. For Kanazawa, this drive to seek evolutionary novel activities and sensations translates to being more open and callous about experimenting with and/or abusing substances in modern culture.


Dementia

A decrease in IQ has also been shown as an early predictor of late-onset
Alzheimer's disease Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...
and other forms of
dementia Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affe ...
. In a 2004 study, Cervilla and colleagues showed that tests of cognitive ability provide useful predictive information up to a decade before the onset of dementia. However, when diagnosing individuals with a higher level of cognitive ability, a study of those with IQs of 120 or more, 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. In 2000, Whalley and colleagues published a paper in the journal ''Neurology'', which examined links between childhood mental ability and late-onset dementia. The study showed that mental ability scores were significantly lower in children who eventually developed late-onset dementia when compared with other children tested.


Health related behaviors


Alcohol

The relationship between alcohol consumption and intelligence is not straightforward. In some cohorts higher intelligence has been linked to a reduced risk of binge drinking. In one Scottish study higher intelligence was linked to a lower chance of binge drinking; however, units of alcohol consumed were not measured and alcohol induced hangovers in middle age were used as a proxy for binge drinking. Several studies have found the opposite effect with individuals of higher intelligence being more likely to drink more frequently, consume more units and have a higher risk of developing a drinking problem, especially in women.


Drugs

In U.S. study the link between drug intake and intelligence suggests that individuals with lower IQ take more drugs. However, in the UK the opposite relationship has been found with higher intelligence being related to greater illegal drug use.


Smoking

The relationship between intelligence and smoking has changed along with public and government attitudes towards smoking. For people born in 1921 there was no correlation between intelligence and having smoked or not smoked; however, there was a relationship between higher intelligence and quitting smoking by adulthood. In another British study, high childhood IQ was shown to inversely correlate with the chances of starting smoking.


Diet

One British study found that high childhood IQ was shown to correlate with one's chance of becoming a
vegetarian Vegetarianism is the practice of abstaining from the consumption of meat (red meat, poultry, seafood, insects, and the flesh of any other animal). It may also include abstaining from eating all by-products of animal slaughter. Vegetarianism m ...
in adulthood. Those of higher intelligence are also more likely to eat a healthier diet including more fruit and vegetables, fish, poultry and wholemeal bread and to eat less fried food.


Exercise

Higher intelligence has been linked to exercising. More intelligent children tend to exercise more as adults and to exercise vigorously. A study of 11,282 individuals in Scotland who took intelligence tests at ages 7, 9 and 11 in the 1950s and 1960s, 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 even after accounting for factors such as the child's socioeconomic background.


