In this study, we examined the relationship between polygenic liability for depression and number of stressful life events (SLEs) as risk factors for early-onset depression treated in inpatient, outpatient or emergency room settings at psychiatric hospitals in Denmark.

Data were drawn from the iPSYCH2012 case-cohort sample, a population-based sample of individuals born in Denmark between 1981 and 2005. The sample included 18 532 individuals who were diagnosed with depression by a psychiatrist by age 31 years, and a comparison group of 20 184 individuals. Information on SLEs was obtained from nationwide registers and operationalized as a time-varying count variable. Hazard ratios and cumulative incidence rates were estimated using Cox regressions.

Risk for depression increased by 35% with each standard deviation increase in polygenic liability (p < 0.0001), and 36% (p < 0.0001) with each additional SLE. There was a small interaction between polygenic liability and SLEs (β = −0.04, p = 0.0009). The probability of being diagnosed with depression in a hospital-based setting between ages 15 and 31 years ranged from 1.5% among males in the lowest quartile of polygenic liability with 0 events by age 15, to 18.8% among females in the highest quartile of polygenic liability with 4+ events by age 15.

These findings suggest that although there is minimal interaction between polygenic liability and SLEs as risk factors for hospital-treated depression, combining information on these two important risk factors could potentially be useful for identifying high-risk individuals.

Epidemiological studies indicate that individuals with one type of mental disorder have an increased risk of subsequently developing other types of mental disorders. This study aimed to undertake a comprehensive analysis of pair-wise lifetime comorbidity across a range of common mental disorders based on a diverse range of population-based surveys.

The WHO World Mental Health (WMH) surveys assessed 145 990 adult respondents from 27 countries. Based on retrospectively-reported age-of-onset for 24 DSM-IV mental disorders, associations were examined between all 548 logically possible temporally-ordered disorder pairs. Overall and time-dependent hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated using Cox proportional hazards models. Absolute risks were estimated using the product-limit method. Estimates were generated separately for men and women.

Each prior lifetime mental disorder was associated with an increased risk of subsequent first onset of each other disorder. The median HR was 12.1 (mean = 14.4; range 5.2–110.8, interquartile range = 6.0–19.4). The HRs were most prominent between closely-related mental disorder types and in the first 1–2 years after the onset of the prior disorder. Although HRs declined with time since prior disorder, significantly elevated risk of subsequent comorbidity persisted for at least 15 years. Appreciable absolute risks of secondary disorders were found over time for many pairs.

Survey data from a range of sites confirms that comorbidity between mental disorders is common. Understanding the risks of temporally secondary disorders may help design practical programs for primary prevention of secondary disorders.

Studies have indicated that the association of urbanicity at birth and during upbringing with schizophrenia may be driven by familial factors such as genetic liability. We used a population-based nested case–control study to assess whether polygenic risk score (PRS) for schizophrenia was associated with urbanicity at birth and at age 15, and to assess whether PRS and parental history of mental disorder together explained the association between urbanicity and schizophrenia.

Data were drawn from Danish population registries. Cases born since 1981 and diagnosed with schizophrenia between 1994 and 2009 were matched to controls with the same sex and birthdate (1549 pairs). Genome-wide data were obtained from the Danish Neonatal Screening Biobank and PRSs were calculated based on results of a separate, large meta-analysis.

Those with higher PRS were more likely reside in the capital compared with rural areas at age 15 [odds ratio (OR) 1.19, 95% confidence interval (CI) 1.01–1.40], but not at birth (OR 1.09, 95% CI 0.95–1.26). Adjustment for PRS produced almost no change in relative risks of schizophrenia associated with urbanicity at birth, but slightly attenuated those for urban residence at age 15. Additional adjustment for parental history led to slight attenuation of relative risks for urbanicity at birth [incidence rate ratio (IRR) for birth in capital = 1.54, 95% CI 1.18–2.02; overall p = 0.016] and further attenuation of relative risks for urbanicity at age 15 (IRR for residence in capital = 1.32, 95% CI 0.97–1.78; overall p = 0.148).

While results regarding urbanicity during upbringing were somewhat equivocal, genetic liability as measured here does not appear to explain the association between urbanicity at birth and schizophrenia.

