Findings from longitudinal studies suggest that poorer performance on tests of cognitive ability in childhood or adolescence may indicate vulnerability to some forms of mental disorder. Lower scores on standard tests of intelligence have consistently been associated with an increased risk of schizophrenia,1 and, more recently, with risk of post-traumatic stress disorder.2–5 There is less evidence on whether cognitive ability in early life is related to risk of other specific mental disorders, although some studies have found links between poorer cognition and later onset of severe depression,3,6,7 generalized anxiety disorder,3,7,8 social phobia,7 nonaffective psychoses,6 and abuse of or dependence on alcohol or drugs.3,9
Observations in a Danish cohort suggest that IQ scores may indicate risk for a wider range of mental disorders.10 In that small study, comparison of IQ test scores at around age 19 years showed that men subsequently admitted to hospital for schizophrenia, other psychoses, personality disorders, adjustment disorders, alcohol, and other substance use disorders had previously achieved lower scores than those who had not been hospitalized.
It is unclear whether cognitive ability in early life is a risk factor not just for the occurrence of mental disorder but also for severity of illness. Among 806 men admitted to hospital for psychoses, no associations were found between IQ scores in late adolescence and subsequent number of hospital admissions.6 However, there is evidence that people with a lower IQ in childhood are more likely to have contact with psychiatric services.11 This observation, coupled with findings linking IQ with several forms of mental disorder,3,10 and with comorbidity,3,7 suggests that lower IQ may be a risk factor for more severe illness. People with psychiatric comorbidity experience higher levels of distress,12 greater functional disability,13 greater use of psychiatric services,12,14 and longer duration of illness.15
We used data on more than one million Swedish men who took tests of cognitive ability in early adulthood and were followed up for psychiatric hospital admissions for an average of 22.6 years. We examined the relation of IQ with risk of admission and with admission rates for mental disorders across the diagnostic spectrum.
METHODS
Study Participants and Record-linkage of Registers
The record linkage methods used to generate this cohort study have been reported previously.16,17 In brief, the cohort comprised all nonadopted men born in Sweden between 1950 and 1976 for whom both biologic parents could be identified in the Multi-Generation Register. Using unique personal identification numbers, we linked the Multi-Generation Register with the Military Service Conscription Register, Population, and Housing Censuses records (1960 and 1970), the Cause of Death Register, and the National Hospital Discharge Register. This resulted in 1,346,545 successful matches. Study approval was obtained from the Regional Ethics Committee, Stockholm.
Conscription Examination
The military service conscription examination involves a structured, standard medical assessment of physical and mental health, and cognitive function. During the years covered by this study, the law required this examination; only men of foreign citizenship or those with severe disability were excused. This dataset covers examinations from 15 September 1969 to 31 December 1994, after which procedures for measuring IQ changed. Average age at examination was 18.3 years (range, 16–25).
IQ was measured by 4 written subtests representing verbal, logical, spatial, and technical abilities.6 All test scores—including a total IQ score derived from summing the subtest results—were standardized to give a Gaussian-distributed score between 1 and 9. Higher values indicate greater intellectual capacity. The logic test measures capacity to understand written instructions and apply them to solving a problem. Items from the spatial test depicted a plan drawing of an object in its preassembled, 2-dimensional state. Respondents were asked to identify, from drawings of fully-assembled, 3-dimensional objects, which it represented. The verbal test measures knowledge of synonyms. Respondents were asked to determine which of 4 alternatives is the synonym of a given word. The technical test measures knowledge of chemistry and physics and implies the assessment of a component of general knowledge. Correlation coefficients among the 4 IQ subtests ranged from 0.48 to 0.70. As associations between the subtests and the study outcomes were very similar to those obtained using the total IQ score, we present results for total IQ score only.
Men underwent a medical examination, including a structured interview by a psychologist. A psychiatrist assessed men who reported psychiatric symptoms. Diagnoses were made according to the International Classification of Disease (ICD) versions 8 or 9.
