INTRODUCTION
Suicide is one of the most important causes of death among young people in Europe.1 Parental risk factors, including parental suicidal behaviour, 2—4 parental psychiatric morbidity2 4 and parental non-suicidal death,4 contribute to suicidal behaviour.
A life-course approach may add to our understanding of the aetiology of suicidal behaviour.5—8 This approach is based on the assumption that the impact of biological and social risk factors as well as protective factors may vary with age at exposure due to a differential impact at different developmental stages.8 Any variations in risk for suicidal behaviour
in the offspring with age at first exposure to parental factors may be due to the accumulation of exposures during the life span, due to a latency in outcome manifestation, or due to specific developmental mechanisms that act exclusively (‘critical period hypothesis’) or predominantly (‘sensitive period hypothesis’) during a specific age window.8 In recent years, the life course paradigm has reached a considerable resonance in chronic disease epidemiology.8 In spite of this development, applications related to suicidal behaviour in the offspring remain sparse.7 In order to explore the effects of age at exposure to parental mortality and markers of morbidity on the risks of suicide and attempted suicide in offspring, we conducted the present study.
METHODS
Study design
We conducted a matched case—control study through record linkages between Swedish national registers. The study base consisted of all individuals, born in Sweden between January 1973 and December 1983, who were singletons and for whom information on both biological parents was available. The cases comprised all individuals recorded in the National Patient Register (NPR) or in the Causes of Death Register (CDR) due to attempted or completed suicide (E950—E959 in the International Classification of Diseases ICD-8 and ICD-9, X60—X84 in ICD-10). The National Board of Health and Welfare provided us with information on the equivalence of ICD-8/ ICD-9 and ICD-10 codes.
Cases included suicides and attempted suicides with uncertainty about intention (E980—E989 in ICD-8 and ICD-9, Y10—Y34 in ICD-10). Uncertain and certain diagnoses were combined to limit temporal and regional variation in ascertainment routines.2 A sensitivity analysis of certain and uncertain suicidal behaviours established the comparability of the estimates.
Attempted suicide in offspring was also analysed with regard to the method used. We distinguished violent methods, including hanging, use of firearms and knives, jumping from heights and in front of moving objects, and drowning (ICD-8 and ICD-9: E953-957 and E983-987; ICD-10: X70-X82 and Y20-Y32) from non-violent methods, which included all forms of poisoning (ICD-8 and ICD-9: E950-952 and E980-982; ICD-10: X60-X69 and Y10-Y19). This classification strategy was in accordance with
related literature investigating violent and non-violent suicide methods.9
Cases comprised 1407 individuals with suicide completion and 17159 individuals with attempted suicide. They were each matched by sex, month, year and county of birth to up to 10 randomly selected controls. Only individuals who were alive and living in Sweden at the time of the index event were eligible to serve as controls. Events coded as attempted suicides and suicides before the age of 10 years might be misclassified and were excluded from the analyses. Suicide victims were assessed from 1 January 1983 until 31 December 2004, and were up to 31 years of age at the end of follow-up. Suicide attempters were assessed from the same date up until 31 December 2006.
Data sources
In Sweden, all residents are identified by a unique identification number. This enables merging of individual information from different national registers.
Children were linked to their biological parents using the Multi-Generation Register (MGR). Of the individuals born after 1950, less than 2% could not be linked to their parents.10
We derived data on completed suicides in parents and offspring and parental deaths due to other causes from the Causes of Death Register (CDR). The National Patient Register (NPR) provided information on attempted suicides in parents and offspring, and also data on the dates and diagnoses of hospital care based on clinical assessments. The Register of the Social Insurance Agency (RSIA) provided information on diagnosis-specific disability pension. All diagnoses in the NPR, CDR and RSIA were classified in accordance with ICD-8, ICD-9 and ICD-10.