Maternal smoking has consistently been associated with multiple adverse childhood outcomes including externalizing disorders. In contrast the association between maternal smoking during pregnancy (MSDP) and internalizing (anxiety and depressive) disorders in offspring has received less investigation.

We conducted a nationwide cohort study including 957635 individuals born in Denmark between 1991 and 2007. Data on MSDP and diagnoses of depression or anxiety disorders were derived from national registers and patients were followed up from the age of 5 years to the end of 2012. Hazard rate ratios (HRRs) were estimated using stratified Cox regression models. Sibling data were used to disentangle individual- and familial-level effects of MSDP and to control for unmeasured familial confounding.

At the population level, offspring exposed to MSDP were at increased risk for both severe depression [HRR 1.29, 95% confidence interval (CI) 1.22–1.36] and severe anxiety disorders (HRR 1.26, 95% CI 1.20–1.32) even when controlling for maternal and paternal traits. However, there was no association between MSDP and internalizing disorders when controlling for the mother's propensity for MSDP (depression: HRR 1.11, 95% CI 0.94–1.30; anxiety disorders: HRR 0.94, 95% CI 0.80–1.11) or comparing differentially exposed siblings (depression: HRR 1.18, 95% CI 0.75–1.89; anxiety disorders: HRR 0.87, 95% CI 0.55–1.36).

The results suggest that familial background factors account for the association between MSDP and severe internalizing disorders not the specific exposure to MSDP.

There is growing interest in the role of childhood adversities, including parental death and separation, in the etiology of psychotic disorders. However, few studies have used prospectively collected data to specifically investigate parental separation across development, or assessed the importance of duration of separation, and family characteristics.

We measured three types of separation not due to death: maternal, paternal, and from both parents, across the ages of 1–15 years among a cohort of 985 058 individuals born in Denmark 1971–1991 and followed to 2011. Associations with narrowly and broadly defined schizophrenia and bipolar disorder in the psychiatric register were assessed in terms of separation occurrence, age of separation, and number of years separated. Interactions with parental history of mental disorder were assessed.

Each type of separation was associated with all three outcomes, adjusting for age, sex, birth period, calendar year, family history of mental disorder, urbanicity at birth and parental age. Number of years of paternal separation was positively associated with both schizophrenia and bipolar disorder. Associations between separation from both parents and schizophrenia were stronger when separation occurred at later ages, while those with bipolar disorder remained stable across development. The first occurrence of paternal separation appeared to increase risk more when it occurred earlier in childhood. Associations differed according to parental history of mental disorder, although in no situation was separation protective.

Effects of parental separation may differ by type, developmental timing and family characteristics. These findings highlight the importance of considering such factors in studies of childhood adversity.

Depression is known to run in families, but the effects of parental history of other psychiatric diagnoses on depression rates are less well studied. Few studies have examined the impact of parental psychopathology on depression rates in older age groups.

We established a population-based cohort including all individuals born in Denmark after 1954 and alive on their 10th birthday (N = 29 76 264). Exposure variables were maternal and paternal history of schizophrenia, bipolar disorder, depression, anxiety or ‘other’ psychiatric diagnoses. Incidence rate ratios (IRRs) were estimated using Poisson regressions.

Parental history of any psychiatric diagnosis increased incidence rates of outpatient (maternal: IRR 1.88, p < 0.0001; paternal: IRR 1.68, p < 0.0001) and inpatient (maternal: IRR 1.99, p < 0.0001; paternal: IRR 1.83, p < 0.0001) depression relative to no parental history. IRRs for parental history of non-affective disorders remained relatively stable across age groups, while IRRs for parental affective disorders (unipolar or bipolar) decreased with age from 2.29–3.96 in the youngest age group to 1.53–1.90 in the oldest group. IRR estimates for all parental diagnoses were similar among individuals aged ⩾41 years (IRR range 1.51–1.90).

Parental history of any psychiatric diagnosis is associated with increased incidence rates of unipolar depression. In younger age groups, parental history of affective diagnoses is more strongly associated with rates of unipolar depression than non-affective diagnoses; however, this distinction disappears after age 40, suggesting that parental psychopathology in general, rather than any one disorder, confers risk for depression in middle life.