Inpatient Episodes
The Swedish National Hospital Discharge Register has covered virtually all in-patient care for psychiatric disorders (including hospital admissions at forensic psychiatric clinics) since 1973, with the exception of admissions in a few counties during some of the early years of data collection.18 Admissions were coded according to the International Classification of Disease versions 8, 9, or 10. We extracted data on psychiatric admissions from 1968 to 31 December 2004. We grouped diagnoses (primary and secondary) into the following categories: schizophrenia (ICD-8/9: 295; ICD-10: F20); other nonaffective psychotic disorders (excluding alcohol or drug psychoses) (ICD-8/9: 297, 298.2–9; ICD-10: F21–F29); mood disorders (ICD-8/9: 296, 298.0, 298.1; ICD-9: 311; ICD-10: F30–F39); neurotic and somatoform disorders (excluding adjustment disorders) (ICD-8: 305; ICD-8/9: 300; ICD-9:306; ICD-10: F40–F42 and F44–F49); adjustment disorders (ICD-8: 307, 308.4; ICD-9: 308–309; ICD-10: F43); personality disorders (ICD-8/9: 301; ICD-10: F60–F61, F69); alcohol-related disorders (including alcoholic psychoses) (ICD-8/9: 291, 303; ICD-9: 305.0; ICD-10: F10); other substance use disorders (including drug psychoses) (ICD-8: 294.3; ICD-9: 292, 305.1–8; ICD-8/9: 304; ICD-10: F11–F19; and all diagnoses of mental disorder not included in the above 8 categories, but excluding diagnoses of mental retardation.
Analytic Sample
Of 1,346,545 men whose records were matched, 1,138,048 (85%) had complete data on IQ and psychiatric diagnoses from the conscription examination, together with data from the Population and Housing Censuses on parental age and socioeconomic status. As prior psychiatric disorder might have affected performance on the IQ test, we excluded men diagnosed with psychiatric disorder at conscription or who had been admitted to the hospital with a psychiatric disorder before conscription (n = 88,385). Analyses are based on the remaining 1,049,663 men.
Statistical Methods
We used ANOVA and the χ2 test to examine the relation between the 9-point total IQ score and study covariates. We used Cox proportional hazards regression to calculate relative risks (RRs) of hospital admission for each diagnostic category per standard deviation (SD) decrease in the 9-point total IQ score and by IQ categories. Survival time in days was calculated from date of conscription to date of first hospital admission for the diagnosis being considered, date of death, date of emigration, or 31 December 2004, whichever occurred first. Ninety percent of participants were followed up to 31 December 2004.
We adjusted for age at conscription, conscription testing center, and year of testing, and then for the potential confounding factors of parental age at birth19 and parental socioeconomic status. The latter variable was based on the highest socioeconomic status of either parent in the 1960 or 1970 Census, classified in 5 categories: nonmanual (high/intermediate), nonmanual (low), skilled, unskilled, and other. (As the presence of severe mental disorder is likely to have affected the men's educational attainment and adult socioeconomic status, we did not adjust for these factors.) We defined comorbidity as having received diagnoses from 2 or more different diagnostic categories, either simultaneously or sequentially. We used logistic regression to examine the relation between IQ and risk of comorbidity. We calculated survival time and total number of admissions in each diagnostic category for each of the 9-point IQ categories. From these, we calculated total admissions per 1000 person-years. We compared observed admissions with those expected (the rate of admissions assuming no difference between IQ categories). We calculated 95% confidence intervals (CIs) on the assumption that the observed values had a Poisson distribution.20
RESULTS
Table 1 shows the distribution of the participants according to the 9-point total IQ scale, and the association of IQ with the study covariates. Men with higher IQ scores were less likely to have parents in unskilled occupations. Higher IQ was also associated with older age at conscription and with older maternal and paternal age at birth.
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TABLE 1: Distribution of IQ Scores in 1,049,663 Men at Conscription and the Relation of IQ Score Category With the Study Covariates
During the follow-up period, 52,487 men (5%) had at least one hospital admission for mental disorders. Median length of stay was 4 days (interquartile range, 1–12). Half (53%) of the men admitted received diagnoses from more than one category. Table 2 lists the number of men in each diagnostic category and, for each category, the 3 most frequent comorbid disorders.
