Sibling comparison designs: bias from non-shared confounders and measurement error

Gestational Age and Birth Weight and the Risk of Childhood Type 1 Diabetes: A Population-Based Cohort and Sibling Design Study

OBJECTIVE

We investigated the effects of gestational age, birth weight, small for gestational age (SGA), and large for gestational age (LGA) on risk of childhood type 1 diabetes.

RESEARCH DESIGN AND METHODS

We conducted a population-based cohort study of all singleton live births in Sweden between 1973 and 2009 and a sibling control study. Perinatal data were extracted from the Swedish Medical Birth Register. Children with type 1 diabetes diagnosis were identified from the Swedish National Patient Register. Log-linear Poisson regression and conditional logistic regression were used for data analysis.

RESULTS

The study cohort consisted of 3,624,675 singleton live births (42,411,054 person-years). There were 13,944 type 1 diabetes cases during the study period. The sibling control study consisted of 11,403 children with type 1 diabetes and 17,920 siblings. Gestational age between 33 and 36 weeks (relative risk [RR] 1.18 [95% CI 1.09, 1.28) and 37 and 38 weeks (RR 1.12 [95% CI 1.07, 1.17]) was associated with type 1 diabetes in the cohort study and remained significant in the sibling control study. SGA (RR 0.83 [95% CI 0.75, 0.93]) and LGA (RR 1.14 [95% CI 1.04, 1.24]) were associated with type 1 diabetes in the cohort study. The SGA association remained unchanged in the sibling study, while the LGA association disappeared. Very low birth weight was associated with a reduced risk of type 1 diabetes.

CONCLUSIONS

The findings suggest a small association between gestational age and type 1 diabetes that is not likely due to familial confounding factors. Gestational age and type 1 diabetes may be related to insulin resistance due to early life growth restriction or altered gut microbiota in preterm babies.

Translational Epidemiologic Approaches to Understanding the Consequences of Early-Life Exposures

Prominent developmental theories posit a causal link between early-life exposures and later functioning. Yet, observed associations with early exposures may not reflect causal effects because of genetic and environmental confounding. The current manuscript describes how a systematic series of epidemiologic analyses that combine several genetically-informative designs and statistical approaches can help distinguish between competing theories. In particular, the manuscript details how combining the use of measured covariates with sibling-comparisons, cousin-comparisons, and additional designs can help elucidate the sources of covariation between early-life exposures and later outcomes, including the roles of (a) factors that are not shared in families, including a potential causal effect of the exposure; (b) carryover effects from the exposure of one child to the next; and (c) familial confounding. We also describe key assumptions and how they can be critically evaluated. Furthermore, we outline how subsequent analyses, including effect decomposition with respect to measured, plausible mediators, and quantitative genetic models can help further specify the underlying processes that account for the associations between early-life exposures and offspring outcomes.

Severe stress following bereavement during pregnancy and risk of pregnancy loss: results from a population-based cohort study

BACKGROUND

Previous findings on the association between stress during pregnancy and pregnancy loss are inconsistent. We aimed to estimate this association using a large prospective cohort.

METHODS

This population-based study included all 1 303 660 clinically recognised pregnancies in Denmark between 1995 and 2008. We categorised women as exposed to severe stress if they lost a child, sibling or parent during pregnancy. Cox Proportional Hazards models were used to study the association between exposure and rate of fetal death, starting with the follow-up on the day of completion of week 4 of pregnancy. In an attempt to control for unknown potential confounders, we also designed a pregnancy-matched analysis in which each woman had her own baseline risk of pregnancy loss and controls therefore for genetic and time-stable environmental factors.

RESULTS

A total of 146 031 pregnancies ended in clinically recognised fetal loss (11.2%) and a total of 10 808 (0.8%) women were categorised as exposed. The overall risk of pregnancy loss was similar in the exposed and unexposed (aHR=1.05, 95% CI 0.95 to 1.15). Results from the pregnancy-matched analysis (performed in 423 women) showed stronger and significant associations (aHR=1.83, 95% CI 1.49 to 2.25). All the analyses indicated a stronger effect of bereavement when the mother lost a child or when the death was unexpected.

CONCLUSIONS

Our main results suggested no strong association between severe stress during pregnancy and risk of pregnancy loss. Results from the pregnancy-matched analyses considered information from a selected and small group of women for whom there may exist a stronger association between stress during pregnancy and pregnancy loss. The fact that an unexpected death or the loss of a child had a stronger effect in both analyses may indicate that severe stressful situations increase the risk of pregnancy loss.

Maternal obesity during pregnancy and cardiovascular development and disease in the offspring

Maternal obesity during pregnancy is an important public health problem in Western countries. Currently, obesity prevalence rates in pregnant women are estimated to be as high as 30 %. In addition, approximately 40 % of women gain an excessive amount of weight during pregnancy in Western countries. An accumulating body of evidence suggests a long-term impact of maternal obesity and excessive weight gain during pregnancy on adiposity, cardiovascular and metabolic related health outcomes in the offspring in fetal life, childhood and adulthood. In this review, we discuss results from recent studies, potential underlying mechanisms and challenges for future epidemiological studies.

Age at immigration and crime in Stockholm using sibling comparisons

Past Swedish research has shown that immigrants arriving in the receiving country at an older age are less likely to commit crime than immigrants arriving at a younger age. Segmented assimilation theory argues that the family and neighborhood may be important factors affecting how age at immigration and crime are related to one another. This study used population-based register data on foreign-background males from Stockholm to test the effect of age at immigration on crime. Potential confounding from the family and neighborhood was addressed using variables and modeling strategies. Initial results, using variables to control for confounding, showed that people who immigrated around age 4 were the most likely to be suspected of a crime. When controlling for unmeasured family characteristics, it seemed that a later age at immigration was tied to a lower likelihood of crime, which does not corroborate past research findings. The effect of age at immigration, however, was not statistically significant. The results imply that future research on entire families may be a worthwhile endeavor.

Maternal smoking and offspring inattention and hyperactivity: results from a cross-national European survey

In utero exposure to tobacco smoke is associated with adverse neonatal outcomes; the association with later childhood mental health outcomes remains controversial. We used a strategy involving comparison of maternal and paternal smoking reports in a sample pooling data from six diverse European countries. Data were drawn from mother (N = 4,517) and teacher (N = 4,611) reported attention deficit and hyperactivity disorder (ADHD) symptoms in school children aged 6-11 in Turkey, Romania, Bulgaria, Lithuania, Germany, and the Netherlands, surveyed in 2010. Mothers report on self and husband's smoking patterns during the pregnancy period. Logistic regression used with control covariates including demographics, maternal distress, live births, region, and post-pregnancy smoking. In unadjusted models, maternal prenatal smoking was associated with probable ADHD based on mother [Odds Ratio (OR) = 1.82, 95 % Confidence Interval (CI) 1.45-2.29], teacher (OR = 1.69, 95 % CI 1.33-2.14) and mother plus teacher (OR = 1.49, 95 % CI 1.03-2.17) report. Paternal prenatal smoking was similarly associated with probable ADHD in unadjusted models. When controlled for relevant confounders, maternal prenatal smoking remained a risk factor for offspring probable ADHD based on mother report (OR = 1.44, 95 % CI 1.06-1.96), whereas the effect of paternal prenatal smoking diminished (e.g., mother report: OR = 1.17, 95 % CI 0.92-1.49). Drawing on data from a diverse set of countries across Europe, we document that the association between maternal smoking and offspring ADHD is stronger than that of paternal smoking during the pregnancy period and offspring ADHD. To the extent that confounding is shared between parents, these results reflect a potential intrauterine influence of smoking on ADHD in children.

Does Traumatic Brain Injury Lead to Criminality? A Whole-Population Retrospective Cohort Study Using Linked Data

Background

Traumatic brain injury (TBI) may be a risk factor for criminal behaviour however multiple factors potentially confound the association.