The Population and Housing Censuses (PHC) provided data on parental socioeconomic status, maternal marital status, and parental education in 1970. Data on parental education in 1990 were retrieved from the Longitudinal Integration Database for Health Insurance and Labour Market Studies (LISA). Information on emigration and immigration was extracted from the Register of the Total Population. Table 1 summarises details of the variables and registers.
Exposure variables
The exposure assessment for each case and control refers to the time up until the event involving the index subject. Parental markers of morbidity, namely diagnosis-specific disability pension, attempted suicide, and inpatient care due to mental disorder, were categorised into age windows corresponding to offspring’s age at first exposure. The categories were exposure before the age of 3 years, between 3 and 10, and over the age of 10. We considered only the main diagnosis, and the first date of
hospital inpatient care or receipt of disability pension. Also, diagnoses before the birth of the child were allocated to the youngest exposure window, since it was deemed likely that chronic illness underlying the markers of morbidity would result in early exposure of the child. Parental markers of mortality, namely suicide completion and death due to other causes, were categorised into exposures before and after the age of 10 years due to relatively small numbers of cases. Whenever the child was exposed to a marker that was positive for both parents, the age window corresponding to the earlier exposure was used. Absence of exposure was employed as the reference category.
Covariates
Maternal and paternal age at offspring’s exposure were used as continuous variables. Parental education was measured as education up to 10 years (primary education), 10 to 13 years (secondary education), and >13 years (university education, reference category). We used either maternal or paternal education, whichever was the higher, in accordance with the dominance principle which has been shown to perform well in classifying families.15 Whenever both parents were born in 1940 or earlier we used educational data from 1970. These parents were at least 30 years old at time of measurement. For all other parents, we used educational data from 1990. Categories for parental socioeconomic status were unskilled workers, skilled workers, low level salaried employees, intermediate or high level salaried employees (reference category), and others. We used either maternal or paternal socioeconomic status, whichever was the higher. Data from the 1980 census were used for events that occurred between 1983 and 1990, 1985 census data for events between 1991 and 1998, and 1990 data for events from 1999 to 2006. In the case of internal missing data, we took information from the preceding census. This approach ensured that all measurements were taken at a point in time preceding offspring’s suicidal behaviour.
Maternal marital status was dichotomised into married and cohabiting versus other status (unmarried, divorced, widowed, non-cohabiting), using the same approach as for socioeconomic status with regard to the choice of census year.
There were missing data on covariates in 4.0% of cases of attempted suicide, and in 4.2% of cases of completed suicide. Missing data were coded as a separate category. A sensitivity analysis showed similar patterns of suicide and attempted suicide risks in cases with complete information and in cases with missing data.
Statistical analysis
The outcome variables were completed suicide and attempted suicide in offspring. Univariate and multivariate ORs for the exposure variables were estimated using conditional logistic
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J Epidemiol Community Health 2012;66:233-239. doi:10.1136/jech.2010.109595
regression. Interactions between offspring’s sex, offspring’s age at onset of suicidal behaviour as well as maternal marital status and the exposure variables were subjected to partial likelihood ratio tests by introducing corresponding product terms into the adjusted models.
Attributable proportions for the fully adjusted model were calculated using the formula AP=S(1—(1/OR)), where S is the number of cases with exposure divided by the total number of cases.
Further, in order to test for a possible linear trend across offspring’s age at exposure, we included offspring’s age at exposure as a continuous variable in the fully adjusted models. These analyses were restricted to exposed individuals only.
Data processing was performed using SPSS V15.0 for Windows.
RESULTS
The numbers of suicide victims and suicide attempters, respectively, were 1019 (72.4%) and 6003 (35.0%) boys/men, and 388 (27.6%) and 11 156 (65.0%) girls/women. Mean age was 22.3 years (SD 3.7) at suicide completion, and 21.1 years (SD 4.4) at first registered attempted suicide. With regard to suicide methods, for certain suicides (N—1077), the main method was hanging (41.0%), followed by poisoning (22.1%) and jumping (19.0%). For uncertain suicides (N—330), poisoning (67.3%) was followed by other methods (10.4%) and jumping (10.0%). For certain attempted suicides (N—13 976), poisoning (88.9%) was the most frequent method, followed by cutting (4.2%). For uncertain attempted suicides (N—3183), the main method was poisoning (76.2%), followed by other methods (17.2%). Of attempted suicides, 14 853 (86.6%) were coded as non-violent attempts, and 2306 (13.4%) as violent.