TABLE 2: Psychiatric Diagnoses Received During Hospital Admission (n = 52,487)
Table 3 shows hazard ratios for having at least one admission for each type of mental disorder per SD decrease in IQ score. In analyses adjusting for age at conscription, birth year, and conscript testing center, lower IQ was associated with an increase in risk of admission for all types of disorder. Further adjustment for parental age had little or no effect on these estimates. Adjustment for parental socioeconomic status slightly strengthened associations between IQ and risk of admission for schizophrenia, other nonaffective psychoses, and disorders not otherwise categorized, but had little or no effect on other associations. The estimates remained unchanged on full adjustment for all covariates. Additional adjustment for parental education in the subset (95%) with these data had little effect on these estimates (data not shown). To investigate whether any of these associations were explained by the link between IQ and comorbid disorders, we reran the fully adjusted models after the exclusion of men with comorbidity. This exclusion had no effect on the association between IQ and risk of admission from other substance-use disorders, and very little attenuating effect on the association between IQ and risk of admission for schizophrenia. Estimates for the relation between IQ and risk of admission for all other diagnostic categories were attenuated to a larger degree but all associations remained strong.
TABLE 3: Hazard Ratios for Admission to Hospital for Each Category of Psychiatric Disorder per SD Decrease in Total IQ Score at Conscription (n = 1,049,663)
Undiagnosed psychiatric disorder present at the time of conscription might have affected IQ scores. We repeated our analyses excluding men who were admitted to the hospital within 5 years of conscription. This had essentially no effect on our findings.
We examined whether the relation between IQ and risk of admission for each type of disorder varied over time by stratifying according to whether first admission occurred up to 1981, 1982–1993, or 1994–2004. Risk estimates differed little according to period of admission.
The relation between IQ and risk of hospital admission for each diagnostic category was evident across the range of IQ scores, with the largest increase in risk observed in men with the lowest IQ (Fig. 1).
FIGURE 1.:
Hazard ratios for admission for various categories of psychiatric disorder according to 9-point IQ scale. Highest IQ score (coded as 9) is the reference group. Estimates are adjusted for age at conscription, birth year, conscription testing center, parental age and parental socioeconomic status.
Lower IQ was associated with an increased risk of dual or multiple diagnoses. For a SD decrease in IQ, the age-adjusted odds ratios for receiving diagnoses in 2 or more, 4 or more, or 6 or more diagnostic categories were 1.71 (95% CI = 1.68–1.73), 1.95 (1.88–2.04), and 2.06 (1.91–2.22), respectively.
Figure 2 shows for each diagnostic category, the total admissions rate per 1000 person-years in each of the IQ categories. In general, there was a consistent dose-response relation, such that the admissions rate for each type of disorder was highest in those with the lowest IQ and fell with each 1-point increment in IQ category. The steepest gradients in rate of admissions across IQ categories were for alcohol-related and other substance use disorders, followed by personality disorders: men with the lowest IQ had risks of admissions for these 3 types of disorder that were respectively 21, 28, and 18 times higher than men with the highest IQ.
FIGURE 2.:
Total number of admissions for various categories of psychiatric disorders per 1000 person-years, by the 9-point IQ scale.
The ratio of the observed rate to the rate that would be expected, assuming no difference by IQ, are displayed in Figure 3. Observed/expected ratios showed that, for all types of disorder, men in the lowest 4 IQ categories had admission rates that were higher than expected from rates in the whole study population, while those in the highest 5 IQ categories had lower-than-expected admission rates. Men with the lowest IQ had admissions for the various types of disorder that were between 2.7 (95% CI = 2.6–2.8) and 4.8 (4.6–5.0) times higher than expected. Men with the highest IQ had admissions that were between 0.13 (0.12–0.14) and 0.60 (0.54–0.65) those expected (eTable, https://links.lww.com/EDE/A346).