Methods

Record linkage and Cox proportional hazards regression analyses were used to examine the association between hospital-recorded TBI (n = 7,694) and subsequent first criminal conviction in a retrospective cohort matched 1:3 with 22,905 unaffected community controls and full-sibling controls (n = 2,397). Aboriginality, substance abuse, social disadvantage, and mental illness were included in analyses as potential confounders

Results

In multivariable models, relative to general population controls, TBI was associated with any conviction (males: Hazard Ratio (HR) = 1·58 (95% CI 1·46 to 1·72); females: HR = 1·52 (95% CI 1·28 to 1·81)); and similar Hazard Ratios were obtained for the sibling analyses in males (HR = 1.68 (95% CI 1.31-2.18)) and females (HR 1.27 (95% CI 0.71-2.29)). TBI was also associated with violent convictions relative to the general population, (males: HR = 1.65 (95% CI 1.42 to 1.92); females HR = 1.73 (95% CI 1.21 to 2.47)), and in analyses with sibling controls in men (HR = 1.89 (95% CI 1.20-3.00)), but not in women (HR 0.73, 95% CI 0.29-1.81)).

Conclusion

The results support a modest causal link between TBI and criminality after comprehensive adjustment for confounding. Reducing the rate of TBI, a major public health imperative, might have benefits in terms of crime reduction.

Optimal design strategies for sibling studies with binary exposures

Sibling studies have become increasingly popular because they provide better control over confounding by unmeasured family-level risk factors than can be obtained in standard cohort studies. However, little attention has been devoted to the development of efficient design strategies for sibling studies in terms of optimizing power. We here address this issue in commonly encountered types of sibling studies, allowing for continuous and binary outcomes and varying numbers of exposed and unexposed siblings. For continuous outcomes, we show that in families with sibling pairs, optimal study power is obtained by recruiting discordant (exposed-control) pairs of siblings. More generally, balancing the exposure status within each family as evenly as possible is shown to be optimal. For binary outcomes, we elucidate how the optimal strategy depends on the variation of the binary response; as the within-family correlation increases, the optimal strategy tends toward only recruiting discordant sibling pairs (as in the case of continuous outcomes). R code for obtaining the optimal strategies is included.

Mortality and work disability in a cohort of Norwegian couples--the HUNT study

BACKGROUND

Poor health is clustered in families, and partners might influence each other. We studied possible consequences of living with a spouse with poor health or unhealthy lifestyle on mortality and work disability.

METHODS

In total, 18 943 couples from the HUNT2 Study (1995-97) were linked to national registries and followed until December 2007, identifying deaths and disability pension retirements. Couple's mean exposures were included together with the individual's deviation from the couple mean in discrete time multilevel logistic regression.

RESULTS

There was weak evidence of associations between partner's health and risk of dying. Associations between couples slightly exceeded associations within couples for smoking [odds ratio (OR) within 1.57 (95% confidence interval (CI): 1.38-1.78); OR between 1.88 (95% CI: 1.70-2.08), P value for difference 0.027] and education [OR within 1.07 (95% CI: 0.99-1.15); OR between 1.17 (1.11-1.23), P value for difference 0.065]. Indicators of partner's health, such as self-rated health [OR within 3.17 (95% CI: 2.80-3.58); OR between 3.92 (95% CI: 3.50-4.40), P value for difference 0.014], insomnia [OR within 1.39 (95% CI: 1.18-1.64); OR between 2.11 (95% CI: 1.86-2.53), P value for difference <0.001] and symptoms of depression [OR within 1.45 (95% CI: 1.22-1.71); OR between 1.98 (95% CI: 1.69-2.31) P value for difference 0.009] were, however, associated with risk of work disability. Self-rated health and symptoms displayed stronger associations with work disability among partners than reported somatic diseases.

CONCLUSIONS

This study did not indicate strong consequences of living with a spouse with poor health or unhealthy lifestyle on mortality. It did, however, indicate associations of partner's health with work disability.

In utero exposure to selective serotonin reuptake inhibitors and risk for autism spectrum disorder

We investigated whether there is an association between increased risk for autism spectrum disorders (ASD) and selective serotonin reuptake inhibitors (SSRIs) used during pregnancy. This study used Denmark's health and population registers to obtain information regarding prescription drugs, ASD diagnosis, and health and socioeconomic status. There were 1.5 % of cases and 0.7 % of controls exposed to SSRIs during the pregnancy period, and higher effect estimates observed with longer use. We found evidence that in utero exposure to SSRIs increases a child's risk associated with ASD. These results, while adding to the limited knowledge on prenatal pharmacological exposures as potential ASD risk factors, need to be balanced against the benefits of indicated medication use by pregnant mothers.

Altered amygdalar resting-state connectivity in depression is explained by both genes and environment

Recent findings indicate that alterations of the amygdalar resting-state fMRI connectivity play an important role in the etiology of depression. While both depression and resting-state brain activity are shaped by genes and environment, the relative contribution of genetic and environmental factors mediating the relationship between amygdalar resting-state connectivity and depression remain largely unexplored. Likewise, novel neuroimaging research indicates that different mathematical representations of resting-state fMRI activity patterns are able to embed distinct information relevant to brain health and disease. The present study analyzed the influence of genes and environment on amygdalar resting-state fMRI connectivity, in relation to depression risk. High-resolution resting-state fMRI scans were analyzed to estimate functional connectivity patterns in a sample of 48 twins (24 monozygotic pairs) informative for depressive psychopathology (6 concordant, 8 discordant and 10 healthy control pairs). A graph-theoretical framework was employed to construct brain networks using two methods: (i) the conventional approach of filtered BOLD fMRI time-series and (ii) analytic components of this fMRI activity. Results using both methods indicate that depression risk is increased by environmental factors altering amygdalar connectivity. When analyzing the analytic components of the BOLD fMRI time-series, genetic factors altering the amygdala neural activity at rest show an important contribution to depression risk. Overall, these findings show that both genes and environment modify different patterns the amygdala resting-state connectivity to increase depression risk. The genetic relationship between amygdalar connectivity and depression may be better elicited by examining analytic components of the brain resting-state BOLD fMRI signals.

Time to consider the risks of caesarean delivery for long term child health

Jan Blustein and Jianmeng Liu examine the evidence linking caesarean delivery with childhood chronic disease and say that guidelines on delivery should be reviewed with these risks in mind

Maternal body mass index during early pregnancy, gestational weight gain, and risk of autism spectrum disorders: Results from a Swedish total population and discordant sibling study

BACKGROUND

Prenatal environmental factors such as maternal adiposity may influence the risk of offspring autism spectrum disorders (ASD), though current evidence is inconsistent. The objective of this study was to assess the relationship of parental BMI and gestational weight gain (GWG) with risk of offspring ASD in a population-based cohort study using family-based study designs.

METHODS

The cohort was based in Stockholm County, Sweden, including 333,057 individuals born 1984-2007, of whom 6420 were diagnosed with an ASD. We evaluated maternal body mass index (BMI) at first antenatal visit, GWG and paternal BMI at the time of conscription into the Swedish military as exposures using general estimating equation (GEE) models with logit link.

RESULTS

At the population level, maternal overweight/obesity was associated with increased risk of offspring ASD [odds ratio (OR)25 ≤ BMI < 30 1.31, 95% confidence interval = 1.21-1.41; ORBMI ≥ 30 1.94, 1.72-2.17], as was paternal underweight (ORBMI < 18.5, 1.19, 1.06-1.33) and obesity (ORBMI ≥ 30 1.47, 1.12-1.92) in mutually adjusted models. However, in matched sibling analyses, the relationship between elevated maternal BMI and ASD risk was not apparent. GWG had a U-shaped association with offspring ASD at the population level (ORinsufficient 1.22, 1.07-1.40; ORexcessive 1.23, 1.08-1.40). Matched sibling analyses were suggestive of elevated risk with excessive GWG (ORinsufficient 1.12, 0.68-1.84; ORexcessive 1.48, 0.93-2.38).