The link between psychotic disorders and violent offending is well established; knowledge about risk of post-illness-onset offending across the full spectrum of psychiatric disorders is lacking. We aimed to compare rates of any offending and violent offending committed after the onset of illness, according to diagnostic group, with population controls.

A 25% random sample of the Danish population (n = 521 340) was followed from their 15th birthday until offending occurred. Mental health status was considered as a time-varying exposure in a Poisson regression model used to examine the duration from service contact to the offence.

Males with any psychiatric contact had an incidence rate ratio (IRR) of 2.91 [95% confidence interval (CI) 2.80–3.02] for any offending; 4.18 (95% CI 3.99–4.38) for violent offending. Associations were stronger for women (IRR 4.17, 95% CI 3.95–4.40 for any offending; 8.02, 95% CI 7.20–8.94 for violent offending). Risk was similar across diagnostic groups for any offending in males, while variation between diagnostic groups was seen for male violent and female offending, both any and violent.

Risk of offending, particularly violent offending, was elevated across a range of mental disorders following first contact with mental health services. The extent of variation in strength of effect across diagnoses differed by gender.

The post-pubertal association of female gender with emotional disorder is a robust finding. However, studies exploring the association of gender and emotional disorders before puberty are few and present diverging results. The aim of this study was to present gender-specific incidence rates of emotional disorders throughout childhood.

This is a population-based cohort study of 907 806 Danish 3- to 18-year-olds. The outcome was assignment of an emotional disorder diagnosis based on in-patient and out-patient data from The Danish Psychiatric Central Register. Outcome measures were incidence rates and cumulative incidences for unipolar depressive disorder (ICD-10: F32–F33), anxiety disorders (ICD-10: F40–F42), and emotional disorders with onset specific to childhood (ICD-10: F93).

Pre-pubertal incidence rates for depressive and anxiety disorders were higher for boys than girls. At age 12 years the pattern reversed. The cumulative incidence for any emotional disorder (F32–F33, F40–F42, F93) on the 11th birthday was 0.52% (95% CI 0.50–0.55) for boys and 0.31% (95% CI 0.29–0.33) for girls. On the 19th birthday cumulative incidence was 2.33% (95% CI 2.24–2.43) for boys and 3.77% (95% CI 3.64–3.90) for girls. The pre-pubertal male preponderance was also significant for depressive disorders (F32–F33, p = 0.00144) and anxiety disorders (F40–F42, F93, p < 0.00001) separately.

Emotional disorders seem to display a male preponderance before the age of 12 years and a female preponderance thereafter. Studies exploring this gender–age interaction are needed. Still, the results question the general assumption that females throughout the lifespan are more at risk for emotional disorders than males.

Cardiovascular (CV) co-morbidity is one of the major modifiable risk factors driving the excess mortality in individuals with schizophrenia or bipolar disorder. Population-based studies in this area are sparse.

We used Danish population registers to calculate incidence rate ratios (IRRs) for CV drug use, and mortality rate ratios comparing subjects with schizophrenia or bipolar disorder with subjects with no prior psychiatric hospitalization.

IRRs for CV prescriptions were significantly decreased in patients with schizophrenia or bipolar disorder compared with the general population. Among persons without previous myocardial infarction (MI) or cerebrovascular disease, persons with schizophrenia or bipolar disorder had an up to 6- and 15-fold increased mortality from all causes or unnatural causes, respectively, compared with the general population, being most pronounced among those without CV treatment (16-fold increase). Among those with previous MI or cerebrovascular disease, excess all-cause and unnatural death was lower (up to 3-fold and 7-fold increased, respectively), but was similar in CV-treated and -untreated persons.

The present study shows an apparent under-prescription of most CV drugs among patients with schizophrenia or bipolar disorder compared with the general population in Denmark. The excess of mortality by unnatural deaths in the untreated group suggests that the association between CV treatment and mortality may be confounded by severity of illness. However, our results also suggest that treatment of CV risk factors is neglected in these patients.

