Department of Health Psychology, Institute of Public Health, University of Copenhagen
Department of Psychiatry (Amager), Copenhagen University Hospital, Copenhagen
Department of Health Psychology, Institute of Public Health, University of Copenhagen, Copenhagen, Denmark
Kinsey Institute for Research in Sex, Gender and Reproduction, Indiana University, Bloomington, Indiana
Social Science Research Institute and Department of Psychology, University of Southern California, Los Angeles, California, USA
Correspondence: Erik Lykke Mortensen, Department of Health Psychology, University of Copenhagen, Øster Farimagsgade 5A, PO Box 2099, 1014 Copenhagen K, Denmark. Tel: +45 3532 7839; fax: +45 3532 7748; e-mail: e.l.mortensen{at}pubhealth.ku.dk
Declaration of interest None. Funding detailed in Acknowledgements
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ABSTRACT |
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Aims To illuminate the relationship between IQ test scores in early adulthood and various mental disorders.
Method For 3289 men from the Copenhagen Perinatal Cohort, military IQ test scores and information on psychiatric hospitalisation were available. We identified 350 men in the Danish Psychiatric Central Register, and compared the mean IQ test scores of nine diagnostic categories withthe mean scores of 2939 unregistered cohort controls.
Results Schizophrenia and related disorders, other psychotic disorders, adjustment, personality, alcohol and substance-use-related disorders were significantly associated with low IQ scores, but this association remained significant for the four non-psychotic disorders only when adjusting for comorbid diagnoses. For most diagnostic categories, test scores were positively associated with the length of the interval between testing and first admission. ICD mood disorders as well as neuroses and related disorders were not significantly associated with low IQ scores.
Conclusions Low IQ may be a consequence of mental disease or a causal factor in psychotic and non-psychotic disorders.
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INTRODUCTION |
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METHOD |
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The Danish Psychiatric Central Register
The Danish Psychiatric Central Register has been computerised since 1 April
1969 (Munk-Jørgensen &
Mortensen, 1997). It contains data on all admissions to Danish
psychiatric in-patient facilities. Until 1994, diagnoses were coded according
to ICD8 (World Health Organization,
1974). Since 1994 diagnoses have been coded according to
ICD10 (World Health Organization,
1992).
Draft records
With the exception of individuals with disqualifying diseases (such as
epilepsy and diabetes) and individuals who volunteer for military service, all
Danish men are required to appear before the draft board when they become
liable for conscription at the age of 18 years. Appearing before the draft
board primarily involves a medical assessment, but also includes IQ testing.
Draft board information including IQ test scores has been collected for all
male members of the cohort by the Prenatal Development Project, a large-scale
study of the effects of prenatal and perinatal factors on human development
(Reinisch et al,
1993).
The IQ test used by the Danish military draft board is the Børge Priens Prøve (BPP). It is a 45 min group test with four sub-tests (letter matrices, verbal analogies, number series and geometric figures) and a total score ranging from 0 to 78. The total score correlates 0.82 with the Full Scale IQ of Wechslers Adult Intelligence Scale (WAIS; Wechsler, 1958), indicating that the BPP is a high-quality measure of general IQ (Mortensen et al, 1989).
Current sample
The Copenhagen Perinatal Cohort originally included 4668 males, but only
4280 survived the first 4 weeks of life. Individual identification numbers
were introduced in Denmark in 1968 and it has been possible to obtain valid
numbers for 4116 males in the cohort (164 individuals were lost to study
because of childhood mortality or emigration). Data from the Danish
Psychiatric Register for these 4116 cohort members were obtained on 31 August
1999. However, BPP scores were only available for about 77% of the 4280 males
who survived the first 4 weeks of life, and the current sample comprises 3289
men who appeared before the draft board at the mean age of 19.2 years
(s.d.=1.3, range 16.426.2).
Diagnostic classification
Of the 3289 men included in our study, 350 (10.6%) were identified in the
Danish Psychiatric Central Register. In order to obtain diagnostic categories
of reasonable size for analysis, ICD8 and ICD10 categories were
combined to form the following nine diagnostic categories:
Data analysis
The unregistered control sample comprised 2939 individuals. The observed
control sample BPP mean score was 40.4, which is close to the 40.9 mean
predicted from a regression formula describing secular trends in BPP scores
(Teasdale & Owen, 1989), and the standard deviation was 11.3, which is also close to the 11.4 observed
for individuals born in 19541958 (the average year of birth of the
control sample was 1960.8). In addition, the distribution of test scores was
close to the normal distribution, and we decided to linearly transform BPP
scores to a standardised scale with a mean of 100 and a standard deviation of
15 in the control sample, corresponding to the most frequently used IQ scales
(Wechsler, 1958).