CONCLUSIONS

Whereas population-level results suggested that maternal BMI was associated with ASD, sibling analyses and paternal BMI analyses indicate that maternal BMI may also be a proxy marker for other familial risk factors. Evidence is stronger for a direct link between GWG and ASD risk.

Early gestation as the critical time-window for changes in the prenatal environment to affect the adult human blood methylome

Background: The manipulation of pregnancy diets in animals can lead to changes in DNA methylation with phenotypic consequences in the offspring. Human studies have concentrated on the effects of nutrition during early gestation. Lacking in humans is an epigenome-wide association study of DNA methylation in relation to perturbations in nutrition across all gestation periods.

Methods: We used the quasi-experimental setting of the Dutch famine of 1944–45 to evaluate the impact of famine exposure during specific 10-week gestation periods, or during any time in gestation, on genome-wide DNA methylation levels at age ∼ 59 years. In addition, we evaluated the impact of exposure during a shorter pre- and post-conception period. DNA methylation was assessed using the Illumina 450k array in whole blood among 422 individuals with prenatal famine exposure and 463 time- or sibling-controls without prenatal famine exposure.

Results: Famine exposure during gestation weeks 1–10, but not weeks 11–20, 21–30 or 31-delivery, was associated with an increase in DNA methylation of CpG dinucleotides cg20823026 (FAM150B), cg10354880 (SLC38A2) and cg27370573 (PPAP2C) and a decrease of cg11496778 (OSBPL5/MRGPRG) (P < 5.9 × 10⁻⁷, P_FDR < 0.031). There was an increase in methylation of TACC1 and ZNF385A after exposure during any time in gestation (P < 2.0 × 10⁻⁷, P_FDR = 0.034) and a decrease of cg23989336 (TMEM105) after exposure around conception. These changes represent a shift of 0.3–0.6 standard deviations and are linked to genes involved in growth, development and metabolism.

Conclusion: Early gestation, and not mid or late gestation, is identified as a critical time-period for adult DNA methylation changes in whole blood after prenatal exposure to famine.

Risk of hypertensive disorders in pregnancies following assisted reproductive technology: a cohort study from the CoNARTaS group

STUDY QUESTION

Is the risk of hypertensive disorders in pregnancies conceived following specific assisted reproductive technology (ART) procedures different from the risk in spontaneously conceived (SC) pregnancies?

SUMMARY ANSWER

ART pregnancies had a higher risk of hypertensive disorders, in particular following cryopreservation, with the highest risk seen in twin pregnancies following frozen-thawed cycles.

WHAT IS KNOWN ALREADY

The risk of hypertensive disorders is higher in ART pregnancies than in SC pregnancies. The increased risk may be partly explained by multiple pregnancies and underlying infertility, but a contribution from specific ART procedures has not been excluded.

STUDY DESIGN, SIZE, DURATION

Population-based cohort study, including sibling design with nationwide data from health registers in Sweden, Denmark and Norway.

PARTICIPANTS/MATERIALS, SETTING, METHODS

All registered ART pregnancies and a sample of SC pregnancies with gestational age ≥22 weeks from 1988 to 2007 were included. ART singleton pregnancies (n = 47 088) were compared with SC singleton pregnancies (n = 268 599), matched on parity and birth year. ART twin pregnancies (n = 10 918) were compared with SC twin pregnancies (46 674). We used logistic regression to estimate adjusted odds ratios and risk differences for hypertensive disorders in pregnancies following IVF, ICSI and fresh or frozen-thawed cycles. We also compared fresh and frozen-thawed cycles within mothers who had conceived following both procedures using conditional logistic regression (sibling analysis).

MAIN RESULTS AND THE ROLE OF CHANCE

Hypertensive disorders were reported in 5.9% of ART singleton and 12.6% of ART twin pregnancies. Comparing singleton pregnancies, the risk of hypertensive disorders was higher after all ART procedures. The highest risk in singleton pregnancies was seen after frozen-thawed cycles [risk 7.0%, risk difference 1.8%, 95% confidence interval (CI) 1.2-2.8]. Comparing twin pregnancies, the risk was higher after frozen-thawed cycles (risk 19.6%, risk difference 5.1%, 95% CI 3.0-7.1), but not after fresh cycles. In siblings, the risk was higher after frozen-thawed cycles compared with fresh cycles within the same mother (odds ratio 2.63, 95% CI 1.73-3.99). There were no clear differences in risk for IVF and ICSI.

LIMITATIONS, REASONS FOR CAUTION

The number of ART siblings in the study was limited. Residual confounding cannot be excluded. In addition, we did not have information on all SC pregnancies in each woman's history, and could therefore not compare risk in ART versus SC pregnancies in the same mother.

WIDER IMPLICATIONS OF THE FINDINGS

Pregnancies following frozen-thawed cycles have a higher risk of hypertensive disorders, also when compared with fresh cycle pregnancies by the same mother. The safety aspects in frozen-thawed cycles merit further attention.

STUDY FUNDING/COMPETING INTERESTS

Funding was received from the European Society for Human Reproduction and Embryology, the University of Copenhagen, the Danish Agency for Science, Technology and Innovation, the Nordic Federation of Societies of Obstetrics and Gynecology and the Liaison Committee between the Central Norway Regional Health Authority and the Norwegian University of Science and Technology. None of the authors has any competing interests to declare.

Maternal tobacco smoking in pregnancy and children's socio-emotional development at age 5: The EDEN mother-child birth cohort study

BACKGROUND

There is debate as to whether maternal tobacco use in pregnancy is related to offspring behaviour later on. We tested this association examining multiple aspects of children's behaviour at age 5 and accounting for parental smoking outside of pregnancy, as well as child and family characteristics.

METHODS

Data come from a prospective community based birth cohort study (EDEN; n=1113 families in France followed since pregnancy in 2003-2005 until the child's 5th birthday). Maternal tobacco use in pregnancy was self-reported. Children's socio-emotional development (emotional symptoms, conduct problems, symptoms of hyperactivity/inattention, peer relationship problems, prosocial behaviour) was assessed by mothers using the Strengths and Difficulties Questionnaire (SDQ) at age 5 years. Logistic regression analyses controlled for Inverse Probability Weights (IPW) of maternal tobacco use calculated based on study center, children's characteristics (sex, premature birth, low birth weight, breastfeeding), maternal characteristics (age at the child's birth, psychological difficulties and alcohol use in pregnancy, post-pregnancy depression, and smoking), paternal smoking in and post-pregnancy, parental educational attainment, family income, parental separation, and maternal negative life events.

RESULTS

Maternal smoking in pregnancy only predicted children's high symptoms of hyperactivity/inattention (sex and study center-adjusted ORs: maternal smoking in the 1st trimester: 1.95, 95%CI: 1.13-3.38; maternal smoking throughout pregnancy: OR=2.11, 95%CI: 1.36-3.27). In IPW-controlled regression models, only children of mothers who smoked throughout pregnancy had significantly elevated levels of hyperactivity/inattention (OR=2.20, 95%CI: 1.21-4.00).

CONCLUSIONS

Maternal tobacco smoking in pregnancy may contribute directly or through epigenetic mechanisms to children's symptoms of hyperactivity/inattention.

Alcohol-related morbidity and mortality within siblings

AIMS

To estimate the association between educational status and alcohol-related somatic and non-somatic morbidity and mortality among full siblings in comparison with non-related individuals.

DESIGN

Cohort study.

SETTING

Denmark.

