Low intelligence test scores in 18 year old men and risk of suicide: c
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《英国医生杂志》
1 Department of Social Medicine, University of Bristol, Bristol BS8 2PR, 2 Department of Genetics and Pathology, Rudbeck Laboratory, Uppsala University, Sweden, 3 Child and Adolescent Public Health Epidemiology Group, Department of Public Health Sciences, Karolinska Institutet, SE-17176 Stockholm, Sweden
Correspondence to: F Rasmussen finn.rasmussen@phs.ki.se
Abstract
Impaired neurodevelopment is thought to increase the risk of several psychiatric disorders. In keeping with this, poor performance on tests of cognitive function is associated with an increased risk of depression1 and psychosis.2 3 Interpretation of these findings is hampered by the possibility that pre-existing mental illness may influence test performance (reverse causality). Furthermore, performance during psychiatric interviews may be influenced by an individual's intelligence.
Few studies have examined the associations of performance in intelligence tests with suicide, and results have conflicted. A study of conscripts in Israel suggested that people who committed suicide had above average intelligence ratings at conscription,4 though the opposite was reported among Australian conscripts.5 6 In a cohort of Swedish men conscripted in 1969-70, "intellectual capacity" was inversely associated with risk of suicide, although this association was attenuated after adjustment for measures of conduct and personality.7 In a more recent follow up of Swedish conscripts, there was an inverse association between intelligence test performance and non-fatal self harm.8
We analysed the association between the results of four intelligence tests, recorded during medical examinations at conscription of Swedish men, and subsequent risk of suicide.
Methods
We found strong linear associations with all four intelligence tests (table 1). Better performance on the tests was associated with a reduced risk of suicide. The strongest associations were with the logic test score, with a threefold difference in risk between high and low scorers (figure). The strength of the associations changed little in models that controlled for parents' socioeconomic index and education. Similar associations were seen in the subset of men with a record of their global test score (fully adjusted hazard ratio per unit increase 0.88 (95% confidence interval 0.86 to 0.90).
Table 1 Associations of four intelligence test scores with suicide in age adjusted and fully adjusted models
Fully adjusted suicide hazard ratios by categories of logic intelligence test score (category 5 is reference)
Correlations between the four test scores ranged from r = 0.43 to r = 0.69. We fitted a model including terms for all four of the test scores to see if mutual adjustment attenuated the strength of any of the associations (table 2). Associations with the synonym, technical, and spatial test scores were all greatly attenuated, but the strength of association with the logic test result was not greatly changed. Subsequent analyses were based on the logic test score alone.
Table 2 Associations with each of four intelligence test scores in fully adjusted model including all four terms simultaneously (hazard ratio suicide per unit increase in test score)
Influence of educational attainment on intelligence test-suicide associations
We examined the possible confounding effect of educational attainment in a restricted dataset of 542 283 men (n = 1027 suicides) born 1950-65 and alive at the age of 25 years, who had therefore had the opportunity to complete their education. In a model adjusted for age and the other confounders, the hazard ratio per unit increase in the logic test score was 0.90 (0.86 to 0.93), further adjustment for educational level attenuated this association to 0.93 (0.90 to 0.97).
Influence of pre-existing psychiatric illness
Men with a psychiatric disorder recorded at conscription tended to perform poorly on the tests of intelligence; 23.4% (6534/27 901) of those with scores of 1 on the logic intelligence test had a psychiatric diagnosis recorded at conscription compared with only 2.7% (1039/38 905) of those with scores of 9. Exclusion of all 59 163 men with psychiatric disorders recorded at baseline had little effect on the association between intelligence and suicide (fully adjusted hazard ratio per unit increase in logic test score 0.89, 0.87 to 0.91).
To investigate whether our findings might have been influenced by the presence of (unrecorded) psychiatric illness at baseline (reverse causality) we assessed the associations after excluding the first five, the first 10, and the first 15 years of follow up (table 3). There is some evidence that associations were strongest in the first five years of follow up, though associations remained after more than 15 years of follow up.