Although transient psychotic disorders are currently classified as a category separate from schizophrenia (SZ) and affective disorders, their distinctive features remain uncertain. This study examines the family psychiatric morbidity of the ICD-10 category of ‘acute and transient psychotic disorders’ (ATPDs), pointing out differences from SZ and bipolar disorder (BD).

From a cohort of 2.5 million persons, we identified all patients enrolled in the Danish Psychiatric Register who were ever admitted with ATPDs (n=2537), SZ (n = 10639) and BD disorder (n=5292) between 1996 and 2008. The relative risk (RR) of ATPDs, SZ and BD associated with psychiatric morbidity in first-degree relatives (FDRs) was calculated as the incidence rate ratio using Poisson regression.

The RR of ATPDs [1.93, 95% confidence interval (CI) 1.76–2.11] was higher if patients with ATPDs had at least one FDR admitted with any mental disorder than patients without family psychiatric antecedents. An additional risk arose if they had FDRs admitted not only with ATPDs (RR 1.60, 95% CI 1.33–1.92) but also with SZ (RR 2.06, 95% CI 1.70–2.50) and/or BD (RR 1.55, 95% CI 1.23–1.96). Despite some overlap, the risk of SZ (RR 2.80, 95% CI 2.58–3.04) and BD (RR 3.68, 95% CI 3.29–4.12) was markedly higher if patients with SZ and BD had FDRs admitted with the same condition.

These findings suggest that family psychiatric predisposition has a relatively modest impact on ATPDs and argue against a sharp differentiation of ATPDs from SZ and BD.

There is a well-established association between psychotic disorders and subsequent offending but the extent to which those who develop psychosis might have a prior history of offending is less clear. Little is known about whether the association between illness and offending exists in non-psychotic disorders. The aim of this study was to determine whether the association between mental disorder and offending is present prior to illness onset in psychotic and non-psychotic disorders.

In a nested case-control study, cases (n=101 890) with a first psychiatric contact during the period 1995 to 2006 were identified and matched by age and gender to population-based controls (n=2 236 195). Exposure was defined as prior criminal and violent offending.

Males with one offence had an incidence rate ratio (IRR) of 2.32 [95% confidence interval (CI) 2.26–2.40] for psychiatric admission whereas two or more convictions yielded an IRR of 4.97 (95% CI 4.83–5.11). For violent offending the associations were stronger and IRRs of 3.97 (95% CI 3.81–4.12) and 6.18 (95% CI 5.85–6.52) were found for one and several offences respectively. Estimates for females were of a similar magnitude. The pattern was consistent across most diagnostic subgroups, although some variability in effect sizes was seen, and persisted after adjustment for substance misuse and socio-economic status (SES).

A prior history of offending is present in almost one in five patients presenting to mental health services, which makes it an important issue for clinicians to consider when assessing current and future risks and vulnerabilities.

Second-generation immigrants have an increased risk of schizophrenia, a finding that still lacks a satisfactory explanation. Various operational definitions of second-generation immigrants have been used, including foreign parental country of birth. However, with increasing global migration, it is not clear that parental country of birth necessarily is informative with regard to ethnicity. We compare two independently collected measures of parental foreign ethnicity, parental foreign country of birth versus genetic divergence, based on genome-wide genotypic data, to access which measure most efficiently captures the increased risk of schizophrenia among second-generation immigrants residing in Denmark.

A case–control study covering all children born in Denmark since 1981 included 892 cases of schizophrenia and 883 matched controls. Genetic divergence was assessed using principal component analyses of the genotypic data. Independently, parental foreign country of birth was assessed using information recorded prospectively in the Danish Civil Registration System. We compared incidence rate ratios of schizophrenia associated with these two independently collected measures of parental foreign ethnicity.

People with foreign-born parents had a significantly increased risk of schizophrenia [relative risk (RR) 1.94 (95% confidence intervals (CI) 1.41–2.65)]. Genetically divergent persons also had a significant increased risk [RR 2.43 (95% CI 1.55–3.82)]. Mutual adjustment of parental foreign country of birth and genetic divergence showed no difference between these measures with regard to their potential impact on the results.

In terms of RR of schizophrenia, genetic divergence and parental foreign country of birth are interchangeable entities, and both entities have validity with regard to identifying second-generation immigrants.