PARTICIPANTS

Approximately 1.4 million full siblings born in Denmark between 1950 and 1979 were followed from age 28-58 years or censoring due to alcohol-related hospitalization and mortality.

MEASUREMENTS

Cox regression analyses were used to estimate associations of educational status with alcohol-related outcomes. Results from cohort analyses based on non-related individuals and inter-sibling analyses were compared.

FINDINGS

A lower educational status was associated with a higher rate of alcohol-related outcomes, especially among the youngest (aged 28-37 years) and individuals born 1970-79. Compared with the cohort analyses, the associations attenuated slightly in the inter-sibling analysis. For example, in the cohort analysis, females with a basic school education born 1970-79 had an increased rate of alcohol-related non-somatic morbidity and mortality [hazard rate ratio (HR) = 4.05, 95% confidence interval (CI) = 3.27-5.02] compared to those with a vocational education. In the inter-sibling analysis, the HR attenuated (HR = 2.66, 95% CI = 1.95-3.63). For alcohol-related somatic outcomes the corresponding figures were HR = 3.47 (95% CI = 2.63-4.58) and HR = 3.36 (95% CI = 2.10-5.38), respectively. In general, the associations were stronger among females than males (aged 28-37) in the analyses of alcohol-related non-somatic outcomes. Health conditions earlier in life explained only a minor part of the associations.

CONCLUSIONS

The association between educational status and alcohol-related somatic and non-somatic morbidity and mortality is only driven by familial factors to a small degree.

Does population density and neighborhood deprivation predict schizophrenia? A nationwide Swedish family-based study of 2.4 million individuals

People living in densely populated and socially disorganized areas have higher rates of psychiatric morbidity, but the potential causal status of such factors is uncertain. We used nationwide Swedish longitudinal registry data to identify all children born 1967-1989 (n = 2361585), including separate datasets for all cousins (n = 1715059) and siblings (n = 1667894). The nature of the associations between population density and neighborhood deprivation and individual risk for a schizophrenia diagnosis was investigated while adjusting for unobserved familial risk factors (through cousin and sibling comparisons) and then compared with similar associations for depression. We generated familial pedigree structures using the Multi-Generation Registry and identified study participants with schizophrenia and depression using the National Patient Registry. Fixed-effects logistic regression models were used to study within-family estimates. Population density, measured as ln(population size/km(2)), at age 15 predicted subsequent schizophrenia in the population (OR = 1.10; 95% CI: 1.09; 1.11). Unobserved familial risk factors shared by cousins within extended families attenuated the association (1.06; 1.03; 1.10), and the link disappeared entirely within nuclear families (1.02; 0.97; 1.08). Similar results were found for neighborhood deprivation as predictor and for depression as outcome. Sensitivity tests demonstrated that timing and accumulation effects of the exposures (mean scores across birth, ages 1-5, 6-10, and 11-15 years) did not alter the findings. Excess risks of psychiatric morbidity, particularly schizophrenia, in densely populated and socioeconomically deprived Swedish neighborhoods appear, therefore, to result primarily from unobserved familial selection factors. Previous studies may have overemphasized the etiological importance of these environmental factors.

Socioeconomic status and sick leave granted for mental and somatic disorders: a prospective study of young adult twins

Background

Low socioeconomic status (SES), indicated by low income and education, has consistently been found to be a strong predictor of sick leave. Several possible pathways from SES to sick leave have been described in previous literature, but there are also evidence indicating that the association can be confounded by common underlying factors. This study utilizes a population-based sample of employed young adult twins to estimate (i) the degree to which education and income are prospectively related to sick leave granted for mental, somatic, and any disorder, and (ii) whether these associations are confounded by familial factors.

Methods

Registry data on educational attainment and income at age 30 and subsequent sick leave were available for 6,103 employed young adult twins, among which there were 2,024 complete twin pairs. The average follow-up time was 6.57 years. Individual-level associations and fixed effects within twin pairs were estimated.

Results

Low education and income were associated with sick leave granted for both mental and somatic disorders, and with sick leave granted for any disorder. Associations were attenuated within dizygotic twin pairs and reduced to non-significance within monozygotic twin pairs, suggesting influence of familial factors on the associations between SES and sick leave.

Conclusions

Low SES is associated with a higher level of sick leave granted for both mental and somatic disorders among young adults, but these associations are confounded by factors that are common to co-twins. Education and income are therefore not likely to strongly affect sick leave in young adulthood.

Tests of the effects of adolescent early alcohol exposures on adult outcomes

AIMS

To determine whether early adolescent alcohol use contributes to adult alcohol use, misuse and other adult substance-related and social outcomes.

DESIGN

In a longitudinal study of twins assessed at target ages 11, 14 and 24 years, two techniques adjusted for confounding factors: a propensity score (PS) adjusting for the effects of measured background covariates and co-twin control (CTC) adjusting for confounding by unmeasured (including genetic) factors shared within early alcohol exposure-discordant pairs.

SETTING

The community-based Minnesota Twin Family Study.

PARTICIPANTS

A total of 1512 (50.3% female) twins.

MEASUREMENTS

Early adolescent alcohol exposures, adult substance-related and social outcomes and background variables reflecting behavioral, familial and environmental characteristics.

FINDINGS

Background covariates unbalanced between those with and without early alcohol exposure were balanced through PS-based weighting, leaving several adult outcomes related to substance use or social functioning remaining significantly associated with early alcohol exposure. Similarly, the within-pair individual-level component of a CTC indicated that early alcohol-exposed twins had higher risk than their non-exposed co-twins for several, but not all, of the same adult outcomes. For example, early alcohol use was associated with an adult index of alcohol use in both PS-weighted (β = 0.57, P < 0.001) and CTC (β = 0.21, P = 0.031) analyses.

CONCLUSIONS

Early alcohol exposures predict adult alcohol problems and related outcomes, despite stringent adjustment for measured and non-measured sources of potential confounding using propensity score and co-twin control. Contrasting the methods indicated that exposure effect estimates from PS application were likely biased by unmeasured confounding factors.

Peak weight and height velocity to age 36 months and asthma development: the Norwegian Mother and Child Cohort Study

Background

The immediate postnatal period is the period of the fastest growth in the entire life span and a critical period for lung development. Therefore, it is interesting to examine the association between growth during this period and childhood respiratory disorders.

Methods

We examined the association of peak weight and height velocity to age 36 months with maternal report of current asthma at 36 months (n = 50,311), recurrent lower respiratory tract infections (LRTIs) by 36 months (n = 47,905) and current asthma at 7 years (n = 24,827) in the Norwegian Mother and Child Cohort Study. Peak weight and height velocity was calculated using the Reed1 model through multilevel mixed-effects linear regression. Multivariable log-binomial regression was used to calculate adjusted relative risks (adj.RR) and 95% confidence intervals (CI). We also conducted a sibling pair analysis using conditional logistic regression.

Results

Peak weight velocity was positively associated with current asthma at 36 months [adj.RR 1.22 (95%CI: 1.18, 1.26) per standard deviation (SD) increase], recurrent LRTIs by 36 months [adj.RR 1.14 (1.10, 1.19) per SD increase] and current asthma at 7 years [adj.RR 1.13 (95%CI: 1.07, 1.19) per SD increase]. Peak height velocity was not associated with any of the respiratory disorders. The positive association of peak weight velocity and asthma at 36 months remained in the sibling pair analysis.

Conclusions

Higher peak weight velocity, achieved during the immediate postnatal period, increased the risk of respiratory disorders. This might be explained by an influence on neonatal lung development, shared genetic/epigenetic mechanisms and/or environmental factors.

Long term mental health outcomes of Finnish children evacuated to Swedish families during the second world war and their non-evacuated siblings: cohort study

OBJECTIVES

To compare the risks of admission to hospital for any type of psychiatric disorder and for four specific psychiatric disorders among adults who as children were evacuated to Swedish foster families during the second world war and their non-evacuated siblings, and to evaluate whether these risks differ between the sexes.