Table 3 Suicide hazard ratios in relation to logic test score over discrete follow up periods
Interactions with own or parents' education
In the subset of men alive at the age of 25 years the association of intelligence test scores with suicide differed depending on an individual's educational achievement (P < 0.005 for interaction, table 4). The gradient of risk in relation to intelligence test score was strongest in those with high or medium levels of education and the gradient of risk in relation to educational levels was highest in those with high intelligence test scores. Among those with only primary level education, intelligence test performance did not seem to be associated with risk of suicide.
Table 4 Hazard ratios (95% confidence intervals) for suicide in relation to logic intelligence test score grouped in three categories and length of conscripts' education in three categories, adjusted for year of birth, parents' socioeconomic position and education, and test centre (542 283 conscripts, 1027 suicides)*
There was no strong evidence that the effects of test performance on risk of suicide differed with parents' education (P = 0.35 for interaction). The greatest risk of suicide was seen among the men who had low intelligence test scores but had highly educated parents (table 5).
Table 5 Hazard ratios (95% confidence intervals) for suicide by logic intelligence test score in three categories and parents' education in three categories, adjusted for year of birth, parents' socioeconomic position, and test centre*
Sensitivity analyses
Associations with undetermined deaths (n = 801) were similar to the associations with suicides. The fully adjusted hazard ratio for the association with the logical test score was 0.83 (0.80 to 0.86). The methods for intelligence testing at conscription changed in 1980.10 The associations with suicide were stronger among those tested in 1981-94 (hazard ratio 0.85, 0.82 to 0.88) than among those tested in 1968-80 (0.90, 0.88 to 0.93). Associations of intelligence test scores with suicide were similar to those with all cause mortality (fully adjusted hazard ratio per unit increase in logic test score based on 12 833 deaths was 0.88 (0.87 to 0.88).
Discussion
Van Os J, Jones P, Lewis G, Wadsworth M, Murray R. Developmental precursors of affective illness in a general population birth cohort. Arch Gen Psychiatry 1997;54: 625-31.
David AS, Malmberg A, Brandt L, Allebeck P, Lewis G. IQ and risk for schizophrenia: a population-based cohort study. Psychol Med 1997;27: 1311-23.
Gunnell D, Harrison G, Rasmussen F, Fouskakis D, Tynelius P. Associations between pre-morbid intellectual performance, early-life exposures and early-onset of schizophrenia. Br J Psychiatry 2003;181: 298-305.
Apter A, Bleich A, King RA, Kron S, Fluch A, Kotler M, et al. Death without warning? A clinical postmortem study of suicide in 43 Israeli adolescent males. Arch Gen Psychiatry 1993;50: 138-42.
O'Toole BI, Stankov L. Ultimate validity of psychological tests. Pers Individ Dif 1992;13: 699-716.
O'Toole BI, Cantor C. Suicide risk factors among Australian Vietnam era draftees. Suicide Life Threat Behav 1995;25: 475-88.
Allebeck P, Allgulander D, Fisher LD. Predictors of completed suicide in a cohort of 50465 young men: role of personality and deviant behaviour. BMJ 1988;297: 176-8.
Jiang GX, Rasmussen F, Wasserman D. Short stature and poor psychological performance: risk factors for attempted suicide among Swedish male conscripts. Acta Psychiatr Scand 1999;100: 433-40.
Otto U. Male youths. A sociopsychiatric study of a total annual population of swedish adolescent boys. Acta Paediatrica Scand Suppl 1976;264: 5-47.
Carlstedt B. Cognitive abilities—aspects of structure, process and measurement. University of Gothenburg , 2000.
Neeleman J, Wessely S, Wadsworth M. Predictors of suicide, accidental death, and premature natural death in a general-population birth cohort. Lancet 1998;351: 93-7.
Paffenbarger RS, King SH, Wing AL. Chronic disease in former college students. Am J Pub Health 1969;59: 900-8.
Hart CL, Taylor MD, Davey-Smith G, Whalley LJ, Starr JM, Hole DJ, et al. Childhood IQ, social class, deprivation, and their relationships with mortality and morbidity risk in later life: prospective observational study linking the Scottish mental survey 1932 and the midspan studies. Psychosom Med 2003;65: 877-83.
Whalley LJ, Deary IJ. Longitudinal cohort study of childhood IQ and survival up to age 76. BMJ 2001;322: 819.