Socioeconomic status

Practically all indicators of
physical health Health, according to the World Health Organization, is "a state of complete physical, mental and social well-being and not merely the absence of disease and infirmity".World Health Organization. (2006)''Constitution of the World Health Organiza ...
and mental competence favour people of higher
socioeconomic status Socioeconomic status (SES) is an economic and sociological combined total measure of a person's work experience and of an individual's or family's economic access to resources and social position in relation to others. When analyzing a family's ...
(SES). Social class attainment is important because it can predict
health Health, according to the World Health Organization, is "a state of complete physical, mental and social well-being and not merely the absence of disease and infirmity".World Health Organization. (2006)''Constitution of the World Health Organiza ...
across the lifespan, where people from
lower social class A social class is a grouping of people into a set of hierarchical social categories, the most common being the upper, middle and lower classes. Membership in a social class can for example be dependent on education, wealth, occupation, inco ...
have higher
morbidity A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
and mortality. SES and health outcomes are general across time, place, disease, and are finely graded up the SES continuum. Gottfredson argues that
general intelligence The ''g'' factor (also known as general intelligence, general mental ability or general intelligence factor) is a construct developed in psychometric investigations of cognitive abilities and human intelligence. It is a variable that summarizes ...
(g) is the fundamental cause for
health inequality Health equity arises from access to the social determinants of health, specifically from wealth, power and prestige. Individuals who have consistently been deprived of these three determinants are significantly disadvantaged from health inequiti ...
. The argument is that g is the fundamental cause of social class inequality in health, because it meets six criteria that every candidate for the cause must meet: stable distribution over time, is replicable, is a transportable form of influence, has a general effect on health, is measurable, and is falsifiable. Stability: Any casual agent has to be persistent and stable across time for its pattern of effects to be general over ages and decades. Large and stable
individual differences Differential psychology studies the ways in which individuals differ in their behavior and the processes that underlie it. This is a discipline that develops classifications (taxonomies) of psychological individual differences. This is distingui ...
in g are developed by adolescence and the dispersion of g in population's intelligence present in every
generation A generation refers to all of the people born and living at about the same time, regarded collectively. It can also be described as, "the average period, generally considered to be about 20–⁠30 years, during which children are born and gr ...
, no matter what social circumstances are present. Therefore, equalizing socioeconomic environments does very little to reduce the dispersion in IQ. The dispersion of IQ in a society in general is more stable, than its dispersion of socioeconomic status.
Replicability Reproducibility, also known as replicability and repeatability, is a major principle underpinning the scientific method. For the findings of a study to be reproducible means that results obtained by an experiment or an observational study or in a ...
: Siblings who vary in IQ also vary in socioeconomic success which can be comparable with strangers of comparable IQ. Also, g theory predicts that if genetic g is the principal mechanism carrying socioeconomic inequality between generations, then the maximum
correlation In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...
between the parent and child SES will be near to their
genetic correlation In multivariate quantitative genetics, a genetic correlation (denoted r_g or r_a) is the proportion of variance that two traits share due to genetic causes, the correlation between the genetic influences on a trait and the genetic influences on a di ...
for IQ (.50). Transportability: The performance and
functional literacy Functional illiteracy consists of reading and writing skills that are inadequate "to manage daily living and employment tasks that require reading skills beyond a basic level". Those who read and write only in a language other than the predominant ...
studies both illustrated how g is transportable across life situations and it represents a set of largely generalizable reasoning and problem-solving skills. G appear to be linearly linked to performance in school, jobs and achievements. Generality: Studies show that IQ measured at the age of 11 predicted longevity, premature death, lung and stomach cancers,
dementia Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affe ...
, loss of functional independence, more than 60 years later. Research has shown that higher IQ at age 11 is significantly related to higher social class in midlife. Therefore, it is safe to assume that higher SES, as well as higher IQ, generally predicts better health. Measurability: g factor can be extracted from any broad set of mental tests and has provided a common, reliable source for measuring general intelligence in any population. Falsifiability: theoretically, if g theory would conceive health self-care as a job, as a set of instrumental tasks performed by the individuals, it could predict g to influence the health performance in the same way as it predicts performance in education and job.
Chronic illnesses A chronic condition is a health condition or disease that is persistent or otherwise long-lasting in its effects or a disease that comes with time. The term ''chronic'' is often applied when the course of the disease lasts for more than three mon ...
are the major illnesses in developed countries today, and their major risk factors are health
habits A habit (or wont as a humorous and formal term) is a routine of behavior that is repeated regularly and tends to occur subconsciously.
and lifestyle. The higher
social strata Social stratification refers to a society's categorization of its people into groups based on socioeconomic factors like wealth, income, race, education, ethnicity, gender, occupation, social status, or derived power (social and political). As ...
knows the most and the lower social strata knows the least, whether class is assessed by education, occupation or income and even when the information seems to be most useful for the poorest. Higher g promotes more learning, and it increases exposure to learning opportunities. So, the problem is not in the lack of access to health-care, but the patient's failure to use it effectively when delivered. Low literacy has been associated with low use of preventive care, poor comprehension of one's illness – even when care is free. Health self-management is important because literacy provides the ability to acquire new information and complete complex tasks and that limited problem solving abilities make low-literacy patients less likely to change their behaviour on the basis of new information. Chronic lack of good judgement and effective reasoning leads to chronically poor self-management.


Explanations of the correlation between intelligence and health

There have been many reasons posited for the links between health and intelligence. Although some have argued that the direction is one in which health has an influence on intelligence, most have focused on the influence of intelligence on health. Although health may definitely affect intelligence, most of the cognitive epidemiological studies have looked at intelligence in childhood when ill health is far less frequent and a more unlikely cause of poor intelligence. Thus most explanations have focused on the effects intelligence has on health through its influence on mediating causes. Various explanations for these findings have been proposed:


System integrity hypothesis vs evolution hypothesis

The System integrity hypothesis posits that childhood intelligence is just one aspect of a well wired and well-functioning body and suggests that there is a latent trait that encompasses intelligence, health and many other factors. This trait indexes how well the body is functioning and how well the body can respond to change and return to a normal balance again (
allostatic load Allostatic load is "the wear and tear on the body" which accumulates as an individual is exposed to repeated or chronic stress. The term was coined by Bruce McEwen and Eliot Stellar in 1993. It represents the physiological consequences of chro ...
). According to the system integrity hypothesis lower IQ does not cause mortality but instead poor system integrity causes lower intelligence and poorer health as well as a range of other traits which can be thought of as markers of system integrity. Professor Ian Deary has proposed that fluctuating asymmetry, speed of information processing, physical co-ordination, physical strength, metabolic syndrome and genetic correlation may be further potential markers of system integrity which by definition should explain a large part of or nullify the relationship between intelligence and mortality. An opposing theory to the system integrity theory is the evolutionary novelty theory which suggests that those with higher intelligence are better equipped to deal with evolutionary novel events. It is proposed that intelligence evolved to tackle evolutionarily novel situations and that those with a higher IQ are better able to process when such a novel situation is dangerous or a health hazard and thus are likely to be in better health. This theory provides a theoretical background for evidence found that supports the idea that intelligence is related to mortality through health behaviours such as wearing a seatbelt or quitting smoking. Evolutionary novelty theory emphasises the role of behaviour in the link between mortality and intelligence whereas system integrity emphasis the role of genes. Thus system integrity predicts that individuals of higher intelligence will be better protected from diseases that are caused primarily by genetics whereas evolutionary adaptive theory suggests that individuals of higher intelligence will be better protected from diseases that are less heritable and are caused by poor life choices. One study which tested this idea looked at the incidence of heritable and non-heritable cancers in individuals of differing levels of intelligence. They found that those of higher intelligence were less likely to have cancer that was not heritable, that was based on lifestyle, thus supporting the evolutionary novelty theory. However this was only a preliminary study and only included the disease cancer, which has been found in previous studies to have an ambiguous relationship with intelligence.