DESIGN

Cohort study.

SETTING

National child evacuation scheme in Finland during the second world war.

PARTICIPANTS

Children born in Finland between 1933 and 1944 who were later included in a 10% sample of the 1950 Finnish census ascertained in 1997 (n = 45,463; women: n = 22,021; men: n = 23,442). Evacuees in the sample were identified from war time government records.

MAIN OUTCOME MEASURE

Adults admitted to hospital for psychiatric disorders recorded between 1971 and 2011 in the Finnish hospital discharge register.

METHODS

We used Cox proportional hazards models to estimate the association between evacuation to temporary foster care in Sweden during the second world war and admission to hospital for a psychiatric disorder between ages 38 and 78 years. Fixed effects methods were employed to control for all unobserved social and genetic characteristics shared among siblings.

RESULTS

Among men and women combined, the risk of admission to hospital for a psychiatric disorder did not differ between Finnish adults evacuated to Swedish foster families and their non-evacuated siblings (hazard ratio 0.89, 95% confidence interval 0.64 to 1.26). Evidence suggested a lower risk of admission for any mental disorder (0.67, 0.44 to 1.03) among evacuated men, whereas for women there was no association between evacuation and the overall risk of admission for a psychiatric disorder (1.21, 0.80 to 1.83). When admissions for individual psychiatric disorders were analyzed, evacuated girls were significantly more likely than their non-evacuated sisters to be admitted to hospital for a mood disorder as an adult (2.19, 1.10 to 4.33).

CONCLUSIONS

The Finnish evacuation policy was not associated with an increased overall risk of admission to hospital for a psychiatric disorder in adulthood among former evacuees. In fact, evacuation was associated with a marginally reduced risk of admission for any psychiatric disorder among men. Among women who had been evacuated, however, the risk of being admitted to hospital for a mood disorder was increased.

The association between cannabis abuse and subsequent schizophrenia: a Swedish national co-relative control study

BACKGROUND

Although cannabis abuse (CA) is known to be associated with schizophrenia, the causal nature of this association is unclear, with prodromal effects complicating its interpretation.

METHOD

From Swedish national registry databases, we used a co-relative case-control design with full-sibling, half-sibling and first-cousin comparisons, alongside a general Swedish population sample. Using ICD codes, 5456 individuals with an initial diagnosis of schizophrenia (2000-2010) were matched with five schizophrenia-free controls. We further identified first-cousin, half-sibling and full-sibling pairs discordant for CA and statistically extrapolated results for discordant monozygotic (MZ) twins.

RESULTS

Within the general Swedish population, CA was strongly associated with later schizophrenia [odds ratio (OR) 10.44, 95% confidence interval (CI) 8.99-12.11]. This association was substantially attenuated both by increasing temporal delays between CA exposure and schizophrenia diagnosis and by controlling for increasing degrees of familial confounding. Extrapolated discordant MZ pairs suggested that fully controlling for confounding familial factors reduced the association between CA and later schizophrenia to more modest levels (ORs of approximately 3.3 and 1.6 with 3- and 7-year temporal delays respectively). Opiate, sedative, cocaine/stimulant and hallucinogen abuse were also strongly associated with subsequent schizophrenia in the general population. After controlling for familial confounding, only cocaine/stimulant exposure remained associated.

CONCLUSIONS

CA has an appreciable causal impact on future risk for schizophrenia. However, population-based estimates of cannabis-schizophrenia co-morbidity substantially overestimate their causal association. Predictions of the cases of schizophrenia that might be prevented by reduced cannabis consumption based on population associations are therefore likely to be considerably overestimated.

The association of high birth weight with intelligence in young adulthood: a cohort study of male siblings

We aimed to explore why, in population studies, the positive association between normal-range birth weight and intelligence becomes negative at the highest birth weights. The study population comprised 217,746 Norwegian male singletons born at term between 1967 and 1976. All had data on birth weight and intelligence quotient (IQ) score at the time of military conscription; 137,574 had data on sibling birth weights; and 62,906 had data on male sibling birth weights. We estimated associations between birth weight and IQ score by ordinary least squares regression for the total study population and by fixed-effects regression for comparisons of brothers. The crude mean IQ score was 1.2 points (95% confidence interval (CI): 0.3, 2.2) lower for those with birth weights of 5,000 g or more compared with the reference group (with birth weights of 4,000-4,499 g). This difference leveled off to 0.0 (95% CI: -0.8, 0.9) in multivariable ordinary least squares regression and reversed to 2.2 points (95% CI: 0.3, 4.2) higher in fixed-effects regression. Results differed mainly because, at a given birth weight, participants who had a sibling with macrosomia had a lower mean IQ score. Nevertheless, within families with 1 or more macrosomic siblings, as in other families, men with higher birth weights tended to have higher IQ scores. Thus, a family-level confounder introduces a cross-level bias that cannot be detected in individual-level studies. We suggest ways in which future studies might elucidate the nature of this confounder.

Approaches for drawing causal inferences from epidemiological birth cohorts: a review

Large-scale population-based birth cohorts, which recruit women during pregnancy or at birth and follow up their offspring through infancy and into childhood and adolescence, provide the opportunity to monitor and model early life exposures in relation to developmental characteristics and later life outcomes. However, due to confounding and other limitations, identification of causal risk factors has proved challenging and published findings are often not reproducible. A suite of methods has been developed in recent years to minimise problems afflicting observational epidemiology, to strengthen causal inference and to provide greater insights into modifiable intra-uterine and early life risk factors. The aim of this review is to describe these causal inference methods and to suggest how they may be applied in the context of birth cohorts and extended along with the development of birth cohort consortia and expansion of "omic" technologies.

Invited commentary: Interpreting associations between high birth weight and later health problems

High birth weight (>4.0 kg) has been associated with a wide range of health problems later in life. The interpretation of these statistical associations may be difficult, however. These difficulties are closely linked to methodological challenges in this research, such as filtering out confounding from family factors, disentangling associations with prenatal processes from associations with postnatal processes, and uncovering what birth weight actually represents. The well-conducted study by Kristensen et al. (Am J Epidemiol. 2014;180(9):876-884), presented in this issue of the Journal, offers an interesting example of how one can filter out confounding from family factors. In an elegant series of analyses, the authors show how an apparent inverse association between birth weight and later intelligence among those in the highest range of the birth weight scale became a positive association when proper adjustment for family factors was made. Sibling comparisons were important here.

College quality and hourly wages: evidence from the self-revelation model, sibling models and instrumental variables

This paper addresses the recent discussion on confounding in the returns to college quality literature using the Norwegian case. The main advantage of studying Norway is the quality of the data. Norwegian administrative data provide information on college applications, family relations and a rich set of control variables for all Norwegian citizens applying to college between 1997 and 2004 (N = 141,319) and their succeeding wages between 2003 and 2010 (676,079 person-year observations). With these data, this paper uses a subset of the models that have rendered mixed findings in the literature in order to investigate to what extent confounding biases the returns to college quality. I compare estimates obtained using standard regression models to estimates obtained using the self-revelation model of Dale and Krueger (2002), a sibling fixed effects model and the instrumental variable model used by Long (2008). Using these methods, I consistently find increasing returns to college quality over the course of students' work careers, with positive returns only later in students' work careers. I conclude that the standard regression estimate provides a reasonable estimate of the returns to college quality.

Childhood family income, adolescent violent criminality and substance misuse: quasi-experimental total population study

BACKGROUND

Low socioeconomic status in childhood is a well-known predictor of subsequent criminal and substance misuse behaviours but the causal mechanisms are questioned. Aims To investigate whether childhood family income predicts subsequent violent criminality and substance misuse and whether the associations are in turn explained by unobserved familial risk factors.