Sacker A, Schoon I, Bartley M. Social inequality in educational achievement and psychosocial adjustment throughout childhood: magnitude and mechanisms. Soc Sci Med 2002;55: 863-80.
((D Gunnell, professor of epidemiology1, P)
Correspondence to: F Rasmussen finn.rasmussen@phs.ki.se
Abstract
Impaired neurodevelopment is thought to increase the risk of several psychiatric disorders. In keeping with this, poor performance on tests of cognitive function is associated with an increased risk of depression1 and psychosis.2 3 Interpretation of these findings is hampered by the possibility that pre-existing mental illness may influence test performance (reverse causality). Furthermore, performance during psychiatric interviews may be influenced by an individual's intelligence.
Few studies have examined the associations of performance in intelligence tests with suicide, and results have conflicted. A study of conscripts in Israel suggested that people who committed suicide had above average intelligence ratings at conscription,4 though the opposite was reported among Australian conscripts.5 6 In a cohort of Swedish men conscripted in 1969-70, "intellectual capacity" was inversely associated with risk of suicide, although this association was attenuated after adjustment for measures of conduct and personality.7 In a more recent follow up of Swedish conscripts, there was an inverse association between intelligence test performance and non-fatal self harm.8
We analysed the association between the results of four intelligence tests, recorded during medical examinations at conscription of Swedish men, and subsequent risk of suicide.
Methods
We found strong linear associations with all four intelligence tests (table 1). Better performance on the tests was associated with a reduced risk of suicide. The strongest associations were with the logic test score, with a threefold difference in risk between high and low scorers (figure). The strength of the associations changed little in models that controlled for parents' socioeconomic index and education. Similar associations were seen in the subset of men with a record of their global test score (fully adjusted hazard ratio per unit increase 0.88 (95% confidence interval 0.86 to 0.90).
Table 1 Associations of four intelligence test scores with suicide in age adjusted and fully adjusted models
Fully adjusted suicide hazard ratios by categories of logic intelligence test score (category 5 is reference)
Correlations between the four test scores ranged from r = 0.43 to r = 0.69. We fitted a model including terms for all four of the test scores to see if mutual adjustment attenuated the strength of any of the associations (table 2). Associations with the synonym, technical, and spatial test scores were all greatly attenuated, but the strength of association with the logic test result was not greatly changed. Subsequent analyses were based on the logic test score alone.
Table 2 Associations with each of four intelligence test scores in fully adjusted model including all four terms simultaneously (hazard ratio suicide per unit increase in test score)
Influence of educational attainment on intelligence test-suicide associations
We examined the possible confounding effect of educational attainment in a restricted dataset of 542 283 men (n = 1027 suicides) born 1950-65 and alive at the age of 25 years, who had therefore had the opportunity to complete their education. In a model adjusted for age and the other confounders, the hazard ratio per unit increase in the logic test score was 0.90 (0.86 to 0.93), further adjustment for educational level attenuated this association to 0.93 (0.90 to 0.97).
Influence of pre-existing psychiatric illness
Men with a psychiatric disorder recorded at conscription tended to perform poorly on the tests of intelligence; 23.4% (6534/27 901) of those with scores of 1 on the logic intelligence test had a psychiatric diagnosis recorded at conscription compared with only 2.7% (1039/38 905) of those with scores of 9. Exclusion of all 59 163 men with psychiatric disorders recorded at baseline had little effect on the association between intelligence and suicide (fully adjusted hazard ratio per unit increase in logic test score 0.89, 0.87 to 0.91).
To investigate whether our findings might have been influenced by the presence of (unrecorded) psychiatric illness at baseline (reverse causality) we assessed the associations after excluding the first five, the first 10, and the first 15 years of follow up (table 3). There is some evidence that associations were strongest in the first five years of follow up, though associations remained after more than 15 years of follow up.
Table 3 Suicide hazard ratios in relation to logic test score over discrete follow up periods
Interactions with own or parents' education
In the subset of men alive at the age of 25 years the association of intelligence test scores with suicide differed depending on an individual's educational achievement (P < 0.005 for interaction, table 4). The gradient of risk in relation to intelligence test score was strongest in those with high or medium levels of education and the gradient of risk in relation to educational levels was highest in those with high intelligence test scores. Among those with only primary level education, intelligence test performance did not seem to be associated with risk of suicide.