Disease and injury prevention

Having higher intelligence scores may mean that individuals are better at preventing disease and injury. Their cognitive abilities may equip them with a better propensity for understanding the injury and health risks of certain behaviours and actions. Fatal and non-fatal accidental injury have been associated with lower intelligence. This may be because individuals of higher intelligence are more likely to take precautions such as wearing seat belts, helmets etc. as they are aware of the risks. Further there is evidence that more intelligent people behave in a healthier way. Individuals with higher cognitive abilities are also better equipped for dealing with stress, a factor that has been implemented in many health problems ranging from anxiety to cardiovascular disease. It has been suggested that higher intelligence leads to a better sense of control over one's own life and a reduction in feelings of stress. One study found that individuals with lower intelligence experienced a greater number of functional somatic symptoms, symptoms that cannot be explained by organic pathology and are thought to be stress related. However most of the relationship was mediated by work conditions.


Disease and injury management

There is evidence that higher intelligence is related to better self-care when one has an illness or injury. One study asked participants to take aspirin or a placebo on a daily basis during a study on cardiovascular health. Participants with higher intelligence persisted with taking the medication for longer than those with lower intelligence indicating that they could care for themselves better. Studies have shown that individuals with lower intelligence have lower health literacy and a study looking at the link between health literacy and actual health found that it was mediated almost entirely by intelligence. It has been claimed that up to a third of medications are not taken correctly and thus jeopardize the patients' health. This is particularly relevant for those with heart problems as the misuse of some heart medications can actually double the risk of death. More intelligent individuals also make use of preventative healthcare more often for example visiting the doctors. Some have argued however that this is an artefact of higher SES; that those with lower intelligence tend to be from a lower social class and have less access to medical facilities. However it has been found that even when access to healthcare is equal, those with lower intelligence still make less use of the services.


Psychiatric illness

A diagnosis of any mental illness, even mild psychological distress is linked to an increased risk of illness and premature death. The majority of psychiatric illness' are also linked to lower intelligence. Thus it has been proposed that psychiatric morbidity may be another pathway through which intelligence and mortality are related. Despite this the direction of causation between Intelligence and mental health issues has been disputed. Some argue that mental health issues such as depression and schizophrenia may cause a decline in mental functioning and thus scores on intelligence tests, while others believe that lower intelligence increases the likelihood of developing a mental health issue. Although evidence for both points of view has been found, most of the cognitive epidemiological studies are carried out using intelligence scores from childhood, when the psychiatric condition was not present, ensuring that it was not the condition which caused the lower scores. This link has been shown to explain part of the relationship between childhood intelligence and mortality, however the amount of variance explained varies from less than 10 percent to about 5 percent.


Socioeconomic position in adulthood

Although childhood economic status may be seen as a confounder in the relationship between intelligence and mortality, as it is likely to affect intelligence, it is likely that adult SES mediates the relationship. The idea is that intelligent children will find themselves getting a better education, better jobs and will settle in a safer and healthier environment. They will have better access to health resources, good nutrition and will be less likely to experience the hazards and health risks associated with living in poorer neighbourhoods. Several studies have found that there is an association between adult SES and mortality.


Proposed general fitness factor of both cognitive ability and health, the ''f''-factor

Because of the above-mentioned findings, some researchers have proposed a general factor of fitness analogous to the g-factor for general mental ability/intelligence. This factor is supposed to combine fertility factors, health factors, and the g-factor. For instance, one study found a small but significant correlation between three measures of sperm quality and intelligence.


See also

*
Outline of human intelligence The following outline is provided as an overview of and topical guide to human intelligence: Human intelligence is, in the human species, the mental capacities to learn, understand, and reason, including the capacities to comprehend ideas, plan ...
* Accident-proneness *
Passive-aggressive behavior Passive-aggressive behavior is characterized by a pattern of passive hostility and an avoidance of direct communication. Inaction where some action is socially customary is a typical passive-aggressive strategy (showing up late for functions, st ...


References


Further reading

* {{Longevity Epidemiology Intelligence