METHOD

Nationwide Swedish quasi-experimental, family-based study following cohorts born 1989-1993 (n(total) = 526 167, n(cousins) = 262 267, n(siblings) = 216 424) between the ages of 15 and 21 years.

RESULTS

Children of parents in the lowest income quintile experienced a seven-fold increased hazard rate (HR) of being convicted of violent criminality compared with peers in the highest quintile (HR = 6.78, 95% CI 6.23-7.38). This association was entirely accounted for by unobserved familial risk factors (HR = 0.95, 95% CI 0.44-2.03). Similar pattern of effects was found for substance misuse.

CONCLUSIONS

There were no associations between childhood family income and subsequent violent criminality and substance misuse once we had adjusted for unobserved familial risk factors.

Increased risk of autism spectrum disorders at short and long interpregnancy intervals in Finland

OBJECTIVE

Both short and long interpregnancy intervals (IPI) are believed to present possible adverse conditions for fetal development. Short IPI has recently been associated with increased risk of autism, but whether long IPI increases risk for autism spectrum disorders (ASD) has not been thoroughly investigated. We investigated the association between short and long IPI in a Finnish population-based study.

METHOD

This study was conducted in the Finnish Prenatal Study of Autism, which is based in a national birth cohort. Children born in Finland in 1987 to 2005 and diagnosed with ASD by 2007 were identified through the Finnish Hospital Discharge Register. A total of 2,208 non-firstborn patients with ASD and 5,163 matched controls identified from the Finnish Medical Birth Register were included in the primary analysis. The association between IPI and ASD was determined using conditional logistic regression and adjusted for potential confounders.

RESULTS

Relative to births with an IPI of 24 to 59 months, those with the shortest IPI (<12 months) had an increased risk of ASD (odds ratio [OR] = 1.50, 95% CI = 1.28, 1.74) in confounder-adjusted models, whereas the ORs for longer IPI births (60-119 months and ≥120 months) were 1.28 (95% CI = 1.08, 1.52) and 1.44 (95% CI = 1.12, 1.85), respectively.

CONCLUSION

This study provides evidence that risk of ASD is increased at long as well as short IPI.

Maternal use of antidepressant or anxiolytic medication during pregnancy and childhood neurodevelopmental outcomes: a systematic review

Antidepressant and anxiolytic medications are widely prescribed and used by pregnant women for acute and maintenance therapy. These drugs are able to pass the placental barrier, and may potentially influence fetal and brain development. It is possible that exposure to prenatal antidepressants or anxiolytic medication may disturb neurotransmitter systems in the brain and have long-lasting consequences on neurodevelopment in the offspring. As all medication during pregnancy may pose a certain risk to the developing fetus, the potential benefits of the medication must be weighed against the risks for both mother and her unborn child. Therefore, information to guide patients and physicians to make a well-balanced decision for the appropriate treatment during pregnancy is needed. In this systematic review, an overview of maternal use of antidepressant or anxiolytic medication during pregnancy and childhood neurodevelopmental outcomes is provided. Some studies indicate a relation between prenatal exposure to antidepressants and adverse neurodevelopmental outcomes such as delayed motor development/motor control, social difficulties, internalizing problems and autism, but cannot rule out confounding by indication. Overall, the results of the observational studies have been inconsistent, which makes translation of the findings into clinical recommendations difficult. More well-designed observational studies and also randomized controlled trials (e.g., maintenance treatment vs. cessation) are needed to move forward and provide a comprehensive evaluation of the risks and benefits of antidepressant and anxiolytic use during pregnancy.

Unaccompanied evacuation and adult mortality: evaluating the finnish policy of evacuating children to foster care during World War II

OBJECTIVES

I examined associations between evacuation of Finnish children to temporary foster care in Sweden during World War II and all-cause mortality between ages 38 and 78 years.

METHODS

I used a Cox proportional hazards model to estimate mortality risk according to whether the individual was evacuated during childhood or not. I used within-sibling analysis to control for all unobserved socioeconomic and genetic characteristics shared among siblings. Individual-level data for Finnish cohorts born in 1933 to 1944 were derived from wartime government records, Finnish census data from 1950 and 1970, and death cause registry from 1971 to 2011.

RESULTS

I found no statistically significant association between evacuation and all-cause mortality when all exposed individuals were included in the analysis. However, subgroup analysis showed that men evacuated before age 4 years had a 1.31 higher mortality risk (95% confidence interval = 1.01, 1.69) than their nonevacuated counterparts.

CONCLUSIONS

In the aggregate, individuals do not have elevated mortality risk as a consequence of foster care during early childhood owing to the onset of sudden external shocks (e.g., wars).

The causal nature of the association between neighborhood deprivation and drug abuse: a prospective national Swedish co-relative control study

BACKGROUND

Risk for drug abuse (DA) is strongly associated with neighborhood social deprivation (SD). However, the causal nature of this relationship is unclear.

METHOD

Three Swedish population-based cohorts were followed up over 5 years for incident registration of DA in medical, legal or pharmacy records. In each cohort, we examined the SD-DA association, controlling carefully for individual socio-economic status (SES) with multiple measures, in the entire sample and among pairs of first cousins, paternal and maternal half-siblings, full siblings and monozygotic (MZ) twins discordant for SD exposure. The number of informative relative pairs ranged from 6366 to 166,208.

RESULTS

In all cohorts, SD was prospectively related to risk for incident DA. In relative pairs discordant for SD exposure, the SD-DA association was similar to that seen in the entire population in cousins, half-siblings, full siblings and MZ twins. Eliminating subjects who were residentially unstable or had DA in the first two follow-up years did not alter this pattern. When divided by age, in the youngest groups, the SD-DA association was weaker in siblings than in the entire population.

CONCLUSIONS

Across three cohorts, controlling for individual SES and confounding familial factors, SD prospectively predicted risk for incident DA registration. These results support the hypothesis that the SD-DA association is in part causal and unlikely to result entirely from personal attributes, which both increase risk for DA and cause selection into high SD environments. At least part of the SD-DA association arises because exposure to SD causes an increased risk of DA.

Mode of obstetrical delivery and type 1 diabetes: a sibling design study

OBJECTIVES

We investigated the association between cesarean section (CS) and type 1 diabetes (T1D), and if the association remains after accounting for familial confounding by using a sibling-control design.

METHODS

We conducted a population-based cohort study of all singleton live births in Sweden between 1982 and 2009, followed by sibling-control analyses. T1D diagnoses were identified from the Swedish National Patient Register. Mode of delivery was categorized into unassisted vaginal delivery (reference group), instrumental vaginal delivery (IVD), emergency CS, and elective CS. The statistical analysis was conducted in 2 steps: firstly log-linear Poisson regression with aggregated person-years by using the full cohort; secondly, conditional logistic regression for sibling-control analyses. The sibling analysis included siblings who were discordant for both mode of delivery and T1D.

RESULTS

In the cohort analyses (N = 2 638 083), there was an increased risk of childhood T1D among children born by elective CS (adjusted relative risk [RR] = 1.15 [95% confidence interval: 1.06-1.25]) and IVD (RR=1.14 [1.06-1.23]) but not emergency CS (RR = 1.02 [0.95-1.11]) when compared with children born by unassisted vaginal birth. However, the effect of elective CS and IVD on childhood T1D almost disappeared and became nonsignificant in the sibling-control analyses.

CONCLUSIONS

The present findings suggest a small association between elective CS and IVD and T1D. The sibling-control results, however, suggest that these findings are not consistent with causal effects of mode of delivery on T1D and may be due to familial confounders such as genetic susceptibility and environmental factors.