FIGURE 3.:
Observed-to-expected ratio of total number of admissions, by the 9-point IQ scale.
DISCUSSION
Lower premorbid IQ is associated with a higher risk of being admitted to the hospital for the whole spectrum of mental disorders. Furthermore, men with lower IQ appear to be at increased risk of more severe illness: for each diagnostic category, total number of admissions per 1000 person-years of follow-up were highest in men with the lowest IQ and fell as IQ increased.
Previous Studies
Most investigations into the relation between IQ in early life and subsequent psychiatric disorder have concentrated on 1 or 2 disorders, limiting insights into specificity of association. However, these studies have demonstrated consistently that lower IQ is linked with an increased risk of schizophrenia1 and post-traumatic stress disorder.2–5,21 There have been fewer studies of other disorders. Lower cognitive ability has been associated with increased risk of depression,3,6,7 generalized anxiety disorder,3,7,8 anxiety disorders in general,7 psychological distress,22,23 bipolar disorder,24 nonaffective psychoses,6,24 and alcohol or other substance abuse or dependence3,9—although some studies have found no such associations.6,25 In 2 UK birth cohorts, a higher IQ in childhood was associated with alcohol problems,23,26 contrary to the results of other studies.3,9
Some indication that IQ in early life might be a risk factor for psychiatric disorders in general has come from observations in Denmark that men admitted to hospital for several types of disorder had poorer prior cognition.10 Mood and neurotic disorders were not linked with IQ, but the study may have lacked power to detect such differences. A further limitation of that study was the possibility that the lower IQ of men later admitted to hospital was a consequence of existing illness. A similar caveat applies to a cross-sectional study of Swedish conscripts born in 1949–1951, in which the prevalence of several types of psychiatric disorder was higher in men with lower IQ.27 The present study, based on considerably larger numbers and excluding men with prior illness, suggests that IQ is a risk factor for the whole range of mental disorders.
Few studies have examined the relation between cognitive ability and illness severity. In a study of 50,087 men, there was no correlation between prior IQ and number of hospital admissions for psychoses, but the authors did not investigate whether admission rates varied according to IQ.6 We found that men with a lower IQ had higher than expected total admission rates for all categories of disorder. This suggests that lower IQ may be a risk factor not just for the presence of mental illness per se, but also for greater clinical severity. Perhaps this is unsurprising, given the strength of the association here between IQ and comorbidity.
Plausible Mechanisms
The mechanisms underlying the link between IQ and later risk of psychiatric disorder are unclear. Although the greatest elevations in risk were seen in men at the lowest end of the distribution, each association showed a clear dose-response, with risk falling as IQ score increased. One explanation might be that IQ provides an indicator of the vulnerability of the brain. Lower IQ might be a reflection of impaired neurodevelopment that itself affects the development of psychiatric disorder.6,28 Adjustment for birthweight has little or no effect on associations between IQ and risk of psychiatric disorder,22,28 though this might be because birthweight is a crude measure of fetal neurodevelopment. Most studies have had limited information on circumstances in early life that might affect cognitive development and future risk of mental illness, but the associations appear robust to adjustment for parental socioeconomic status,6,8,10,28 parental psychiatric history,6,8,10 and paternal age.6 A further potential mechanism might be that men with lower IQ are more likely as adults to experience socioeconomic disadvantage.29
The apparent vulnerability associated with lower IQ may have genetic causes. There is considerable sharing of genetic risk factors across psychiatric disorders.30–32 An investigation into the structure of genetic and environmental risk factors for 10 psychiatric syndromes in twins concluded that, whereas shared environmental factors contributed to risk of conduct disorder and adult antisocial behavior, comorbidity of common psychiatric and substance use disorders resulted predominantly from genetic factors.33 One twin study5 has examined the extent to which genetic factors might account for the association between IQ and psychiatric disorder. In this study, the variance in post-traumatic stress disorder explained by premorbid IQ was accounted for entirely by common genetic factors.5
Another possible explanation for the association in our data might be that people with a lower IQ are less well equipped to deal with potential stressors. Evidence in children suggests that those with higher intelligence have a more internal locus of control.34 Having a lower perception of control over one's life increases the risk of disorders such as anxiety, depression, and post-traumatic stress disorder,35,36 and has been linked with greater use of mental health services.37 It is also possible that people with greater cognitive resources are better able to find a cognitive way out of potentially stressful situations, thus obviating the emotional response, because the stressor has been resolved. Children and adolescents of higher intelligence tend to be more resilient to stress.38,39 Greater ability to deal with potential stressors may not only reduce the risk of mental health problems, but also enable individuals to deal with acute psychiatric crises without the need for hospital admission. This might help to explain the link between lower IQ and increased admissions.
