The association of intelligence with morbidity, mortality and disability pension : epidemiological studies in a cohort of Swedish men

Intelligence measured early in life is associated with various health outcomes later in life, but the mechanisms underlying the associations are not fully known. The overarching aim of the thesis was to increase the understanding of such associations, and in particular to investigate some potential mechanisms that might explain them. The Swedish conscription cohort 1969-70 was used in all four studies. This cohort of 49 321 men, born 1949-51, was linked to several nation registers with information spanning from the men‘s childhood up to age 59. The exposure variable, intelligence, was measured at conscription when the men were 18-20 years old. Studies I-III were based on the full cohort. Study IV was based on a subsample comprising the 2156 men who were diagnosed with coronary heart disease (CHD) after 1990. The associations between intelligence and the primary outcomes were analysed in survival analysis, using Cox proportional hazards model, or by logistic regression. Potential explanatory factors (confounders and mediators) were retracted from conscription, national populations and housing censuses, the LOUISE/LISA registers and other national registers. In general, lower intelligence in early adulthood was associated with poorer outcomes. This includes most indicators of health status in early adulthood (study III, in cross-sectional analyses), suicide and suicide attempt up to age 57 (study I), disability pension granted between age 40 and 59 (study II), case-fatality in first CHD between age 40 and 58 (study IV) and mortality up to age 59 among the men who survived a first CHD (study (IV). In studies I and II, the associations were graded across the intelligence span. However, no association between intelligence and recurrence in CHD was found (study IV), and some diagnoses in early adulthood were not, or positively, associated with intelligence (study III). Psychiatric diagnoses showed the strongest association with intelligence compared to all somatic diagnoses and health indicators. Characteristics and health indicators measured in early adulthood, including personality aspects, an indicator of social problems, substance use and BMI, attenuated the associations in all analyses where such data were introduced. In addition, socioeconomic and social factors in adulthood contributed in attenuating the associations. However, childhood socioeconomic factors had no or minimal impact on the associations. The results add to, and extend, previous findings of associations between intelligence early in life and health outcomes across the life span. The associations between intelligence and disability pension in late middle age and several diagnoses and health indicators in youth had not been investigated previously. Of the covariates studied, individual characteristics and health behaviours in youth, and socioeconomic circumstances in adulthood, were the most important contributors to the associations in this male cohort. Although causality cannot be determined, the findings are compatible with the notion of an accumulation of risks associated with intelligence across the life course. The findings also suggest that intelligence might not be as important when a chronic disease such as CHD is already established

Personality by intelligence interaction effects on grades tend to be synergistic

Earlier research has identified personality traits × intelligence interaction effects on academic performance. The present study, employing data from the 1997 National Longitudinal Survey of Youth (NLSY97, N = 8984), identified some interaction effects, mainly involving indicators of dependability, on high school grades. The interaction effects tended to be synergistic, meaning that the association between the trait and grades tended to strengthen with increased intelligence. This could be due to a sufficient level of intelligence being necessary for higher grades, and that lack thereof cannot be compensated for by a high degree of some advantageous personality trait. However, a positive association between intelligence and reliability in the measurement of personality, as well as a reversed causal effect of grades on self-rated personality moderated by intelligence, could also be contributing causes to the tendency for synergistic interaction effects

Dangers of residual confounding: A cautionary tale featuring cognitive ability, socioeconomic background, and education

Cognitive ability and socioeconomic background (SEB) have been previously identified as determinants of achieved level of education. According to a “discrimination hypothesis”, higher cognitive ability is required from those with lower SEB in order to achieve the same level of education as those with higher SEB. Support for this hypothesis has been claimed from the observation of a positive association between SEB and achieved level of education when adjusting for cognitive ability. Here we propose a competing hypothesis that the observed association is due to residual confounding. To adjudicate between these hypotheses, we tested both on data from the 1997 National Longitudinal Survey of Youth (NLSY97, N = 8984), including a check of the logic where we switched places between predictor and outcome. We found the expected positive association between SEB and achieved level of education when adjusting for cognitive ability (predicted by both hypotheses), but we also observed a positive association between cognitive ability and SEB when adjusting for level of education (predicted only by the residual confounding hypothesis). Under the logic of the discrimination hypothesis, a contradictory interpretation emerges: higher cognitive ability is required from those with higher SEB in order to achieve the same level of education as those with lower SEB. These results highlight the potential use of reversing predictors and outcomes to test the logic of hypothesis testing, and support a residual confounding hypothesis over a discrimination hypothesis in explaining associations between SEB, cognitive ability, and educational outcome

Intelligence and Disability Pension in Swedish Men and Women Followed from Childhood to Late Middle Age.