A majority of all exposures to disability pensions attributed to parental mental disorder (N—9145) were coded under the headings neurotic, stress-related and somatoform disorders (48.0%), and affective disorders (22.9%).
In exposures to disability pensions granted due to somatic disorders (N—25432), disorders of the musculoskeletal system and connective tissue were the most frequent diagnoses (69.2%).
Tables 2—5 show the results of the univariate and multivariate analyses.
Parental suicidal behaviour
Exposure to parental attempted suicide was associated with a 3.4-fold increase in suicide risk (table 2), and a 3.5-fold increase
in risk of attempted suicide (table 3) in offspring. The ratios decreased to 2.6 and 2.6, respectively, in the fully adjusted models.
Exposure to parental suicide was associated with a 3.5-fold increase in suicide risk (table 2), and a 2.6-fold increase in risk of attempted suicide (table 3). The ratios decreased to 2.5 and 1.8, respectively, in the fully adjusted models. Adjustment for parental age alone had little effect on the estimates (tables 2 and 3). At the second adjustment step, parental education and marital status had the largest effects.
The risk of completed suicide was most pronounced among offspring exposed to parental completed suicide in the earliest age window (table 4). There was a constant and an increasing risk of attempted suicide among offspring across exposure windows in relation to parental attempted suicide and parental completed suicide, respectively (table 5). Separate analyses of violent and non-violent attempted suicides in offspring revealed that there was one slight alteration to the general pattern: the risk of violent attempted suicide after parental suicide was 1.6 (95% CI 1.0 to 2.7) when exposed up to the age of 10 years, and 1.4 (95% CI 0.9 to 2.3) when exposed over the age of 10, indicating a more pronounced risk of violent attempted suicide in offspring in the case of early exposure to parental suicide.
Parental diagnosis-specific disability pension
Parental disability pension due to a psychiatric disorder was associated with a 2.9- and 2.7-fold risk of completed suicide (table 2) and attempted suicide (table 3) in offspring, respectively. The estimates fell to 1.9 and 1.7, respectively, after full adjustment. Exposure to parental somatic disability pension had a smaller effect than exposure to psychiatric disability pension, and was associated with a 1.5-fold risk of suicide completion (table 2), and a 1.7-fold risk of attempted suicide (table 3). The estimates decreased to 1.3 and 1.5, respectively, in the adjusted models. Offspring’s risk of completed and attempted suicide showed a similar pattern with regard to the timing of first exposure to parental diagnosis-specific disability pension. The earlier the exposure, the greater was the risk of suicidal behaviour (tables 4 and 5). There was a significant increase in suicide risk with decreasing age at exposure to psychiatric disability pension (p—0.027) (table 4). Further, there was a significant general increase in attempted suicide risk with decreasing age at exposure to somatic disability pension (p—0.001, table 5).
Parental inpatient care due to mental disorder
Parental inpatient care due to mental disorder was associated with a 2.6-fold risk of completed suicide (table 2) and a 3.0-fold risk of attempted suicide (table 3). The estimates decreased to 1.8 and 2.1, respectively, in the fully adjusted models, largely due to the effects of parental education and maternal marital status. The risks of both completed and attempted suicide were highest in
the case of exposure in the youngest age window (tables 4 and 5). There was an increase in attempted suicide risks with decreasing age at first exposure (p<0.001, table 5).