Maternal smoking during pregnancy and adverse outcomes in offspring: genetic and environmental sources of covariance

Maternal smoking during pregnancy (SDP) has been associated with several psychiatric outcomes in the offspring; studies have questioned whether the associations are causal, however. We analyzed all children born in Sweden between 1983 and 2009 to investigate the effect of SDP on multiple indicators of adverse outcomes in three areas: pregnancy outcomes (birth weight, preterm birth and being born small for gestational age), long-term cognitive abilities (low academic achievement and general cognitive ability) and externalizing behaviors (criminal conviction, violent criminal conviction and drug misuse). SDP was associated with all outcomes. Within-family analyses of the pregnancy outcomes were consistent with a causal interpretation as the associations persisted when siblings discordant for SDP were compared. For the cognitive and externalizing outcomes, the results were not consistent with causal effects; when comparing differentially exposed siblings none of the associations remained significant. In quantitative genetic models genetic factors explained the majority of the associations between SDP and cognitive and externalizing outcomes. The results suggest that the associations between SDP in mothers and cognition and externalizing behaviors in their offspring is primarily due to genetic effects that influence the behaviors in both generations.

Applying measures of discriminatory accuracy to revisit traditional risk factors for being small for gestational age in Sweden: a national cross-sectional study

OBJECTIVES

Small for gestational age (SGA) is considered as an indicator of intrauterine growth restriction, and multiple maternal and newborn characteristics have been identified as risk factors for SGA. This knowledge is mainly based on measures of average association (ie, OR) that quantify differences in average risk between exposed and unexposed groups. Nevertheless, average associations do not assess the discriminatory accuracy of the risk factors (ie, its ability to discriminate the babies who will develop SGA from those that will not). Therefore, applying measures of discriminatory accuracy rather than measures of association only, our study revisits known risk factors of SGA and discusses their role from a public health perspective.

DESIGN

Cross-sectional study. We measured maternal (ie, smoking, hypertension, age, marital status, education) and delivery (ie, sex, gestational age, birth order) characteristics and performed logistic regression models to estimate both ORs and measures of discriminatory accuracy, like the area under the receiver operating characteristic curve (AU-ROC) and the net reclassification improvement.

SETTING

Data were obtained from the Swedish Medical Birth Registry.

PARTICIPANTS

Our sample included 731 989 babies born during 1987-1993.

RESULTS

We replicated the expected associations. For instance, smoking (OR=2.57), having had a previous SGA baby (OR=5.48) and hypertension (OR=4.02) were strongly associated with SGA. However, they show a very small discriminatory accuracy (AU-ROC≈0.5). The discriminatory accuracy increased, but remained unsatisfactorily low (AU-ROC=0.6), when including all variables studied in the same model.

CONCLUSIONS

Traditional risk factors for SGA alone or in combination have a low accuracy for discriminating babies with SGA from those without SGA. A proper understanding of these findings is of fundamental relevance to address future research and to design policymaking recommendations in a more informed way.

Testing the Developmental Origins of Health and Disease Hypothesis for Psychopathology Using Family-Based Quasi-Experimental Designs

The Developmental Origin of Health and Disease (DOHaD) hypothesis is a broad theoretical framework that emphasizes how early risk factors have a causal influence on psychopathology. Researchers have raised concerns about the causal interpretation of statistical associations between early risk factors and later psychopathology because most existing studies have been unable to rule out the possibility of environmental and genetic confounding. In this paper we illustrate how family-based quasi-experimental designs can test the DOHaD hypothesis by ruling out alternative hypotheses. We review the logic underlying sibling-comparison, co-twin control, offspring of siblings/twins, adoption, and in vitro fertilization designs. We then present results from studies using these designs focused on broad indices of fetal development (low birth weight and gestational age) and a particular teratogen, smoking during pregnancy. The results provide mixed support for the DOHaD hypothesis for psychopathology, illustrating the critical need to use design features that rule out unmeasured confounding.

Use of corticosteroids during pregnancy and risk of asthma in offspring: a nationwide Danish cohort study

OBJECTIVE

To examine whether in utero exposure to local and systemic corticosteroids is associated with asthma development in offspring.

DESIGN

Cohort study.

SETTING

Denmark.

PARTICIPANTS

We included all singletons born alive in Denmark between 1996 and 2009. Data on maternal corticosteroid use, asthma in offspring and covariates were obtained from medical registries.

MAIN OUTCOME MEASURES

We compared asthma risks of children prenatally exposed to corticosteroids and of children of former corticosteroid users with that of unexposed children. We computed absolute risks and used proportional-hazards regression to compute adjusted HRs (aHRs). Using logistic regression we compared exposed children with unexposed siblings in a 'within-mother-between-pregnancy' analysis. Adjustment addressed varying length of follow-up.

RESULTS

We identified 877 778 children, 3.6% of whom were prenatally exposed to systemic (n=5327) or local (n=24 436) corticosteroids. A total of 105 677 children developed asthma during follow-up with a 10-year risk of 18.4% among the exposed and 13.5% among the unexposed. The aHR was 1.54 (95% CI 1.45 to 1.65) for systemic use, 1.45 (95% CI 1.40 to 1.50) for local use and 1.32 (95% CI 1.30 to 1.34) for former use. The adjusted OR of the 'within-mother-between-pregnancy' analysis was 1.11 (95% CI 0.98 to 1.25).

CONCLUSIONS

These population-based data do not support a strong causal association between maternal corticosteroid use during pregnancy and increased asthma risk in offspring.

In-utero exposure to maternal smoking is not linked to tobacco use in adulthood after controlling for genetic and family influences: a Swedish sibling study

Previous studies have linked maternal smoking during pregnancy with regular tobacco use in offspring, but findings are not consistent and confounding from genetic and environmental factors have not fully been taken into account. A comparison between siblings discordant for prenatal smoking exposure adjusts for confounding by shared familial (i.e., genetic and environmental) factors. We investigated the association between prenatal exposure to maternal smoking during pregnancy and the risk of regular smoking or snus (Swedish moist smokeless tobacco) use in young adult offspring, using a population based matched cohort study. The cohort consisted of 1,538 randomly sampled same-sex sibling pairs, discordant for maternal smoking during pregnancy, 19-27 years old, participating in a survey conducted in Sweden 2010-2011. Lifetime and current history of tobacco use was self-reported in the survey, and information about maternal smoking during pregnancy was retrieved from the Medical Birth Register. Conditional logistic regression and stratified Cox proportional hazards regression were used to calculate odds ratios, hazard ratios, and corresponding 95% confidence intervals. Analyses of exposure-discordant siblings did not reveal significant associations between prenatal exposure to maternal smoking and lifetime or current daily tobacco use, intensity of use, or time to onset of daily tobacco use. These findings suggest that the previously reported higher risks of tobacco use in offspring of mothers who smoked during pregnancy, compared with offspring of non-smoking mothers, were likely due to confounding from genetic or environmental factors.

Genetic and environmental contributions to the relationship between internalizing disorders and sick leave granted for mental and somatic disorders

This study investigates the degree to which internalizing disorders (anxiety and mood disorders) are prospectively associated with sick leave granted for mental and somatic disorders, and the extent to which common genetic and environmental risk factors influence these relationships. Data include self-reported symptoms of psychological distress from 7,598 young adult twins and diagnostic interviews on a subsample of 2,766 adult twins, subsequently linked to registry data on sick leave. Regression analyses and multivariate twin models were used to investigate the relationship between internalizing disorders and sick leave. Internalizing disorders were associated with sick leave granted for both mental disorders and somatic disorders. The association between internalizing disorders and sick leave granted for mental disorders was influenced by genetic and non-shared environmental factors, while the association between internalizing disorders and sick leave granted for somatic disorders could be explained by common genetic factors alone. Monozygotic twins discordant for internalizing disorders differed significantly in rates of sick leave granted for mental but not somatic disorders. In conclusion, internalizing disorders in young adults predict sick leave granted for both mental and somatic disorders. Environmental risk factors for internalizing disorders seem to influence sick leave granted for mental disorders, but not sick leave granted for somatic disorders.