ObjectiveTo investigate the association between intelligence and disability pension due to mental, musculoskeletal, cardiovascular, and substance-use disorders among men and women, and to assess the role of childhood social factors and adulthood work characteristics.MethodsTwo random samples of men and women born 1948 and 1953 (n = 10 563 and 9 434), and tested for general intelligence at age 13, were followed in registers for disability pension until 2009. Physical and psychological strains in adulthood were assessed using job exposure matrices. Associations were examined using Cox proportional hazard regression models, with increases in rates reported as hazard ratios (HRs) with 95% confidence intervals (95%CI) per decrease in stanine intelligence.ResultsIn both men and women increased risks were found for disability pension due to all causes, musculoskeletal disorder, mental disorder other than substance use, and cardiovascular disease as intelligence decreased. Increased risk was also found for substance use disorder in men. In multivariate models, HRs were attenuated after controlling for pre-school plans in adolescence, and low job control and high physical strain in adulthood. In the fully adjusted model, increased HRs remained for all causes (male HR 1.11, 95%CI 1.07-1.15, female HR 1.06, 95%CI 1.02-1.09) and musculoskeletal disorder (male HR 1.16, 95%CI 1.09-1.24, female HR 1.08, 95%CI 1.03-1.14) during 1986 to 2009.ConclusionRelatively low childhood intelligence is associated with increased risk of disability pension due to musculoskeletal disorder in both men and women, even after adjustment for risk factors for disability pension measured over the life course

Academic performance, externalizing disorders and depression: 26 000 adolescents followed into adulthood

Background The incidence of major depression among adults has been shown to be socially differentiated, and there are reasons to seek explanations for this before adulthood. In this cohort study, we examined whether academic performance in adolescence predicts depression in adulthood, and the extent to which externalizing disorders explain this association. Methods We followed 26,766 Swedish women and men born 1967–1982 from the last year of compulsory school, at age about 16, up to 48 years of age. We investigated the association between grade point average (GPA, standardized by gender) and first diagnosis of depression in national registers of in- or out-patient psychiatric care. We used Cox proportional hazards models, adjusting for lifetime externalizing diagnoses and potential confounders including childhood socioeconomic position and IQ. Results During follow-up, 7.0% of the women and 4.4% of the men were diagnosed with depression. A GPA in the lowest quartile, compared with the highest, was associated with an increased risk in both women (hazard ratio 95% confidence interval 1.7, 1.3–2.1) and men (2.9, 2.2–3.9) in models controlling for potential confounders. Additional control for externalizing disorders attenuated the associations, particularly in women. Conclusions The findings suggest that poor academic performance is associated with depression in young adulthood and that the association is partly explained by externalizing disorders. Our results indicate the importance of early detection and management of externalizing disorders among children and adolescents

Study Components of the Evaluation Through Follow-up 1948 and 1953 cohorts.

<p>The figure shows the sample frame, initial participants, deaths between participation and the 1986 census, and the study base in which disability pension was studied.</p

Distribution of potential explanatory risk factors across groups based on intelligence.

<p>^<i>a</i> = Participants with information on occupation in 1985 census (n = 8218 men, 7213 women).</p><p>Distribution of potential explanatory risk factors across groups based on intelligence.</p

Associations between potential explanatory risk factors and disability pension, hazard ratios (HRs) and 95% confidence intervals (CIs).

<p>^<i>a</i> = Participants with information on occupation in 1985 census (n = 8218 men, 7213 women).</p><p>Associations between potential explanatory risk factors and disability pension, hazard ratios (HRs) and 95% confidence intervals (CIs).</p