Table 4 Hazard ratios (95% confidence intervals) for suicide in relation to logic intelligence test score grouped in three categories and length of conscripts' education in three categories, adjusted for year of birth, parents' socioeconomic position and education, and test centre (542 283 conscripts, 1027 suicides)*
There was no strong evidence that the effects of test performance on risk of suicide differed with parents' education (P = 0.35 for interaction). The greatest risk of suicide was seen among the men who had low intelligence test scores but had highly educated parents (table 5).
Table 5 Hazard ratios (95% confidence intervals) for suicide by logic intelligence test score in three categories and parents' education in three categories, adjusted for year of birth, parents' socioeconomic position, and test centre*
Sensitivity analyses
Associations with undetermined deaths (n = 801) were similar to the associations with suicides. The fully adjusted hazard ratio for the association with the logical test score was 0.83 (0.80 to 0.86). The methods for intelligence testing at conscription changed in 1980.10 The associations with suicide were stronger among those tested in 1981-94 (hazard ratio 0.85, 0.82 to 0.88) than among those tested in 1968-80 (0.90, 0.88 to 0.93). Associations of intelligence test scores with suicide were similar to those with all cause mortality (fully adjusted hazard ratio per unit increase in logic test score based on 12 833 deaths was 0.88 (0.87 to 0.88).
Discussion
Van Os J, Jones P, Lewis G, Wadsworth M, Murray R. Developmental precursors of affective illness in a general population birth cohort. Arch Gen Psychiatry 1997;54: 625-31.
David AS, Malmberg A, Brandt L, Allebeck P, Lewis G. IQ and risk for schizophrenia: a population-based cohort study. Psychol Med 1997;27: 1311-23.
Gunnell D, Harrison G, Rasmussen F, Fouskakis D, Tynelius P. Associations between pre-morbid intellectual performance, early-life exposures and early-onset of schizophrenia. Br J Psychiatry 2003;181: 298-305.
Apter A, Bleich A, King RA, Kron S, Fluch A, Kotler M, et al. Death without warning? A clinical postmortem study of suicide in 43 Israeli adolescent males. Arch Gen Psychiatry 1993;50: 138-42.
O'Toole BI, Stankov L. Ultimate validity of psychological tests. Pers Individ Dif 1992;13: 699-716.
O'Toole BI, Cantor C. Suicide risk factors among Australian Vietnam era draftees. Suicide Life Threat Behav 1995;25: 475-88.
Allebeck P, Allgulander D, Fisher LD. Predictors of completed suicide in a cohort of 50465 young men: role of personality and deviant behaviour. BMJ 1988;297: 176-8.
Jiang GX, Rasmussen F, Wasserman D. Short stature and poor psychological performance: risk factors for attempted suicide among Swedish male conscripts. Acta Psychiatr Scand 1999;100: 433-40.
Otto U. Male youths. A sociopsychiatric study of a total annual population of swedish adolescent boys. Acta Paediatrica Scand Suppl 1976;264: 5-47.
Carlstedt B. Cognitive abilities—aspects of structure, process and measurement. University of Gothenburg , 2000.
Neeleman J, Wessely S, Wadsworth M. Predictors of suicide, accidental death, and premature natural death in a general-population birth cohort. Lancet 1998;351: 93-7.
Paffenbarger RS, King SH, Wing AL. Chronic disease in former college students. Am J Pub Health 1969;59: 900-8.
Hart CL, Taylor MD, Davey-Smith G, Whalley LJ, Starr JM, Hole DJ, et al. Childhood IQ, social class, deprivation, and their relationships with mortality and morbidity risk in later life: prospective observational study linking the Scottish mental survey 1932 and the midspan studies. Psychosom Med 2003;65: 877-83.
Whalley LJ, Deary IJ. Longitudinal cohort study of childhood IQ and survival up to age 76. BMJ 2001;322: 819.
Sacker A, Schoon I, Bartley M. Social inequality in educational achievement and psychosocial adjustment throughout childhood: magnitude and mechanisms. Soc Sci Med 2002;55: 863-80.
((D Gunnell, professor of epidemiology1, P)