The impact of neighbourhood deprivation on adolescent violent criminality and substance misuse: a longitudinal, quasi-experimental study of the total Swedish population

BACKGROUND

A number of studies suggest associations between neighbourhood characteristics and criminality during adolescence and young adulthood. However, the causality of such neighbourhood effects remains uncertain.

METHODS

We followed all children born in Sweden from 1975-1989 who lived in its three largest cities by the age of 15 years and for whom complete information was available about individual and contextual factors (N = 303 465). All biological siblings were identified in the sample (N = 179 099). Generalized linear mixed-effects models were used to assess the effect of neighbourhood deprivation on violent criminality and substance misuse between the ages of 15 and 20 years, while taking into account the cross-classified data structure (i.e. siblings in the same families attending different schools and living in different neighbourhoods at age 15).

RESULTS

In the crude model, an increase of 1 SD in neighbourhood deprivation was associated with a 57% increase in the odds of being convicted of a violent crime (95% CI 52%-63%). The effect was greatly attenuated when adjustment was made for a number of observed confounders (OR 1.09, 95% CI 1.06-1.11). When we additionally adjusted for unobserved familial confounders, the effect was no longer present (OR 0.96, 95% CI 0.84-1.10). Similar results were observed for substance misuse. The results were not due to poor variability either between neighbourhoods or within families.

CONCLUSIONS

We found that the adverse effect of neighbourhood deprivation on adolescent violent criminality and substance misuse in Sweden was not consistent with a causal inference. Instead, our findings highlight the need to control for familial confounding in multilevel studies of criminality and substance misuse.

Family income in early childhood and subsequent attention deficit/hyperactivity disorder: a quasi-experimental study

BACKGROUND

Studies have found negative associations between socioeconomic position and attention deficit/hyperactivity disorder (ADHD), but it remains unclear if this association is causal. The aim of this study was to determine the extent to which the association between family income in early childhood and subsequent ADHD depends on measured and unmeasured selection factors.

METHODS

A total of 811,803 individuals born in Sweden between 1992 and 2000 were included in this nationwide population-based cohort study. Diagnosis of ADHD was assessed via the Swedish national Patient Register and the Swedish Prescribed Drug Register. Annual family income during offspring's first 5 years in life was collected prospectively from the Swedish Integrated Database for Labour Market Research and divided into quartiles by (lower) family disposable income. We predicted ADHD from family income while controlling for covariates and also comparing differently exposed cousins and siblings to control for unmeasured familial confounding.

RESULTS

The crude analyses suggested that children exposed to lower income levels were at increased risk for ADHD (HRQ uartile1  = 2.52; 95% CI, 2.42-2.63; HRQ uartile2  = 1.52; 95% CI, 1.45-1.58; HRQ uartile3  = 1.20; 95% CI, 1.14-1.15). This dose-dependent association decreased after adjustment for measured covariates (HRQ uartile1  = 2.09; 95% CI, 2.00-2.19; HRQ uartile2  = 1.36; 95% CI, 1.30-1.42; HRQ uartile3  = 1.13; 95% CI, 1.08-1.18). Although the association was attenuated in cousin comparisons (HRQ uartile1  = 1.61; 95% CI, 1.40-1.84; HRQ uartile2  = 1.28; 95% CI, 1.12-1.45; HRQ uartile3  = 1.14; 95% CI, 1.01-1.28) and sibling comparison models (HRQ uartile1  = 1.37; 95% CI, 1.07-1.75; HRQ uartile2  = 1.37; 95% CI, 1.12-1.68; HRQ uartile3  = 1.23; 95% CI, 1.04-1.45), it remained statistically significant across all levels of decreased disposable family income.

CONCLUSIONS

Our results indicated that low family income in early childhood was associated with increased likelihood of ADHD. The link remained even after controlling for unmeasured selection factors, highlighting family income in early childhood as a marker of causal factors for ADHD.

Are the educational differences in incidence of cardiovascular disease explained by underlying familial factors? A twin study

To isolate the effect of education from the influence of potential underlying factors, we investigated the association of education with the risk of cardiovascular disease (CVD) and ischemic heart disease (IHD) using twin data to adjust for familial factors shared within twins, including genetic make-up and childhood environment. The study was based on data from the Danish Twin Registry linked to administrative and heath registers in Statistics Denmark. A total of 11,968 monozygotic and 20,464 dizygotic same sexed twins were followed from 1980 to 2009, including more than 8000 events of CVD. Unpaired and intra-pair analyses were compared. In the unpaired analyses, an inverse educational gradient in CVD- and IHD risk was observed. This association was not replicated in the intra-pair analyses that control for shared familial factors exploiting that twins share their intrauterine- and childhood environment and are matched partly or fully on genetic setup. The attenuation of association of education with CVD and IHD in the intra-pair analyses suggests that shared familial factors account for a substantial part of the observed association of education with CVD and IHD in Denmark.

Paternal age at childbearing and offspring psychiatric and academic morbidity

IMPORTANCE

Advancing paternal age is associated with increased genetic mutations during spermatogenesis, which research suggests may cause psychiatric morbidity in the offspring. The effects of advancing paternal age at childbearing on offspring morbidity remain unclear, however, because of inconsistent epidemiologic findings and the inability of previous studies to rigorously rule out confounding factors.

OBJECTIVE

To examine the associations between advancing paternal age at childbearing and numerous indexes of offspring morbidity.

DESIGN, SETTING, AND PARTICIPANTS

We performed a population-based cohort study of all individuals born in Sweden in 1973-2001 (N = 2,615,081), with subsets of the data used to predict childhood or adolescent morbidity. We estimated the risk of psychiatric and academic morbidity associated with advancing paternal age using several quasi-experimental designs, including the comparison of differentially exposed siblings, cousins, and first-born cousins.

EXPOSURE

Paternal age at childbearing.

MAIN OUTCOMES AND MEASURES

Psychiatric (autism, attention-deficit/hyperactivity disorder, psychosis, bipolar disorder, suicide attempt, and substance use problem) and academic (failing grades and low educational attainment) morbidity.

RESULTS

In the study population, advancing paternal age was associated with increased risk of some psychiatric disorders (eg, autism, psychosis, and bipolar disorders) but decreased risk of the other indexes of morbidity. In contrast, the sibling-comparison analyses indicated that advancing paternal age had a dose-response relationship with every index of morbidity, with the magnitude of the associations being as large or larger than the estimates in the entire population. Compared with offspring born to fathers 20 to 24 years old, offspring of fathers 45 years and older were at heightened risk of autism (hazard ratio [HR] = 3.45; 95% CI, 1.62-7.33), attention-deficit/hyperactivity disorder (HR = 13.13; 95% CI, 6.85-25.16), psychosis (HR = 2.07; 95% CI, 1.35-3.20), bipolar disorder (HR = 24.70; 95% CI, 12.12-50.31), suicide attempts (HR = 2.72; 95% CI, 2.08-3.56), substance use problems (HR = 2.44; 95% CI, 1.98-2.99), failing a grade (odds ratio [OR] = 1.59; 95% CI, 1.37-1.85), and low educational attainment (OR = 1.70; 95% CI, 1.50-1.93) in within-sibling comparisons. Additional analyses using several quasi-experimental designs obtained commensurate results, further strengthening the internal and external validity of the findings.

CONCLUSIONS AND RELEVANCE

Advancing paternal age is associated with increased risk of psychiatric and academic morbidity, with the magnitude of the risks being as large or larger than previous estimates. These findings are consistent with the hypothesis that new genetic mutations that occur during spermatogenesis are causally related to offspring morbidity.