Offspring of parents with mental disorder are at risk of a range of adverse outcomes. We sought to establish whether such risks extend to offending by examining rates of criminal conviction, including conviction for violent and sexual offences, among offspring of parents with mental disorder compared to offspring without parental disorder.

From a random sample of the Danish population, a cohort aged ⩽15 years (n=412 117) was followed for the occurrence of conviction between January 1981 and December 2006. Incidence rate ratios (IRRs) and cumulative incidences for offspring conviction by parental mental disorder status were calculated using a Cox regression model. Analyses were repeated for conviction for a serious first offence.

Offspring with history of parental mental disorder had higher rates of conviction than those without parental disorder; rates were highest for those with two affected parents [IRR 3.39, 95% confidence interval (CI) 3.08–3.73]. The association persisted when parental gender, offspring gender and the nature of parental disorder were considered. Absolute rates were lower but relative rates higher for female offspring (IRR 3.26 for males with two affected parents, 4.52 for females). Similar patterns were seen for conviction for serious offences. Associations were attenuated after adjustment was made for family socio-economic position (SEP) and parental criminality.

Offspring of parents with mental disorder represent a group at elevated risk of criminality. This raises the possibility of shared familial vulnerability for mental disorder and criminal behaviour, and highlights the need to consider early identification and intervention in this group.

A family history of schizophrenia is the strongest single indicator of individual schizophrenia risk. Bipolar affective disorder and schizo-affective disorders have been documented to occur more frequently in parents and siblings of schizophrenia patients, but the familial occurrence of the broader range of mental illnesses and their role as confounders have not been studied in large population-based samples.

All people born in Denmark between 1955 and 1991 (1.74 million) were followed for the development of schizophrenia (9324 cases) during 28 million person-years at risk. Information of schizophrenia in cohort members and psychiatric history in parents and siblings was established through linkage with the Danish Psychiatric Central Register. Data were analysed using log-linear Poisson regression.

Schizophrenia was, as expected, strongly associated with schizophrenia and related disorders among first-degree relatives. However, almost any other psychiatric disorder among first-degree relatives increased the individual's risk of schizophrenia. The population attributable risk associated with psychiatric family history in general was 27.1% whereas family histories including schizophrenia only accounted for 6.0%. The general psychiatric family history was a confounder of the association between schizophrenia and urbanization of place of birth.

Clinically diagnosed schizophrenia is associated with a much broader range of mental disorders in first-degree relatives than previously reported. This may suggest risk haplotypes shared across many disorders and/or shared environmental factors clustering in families. Failure to take the broad range of psychiatric family history into account may bias results of all risk-factor studies of schizophrenia.

Poisoning with weak analgesics is a major public health problem because of easy accessibility of the compounds; however, few studies have investigated their influence on subsequent suicide in the context of subjects' psychiatric status and other factors.

This nested case-control study was based on the entire Danish population including all 21 169 suicide cases and 423 128 matched population controls. Data on hospital admissions for poisoning and confounding factors were retrieved from national medical and administrative registries. Conditional logistic regression was used to compute relative risk.

A prior hospital admission for poisoning with weak non-opioid analgesics significantly increased the risk of subsequent suicide [crude incidence rate ratio (IRR) 24.7, 95% confidence interval (CI) 22.1–27.6], and the effect of paracetamol poisoning was substantially stronger than that of poisoning with salicylates or non-steriodal anti-inflammatory drugs (NSAIDs). This association could not be explained by confounding from socio-economic or psychiatric factors. The elevated risk was extremely high during the first week following the overdose (adjusted IRR 738.9, 95% CI 173.9–3139.1), then declined over time but still remained significantly high 3 years later (adjusted IRR 4.2, 95% CI 3.5–5.0). Moreover, a history of weak analgesic poisoning significantly interacted with a person's psychiatric history, increasing the risk for subsequent suicide substantially more for persons with no history of psychiatric hospitalization than did it for those with such a history.

A history of non-fatal poisoning with weak analgesics is a strong predictor for subsequent suicide. These results emphasize the importance of intensive psychiatric care of patients following overdose.