Strengths and Limitations
The main strength of this study is its size. This, coupled with the length of follow-up, gave us more statistical power than previous studies, enabling us to examine the relation of premorbid IQ not only with risk of hospital admission for the whole spectrum of mental disorders, but also with admission rates.
The study also has limitations. First, cases of psychiatric disorder during the follow-up period were defined on the basis of hospital admission. While most people with schizophrenia are likely to be admitted to hospital at some stage,40 inpatient care for nonpsychotic disorders is less common. The men identified as having nonpsychotic mental disorders in our study are therefore likely to be predominantly those with more severe illness, and, perhaps, those whose coping skills or lack of support networks make them more liable to be offered inpatient treatment. One interpretation of our findings linking IQ to risk of admission for all types of psychiatric disorder might be that people with lower IQ are more likely than those of higher IQ to be offered inpatient care, regardless of the nature of their illness. However, the fact that IQ predicts risk of depression and anxiety,3,7,8,22 post-traumatic stress disorder,2–5 and alcohol abuse or dependence,3 as defined by structured diagnostic interview or validated questionnaire in nonclinical samples, suggests that our findings are a reflection of the greater vulnerability of people with lower IQ to mental disorder in general, and not solely a consequence of the treatment options offered to them.
The second limitation of our study is that it is based on men only. The extent to which these findings can be extrapolated to women is uncertain, although similar associations between IQ and subsequent psychological distress have previously been observed in both men and women.22 Third, despite the exclusion of men with psychiatric disorder at conscription, undiagnosed illness might have affected performance on the cognitive tests. However, exclusion of men who were admitted to hospital in the first 5 years of follow-up had little effect, suggesting that reverse causality is unlikely. Fourth, a subset of this cohort had been the subject of previous research into IQ and psychiatric disorder.6,27 Although the present study uses the same conscription register, there is virtually no overlap between the 2 study populations: the earlier studies were based on men born in 1949–1951 and represent around 2% of the present study population, who were born between 1950 and 1976. Finally, 208,497 men were excluded due to missing data. However, in the 17,169 excluded men with data on IQ, associations were similar to those found in the analytic sample. (For example, the age-adjusted HR for alcohol-related disorders, the most common cause of admittance, was 1.82 [95% CI = 1.64–2.00] vs. 1.75 [1.73–1.77] per SD decrease in IQ.) This suggests that selection bias is not a major concern.
Our findings imply that many individuals who seek help for mental health problems may have poorer cognitive function that is a reflection of premorbid ability, rather than a consequence of their disorder. Prior cognitive ability might, in the course of mental disorder, affect the success with which people access professional help, the ability to cope with symptoms, and the compliance with treatment.41
The observation that intelligence is linked to risk of the whole range of mental disorder underlines the importance of investigating whether this association is causal or due to shared etiology. This shared etiology might be genes or early experiences that make the brain more vulnerable. To explore this question would require cohort studies that include detailed data on early life circumstances, as well as genetic information. Studies of twins may be particularly informative. To understand whether intelligence is causally linked to mental disorder, we need to explore psychosocial factors in adult life that might mediate the association. Understanding the extent to which intelligence might influence risk of mental disorder via exposure to the various stresses of socioeconomic disadvantage, or through cognitive processes in response to potentially stressful circumstances, may in turn point the way to interventions.
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