Using t-tests for independent samples, the mean transformed BPP score of each diagnostic category was compared with the mean of all unregistered control individuals. Analysis of covariance (ANCOVA) was conducted to adjust the BPP scores for parental social status and parental registration in the Psychiatric Register. Many patients were registered in several of the main diagnostic categories, and therefore t-tests and ANCOVA analyses of the diagnostic categories are based on partly overlapping patient samples. Consequently, we also conducted a multiple regression analysis with BPP score as the dependent variable and the nine diagnostic categories as independent variables.
In addition, the approximate length of time between appearing before the draft board and the date of first admission was calculated for each individual (the exact date of appearing before the draft board was not available, but the difference between the estimated and the exact date did not exceed 3 months). According to this interval, the individuals in each diagnostic category were classified into three subgroups: individuals who had already been admitted to an in-patient facility when they appeared before the draft board or who were admitted within the subsequent year; individuals who were admitted 15 years after the draft board examination; and individuals who were admitted more than 5 years after appearing before the draft board. Using ANOVA, the mean BPP scores in the three subgroups were compared. In addition to overall F-tests, contrasts were computed to evaluate differences between the three subgroup means (Rosenthal et al, 2000). Finally, those in the first of the three subgroups were excluded from the sample, and the multiple regression analysis with BPP score as the dependent variable and the nine diagnostic categories as independent variables was repeated on this reduced sample.
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RESULTS |
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Diagnoses of schizophrenia, personality disorder and substance use disorders seem to be associated with first hospital admission at a relatively early age. Mood disorders, adjustment disorders and alcohol-related disorders, on the other hand, are associated with older age at first admission. A corresponding pattern was observed for the mean intervals between appearing before the draft board and the time of first admission. The mean age at first admission was particularly low for the other diagnoses category, and it is remarkable that this category almost exclusively consisted of patients who had only been admitted once.
Number of admissions and co-diagnoses
Diagnoses of schizophrenia and of substance use disorder were both
associated with relatively many admissions. Adjustment disorder, on the other
hand, was associated with relatively few admissions
(Table 1). For most diagnostic
categories the average cohort member was registered in one additional
category, but the average individual with the diagnosis of schizophrenia,
other psychotic disorder or substance use disorder was registered in two
additional categories. Table 2
shows the two most frequently diagnosed comorbid conditions. Chi-squared tests
showed that diagnoses of schizophrenia and other psychotic disorder were
positively associated (P<0.0001) and that both these diagnoses
were positively associated with substance use disorder (P<0.0001
and P<0.001 respectively). Adjustment disorder was negatively
associated with schizophrenia (P<0.05) and with personality
disorder (P<0.03), whereas personality disorder and substance use
disorder showed a positive association (P<0.02).
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Comparison with controls
Table 3 shows the BPP mean
scores for the control group and the nine diagnostic categories. Registration
in the Psychiatric Register was associated with significantly lower BPP scores
for all diagnostic categories except mood disorders and neuroses and related
disorders. The differences between the control group mean and the means of
most diagnostic categories were substantial, when compared with the size of
the control group standard deviation. Among the six diagnostic categories
significantly associated with low BPP score, the highest mean was for the
category other psychotic disorder and the lowest mean was for the substance
use disorder category. Table 3
shows that the mean of the category other psychotic disorder corresponds to a
BPP mean of 92.4 and that the mean of the substance use category corresponds
to a BPP mean of 86.5. Thus, in terms of standard deviation units, the mean of
the former category was 0.5 s.d. below the mean of the controls, whereas the
mean of the latter category was 0.9 s.d. below the control mean (7.6/15 and
13.5/15 respectively).
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Parental social status and psychiatric history
Table 3 also shows that the
pattern of mean parental social status scores corresponds to the pattern of
BPP scores. Thus, registration in the Psychiatric Register was associated with
significantly lower parental social status, except for mood disorders and for
neuroses and related disorders. Table
4 shows that adjusting BPP scores for parental social status or
for both parental social status and parental registration in the Psychiatric
Register did not change the levels of significance substantially. It is
noteworthy, however, that the adjusted regression coefficients for the
categories of schizophrenia and schizotypal disorders and of other psychotic
disorders indicated significantly lower BPP scores than for the control group
only at the 5% level of significance.
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Comorbidity
Table 5 shows the results of
analysing the data of all cohort members in the study together and using
indicator variables to code registration in diagnostic categories. Four
diagnostic categories were not significantly associated with low BPP scores:
schizophrenia and schizotypal disorders, other psychotic disorders, mood
disorders, and neuroses and related disorders. The association with low BPP
scores remained significant for five other diagnostic categories: adjustment
disorders, personality disorders, alcohol-related disorders, substance use
disorders and other diagnoses). Table
2 shows substantial overlap between the schizophrenia and other
psychotic disorders categories, but none of these diagnoses showed significant
associations with BPP scores when the other category was excluded from the
regression model (P=0.89 and P=0.64, respectively). The
categories alcohol-related disorders and (in particular) substance use
disorders were frequent co-diagnoses of schizophrenia and other psychotic
disorders. Since Tables 3 and
4 show that alcohol and
substance use disorders were associated with very low BPP scores, we conducted
a regression analysis without indicator variables for these diagnoses. This
analysis also showed no significant association between BPP scores and
schizophrenia or other psychotic disorders (P=0.26 and
P=0.36, respectively).
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Interval between testing and first admission
Table 6 shows the results of
classifying all 350 individuals recorded in the Psychiatric Register into
three subgroups according to the interval between appearing before the draft
board and the time of first admission. One-way ANOVA showed significant
overall F-test only for the categories of schizophrenia and
schizotypal disorders, adjustment disorders and substance use disorders. For
these categories, contrasts showed that the mean of the >5 year subgroup
was significantly higher than the means of the subgroups with intervals of
<1 year and 15 years. The table shows similar trends for the
categories of other psychotic disorders and personality disorders, whereas the
inconsistent results for mood disorders and neuroses and related disorders
most probably reflect small subgroup sample sizes and low power.
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Differences between the <1 year and the 15 year subgroups were insignificant and for most categories small. The results of excluding the 75 men in the 51 year subgroup and adjusting for comorbid diagnoses are presented in Table 7. The adjusted regression coefficients in this table do not deviate substantially from the coefficients in Table 5, and the higher P-values in Table 7 seem to reflect loss of power due to smaller group sizes.
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DISCUSSION |
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When comorbid diagnoses were included as covariates, the adjusted regression coefficients for five categories (adjustment disorders, personality disorders, alcohol-related disorders, substance use disorders and other diagnoses) indicated significantly lower mean BPP scores than the mean of the unregistered controls, whereas this was not the case for schizophrenia and schizotypal disorders and other psychotic disorders. Except for mood disorders, neuroses and related disorders and other diagnoses, BPP scores were higher although not necessarily significantly so for individuals first admitted more than 5 years after BPP testing. Differences in BPP scores between those admitted within 1 year and those admitted 15 years after testing were small and inconsistent, and an analysis excluding those admitted within 1 year essentially confirmed the results for the full sample.
It should be kept in mind that these observations refer to associations between the possibly unreliable hospital diagnoses in the Danish Psychiatric Register and BPP performance. However, noise introduced by unreliable diagnoses may weaken rather than strengthen associations between diagnostic categories and BPP scores. In addition, although unreliable diagnoses and misclassification of patients might affect results for individual diagnostic categories (e.g. personality disorders), it is unlikely that such factors explain the overall picture of associations between several broad diagnostic categories and BPP score. Strong evidence that low IQ test scores are not only associated with psychiatric hospitalisation and hospital diagnoses is provided by a recent cross-sectional study of associations between cognitive test scores and psychopathological screening diagnoses in male adolescents appearing before the Israeli draft board. In that study, mean cognitive test scores were significantly poorer for almost all diagnostic groups than for controls, and observed effect sizes were in the range 0.31.6 (Weiser et al, 2004).
Explaining the association
Several mechanisms may explain the association between IQ and mental
disease. An aetiological hypothesis assumes that low IQ has a direct or
indirect role in the aetiology of a mental disease; thus, it is possible that
low IQ plays a part in the development of risk drinking
(Windle & Blane, 1989;
Mortensen et al,
2005) or antisocial personality traits
(Loney et al, 1983;
Kandel et al, 1988).
For schizophrenia, it might be assumed that low IQ mediates the effects of
adverse prenatal factors and birth complications, although there is evidence
that IQ may be an independent risk factor
(Gunnell et al, 2002).
Second, common genetic and environmental factors may affect both the
development of mental disease and cognitive development: for example,
obstetrical complications might affect both the risk of schizophrenia and
cognitive development (Boog,
2004). Thus, we observed that parental social status was
associated with both offspring IQ and mental disease but we also
observed that neither parental social status nor genetic (family) disposition
to mental disease could explain the link between IQ and disease (parental
psychiatric admission is obviously a general and nonspecific indicator of
genetic disposition).
A third possibility is that mental disease affects IQ test scores, which might occur in several ways. A developmental hypothesis assumes that personality traits associated with the risk of developing a mental disease adversely affect cognitive development. Thus, deviant personality traits in people who go on to develop schizophrenia may be associated with less optimal cognitive development (Malmberg et al, 1998). A test performance hypothesis assumes that premorbid traits or early symptoms affect motivation and test performance when young men appear before the draft board. Thus, some personality disorders might be associated with low motivation, and taking the BPP under the influence of alcohol or other substances might lead to low test scores although in our sample this seems an unlikely explanation of the association between BPP scores and alcohol-related and other substance use disorders, because a medical examination is part of appearing before the draft board, and because of the average 10-year and 8.9-year intervals between taking the BPP and the first admissions with these diagnoses (see Table 1). Finally, there is a dementia hypothesis, according to which cognitive impairment may be the direct result of mental diseases such as alcoholism or schizophrenia. Because of the young age of the individuals in our study, it is doubtful that such a mechanism explains the association between alcoholism and low BPP scores, but it could have a role in very early cases of schizophrenia.
A fourth possibility is that IQ is primarily associated with the risk of hospitalisation, or the likelihood of obtaining a certain diagnosis when hospitalised. Thus, one study has reported that IQ is an independent predictor of lifetime psychiatric contact (Walker et al, 2002), and it is likely that IQ is a strong determinant of the ability to cope with crises and life events and that this ability affects the risk of admission to a psychiatric department. This may partly explain the association between adjustment disorders and low IQ and the similar results for the other diagnoses category (typically admitted only once to psychiatric treatment). A link between IQ and risk of admission may also have a role in personality disorders, but it is also possible that low IQ affects the risk of receiving a diagnosis of personality disorder or alcoholism relatively early (i.e. patients with a higher IQ may be able to hide their antisocial behaviour or alcohol problems). However, a recent follow-up study of a non-hospitalised subsample from the Copenhagen Perinatal Cohort showed an association between low IQ and self-reported risk drinking, suggesting that the association between low IQ and alcohol-related disorder does not only reflect a link between IQ and risk of hospitalisation with an alcohol diagnosis (Mortensen et al, 2005).
Our results indicate significant premorbid cognitive deficits in several psychiatric diagnostic categories, and it is likely that many if not all of the mechanisms described above contribute to the low premorbid IQ observed for most diagnostic categories. Several of these mechanisms (the aetiological hypothesis, common genetic and environmental factors and the developmental hypothesis) may explain low IQ in neurodevelopmental disorders (Bradshaw, 2001). However, low BPP scores were observed for both psychotic disorders (schizophrenia, and the category of other psychotic disorders, which included paranoid psychosis) and for non-psychotic disorders (adjustment disorders, personality disorders, alcohol-related disorders and other substance use disorders). In interpreting the lack of evidence for low premorbid IQ in mood disorder (affective illness) and neuroses and related disorders, the small number of individuals in these categories should be borne in mind. Thus, although we were able to confirm low IQ in those later developing schizophrenia, our results suggest that low premorbid IQ is not specific for schizophrenia and related disorders, and may indeed show a stronger association with adjustment disorders, personality disorders, and alcohol-related and substance use disorders. The cohort members were 3840 years old when the follow-up was completed, thus ensuring that the vast majority of incident cases with schizophrenia would have been identified. The in-patient register diagnoses of schizophrenia can be considered to be reliable in Denmark (Munk-Jørgensen, 1995).
Interval between testing and first admission
In this register-based study the first admission to a psychiatric
department was the earliest available indicator of mental disorder. In
general, the longer the interval between IQ testing and first admission, the
greater the likelihood that a low IQ score is not an effect of mental
disorder. Table 1 shows that
for all diagnostic categories the average individual was admitted several
years after appearing before the draft board, and
Table 6 shows that the majority
of patients were admitted more than 5 years after BPP testing. For most
diagnostic categories these patients obtained higher BPP mean scores than
those admitted sooner, but perhaps it is more remarkable that differences
between those admitted within 1 year and those admitted 15 years after
testing were small and inconsistent. This pattern suggests that previous and
existing mental disorders do not have substantial negative effects on BPP
scores. For schizophrenia and related disorders, the low BPP mean for those
admitted within 5 years of testing may reflect the effects of prodromal
symptoms on test performance (Rabinowitz
et al, 2000), but perhaps more probably a complex
development with premorbid traits leading to low cognitive ability, which in
turn leads to vulnerability and early psychosis. For schizophrenia as well as
adjustment disorders and substance use disorders, the differences between
those admitted within 5 years and those admitted later may also reflect links
between cognitive ability, coping strategies and risk of hospitalisation. In
this perspective it is, however, remarkable that the difference between early
and late admission is relatively small for alcohol-related disorders,
suggesting that the association between low IQ and alcohol-related disorders
is not entirely explained by a link between low IQ and the likelihood of
hospitalisation and early alcohol diagnosis.
Comorbidity
Attempts to interpret our results should take into account the facts that
individual diagnoses often change over the life span (e.g. from personality
disorder to schizophrenia) and that admission to a psychiatric department
often results in several discharge diagnoses. Thus, there is evidence from the
Swedish conscript study (Lewis et
al, 2000) that nearly 40% of men who developed schizophrenia
would have received a psychiatric diagnosis of a non-psychotic disorder at the
age of 18 years. In our study sample, 46% of the men with schizophrenia were
also registered in the category of other psychotic disorders, and 38% were
registered in the category of other substance use disorders. On the other
hand, alcohol-related disorders and personality disorders tended to cluster
together (about 33% diagnostic overlap). When we adjusted for all diagnostic
categories (Tables 5 and
7), the regression coefficients
indicated particularly low BPP scores for alcohol-related disorders and other
diagnoses (followed by substance use and adjustment disorders). We believe
that adjusting for co-diagnoses is more realistic than constructing a
diagnostic hierarchy and assigning each patient to only one diagnostic
category (this procedure has to be based on strong assumptions about
relationships among mental disorders, and for individual patients these
assumptions cannot be verified in a register-based study). However, the
results of adjusting for co-diagnoses may be ambiguous, since some diagnostic
categories may typically be associated with more co-diagnoses than other
categories and there must be much overlapping variance between some of the
categories. Our analyses suggested that collinearity with alcohol and
substance use disorders or other psychotic disorders did not explain the
results for schizophrenia. When interpreting the lack of significance for
schizophrenia in Tables 5 and
7, it should also be borne in
mind that the unadjusted mean in Table
3 confirms other studies that have not adjusted for co-diagnoses
and observed an association between low IQ scores and schizophrenia. The
unadjusted mean describes the observed IQ of patients with a diagnosis of
schizophrenia, whereas the adjusted regression coefficients reflect both the
pattern of associations among the diagnostic categories and between each
diagnostic category and IQ.
Implications of our findings
We have demonstrated that most broad diagnostic categories of mental
disorder are associated with relatively low premorbid IQ. Both low IQ and
mental disorder may be considered to be indicators of suboptimal brain
development, but many mechanisms may contribute to explain the association
between IQ and mental disorder. The range of mental disorders and the observed
effect sizes should be considered. If our results can be generalised, it is
unlikely that any personality trait or other psychological characteristic
perhaps except the broad personality dimension of
neuroticism has a stronger association with mental
disorder than IQ. This has important implications for both research and
clinical practice. Scores on IQ tests correlate with many neuropsychological
tests of specific cognitive functions and with many other psychological and
social variables (Gottfredson,
2002). Consequently, the association with IQ should be considered
in any theoretical account of the development of the relevant disorders, and
if possible premorbid IQ should be controlled for in studies
comparing patient samples (e.g. people with schizophrenia or alcoholism) and
normal controls. In clinical practice, the relatively high frequency of low
premorbid IQ in many diagnostic categories should also be taken into account
when choosing appropriate therapy and when interpreting the cognitive
performance of individual patients (e.g. neuropsychological test scores in
cases of long-term alcoholism). It should, however, not be forgotten that
patients show large individual differences in IQ scores and that a substantial
number of patients in all diagnostic categories are of normal or above-normal
IQ (in our study sample about 28% of the 350 patients obtained BPP scores
corresponding to an IQ of 100 or higher). Finally, our study describes
associations between diagnostic categories and scores on a Danish IQ test, and
to evaluate whether our findings can be generalised, similar studies in other
countries and cultures are needed.
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Clinical Implications and Limitations |
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LIMITATIONS
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ACKNOWLEDGMENTS |
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Thanks are due to Vibeke Munk, MA, for help with the manuscript and critical comments.
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Received for publication May 18, 2004. Revision received November 9, 2004. Accepted for publication December 14, 2004.
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