Parental death due to causes other than suicide
Parental death due to causes other than suicide was associated with a 1.7- and 1.6-fold risk for suicide and attempted suicide
among offspring, respectively (tables 2 and 3). ORs decreased to 1.3 and 1.3, respectively, in the adjusted models. In contrast to other markers of parental morbidity, exposure to parental non-suicidal death after the age of 10 years was associated with an increased risk of attempted and completed suicide only in offspring exposed over the age of 10 (tables 4 and 5). There was a significant increase in attempted suicide risk with increasing age at exposure to parental non-suicidal death (p=0.018) (table 5).
Interaction effects of offspring’s sex, offspring’s age at onset of suicidal behaviour and maternal marital status with the exposure variables
Partial likelihood ratio tests indicated that exposure to parental death due to other causes than suicide increased the risk of attempted suicide more in boys/men than in girls/women (p=0.006). In comparison to non-exposed girls/women, the estimates for exposed boys/men and exposed girls/women were 1.40 (1.26 to 1.55) and 1.15 (1.05 to 1.26), respectively.
Furthermore, exposure to parental somatic disability pension increased the risk of attempted suicide more when onset of suicidal behaviour was before the age of 20 years, than in the case of later onset (p=0.012). Compared with unexposed offspring with onset after the age of 20 years, the respective estimates were 1.66 (1.54 to 1.80) for exposed offspring with onset before 20 years and 1.46 (1.39 to 1.55) for exposed offspring with onset after the age of 20 years.
With regard to suicide attempt in offspring, there were several interaction effects between parental markers of morbidity and
maternal marital status. Specifically, exposure to parental attempted suicide (p<0.001), parental inpatient care due to mental disorders (p=0.002), parental psychiatric disability pension (p=0.05) and parental somatic disability pension (p=0.001) interacted with maternal marital status. In comparison with unexposed offspring of married and cohabiting mothers, the suicide attempt risk for offspring of unmarried, divorced, widowed and non-cohabiting mothers exposed to parental attempted suicide were 2.51 (2.11 to 2.99), and the estimates for exposed offspring of married and cohabiting mothers were 1.87 (1.56 to 2.24). The respective estimates for exposure to parental inpatient care due to mental disorders, parental psychiatric and somatic disability pension were 3.17 (2.99 to 3.35), 2.69 (2.49 to 2.91) and 2.33 (2.18 to 2.49) for offspring of unmarried, divorced, widowed and non-cohabiting mothers, respectively, and 2.22 (2.10 to 2.36), 1.83 (1.67 to 2.02) and 1.58 (1.49 to 1.68) for exposed offspring of married/cohab-iting mothers, respectively.
DISCUSSION
Main findings
A general pattern of increasing risk of suicide and attempted suicide in offspring with decreasing age at exposure to parental risk factors emerged. This pattern was present for parental somatic disability pension, parental inpatient care due to mental disorders and parental psychiatric disability pension. With regard to offspring’s risk of completed suicide, the pattern was
also present for parental suicide and attempted suicide. For parental non-suicidal deaths, the pattern was the opposite.
Strengths and limitations
The main strengths of the present register study were the large number of participants, the population-based design, the full coverage of cases, the opportunity to control for potential confounders, the high quality of the data10-14 and the very low dropout. Difficulties, such as recall bias, which is often present in studies based on data from clinical settings, could be avoided due to the use of national registers.
The study also has some limitations. For the Register of the Social Insurance Agency, which provided data on disability pension, misclassifications within the group of psychiatric diagnoses, entailing an under-reporting of psychotic disorders, have been discussed previously.16 This register has not yet been evaluated in detail. Furthermore, we could not address the effect of cumulative exposure to morbidity and mortality in both parents due to the relatively small number of cases in several exposure windows. A recent study identified a clear increase in attempted suicide risk in the offspring when suicidality or psychiatric disorders occurred in several family members.2 Further, the present data on attempted suicides and mental disorders only covered individuals who were hospitalised. It is estimated that approximately 25% of suicide attempters receive inpatient care.17 The findings, therefore, only apply to attempts that resulted in hospitalisation.
Comparison with previous studies
Parental suicidal behaviour2 4 7 and parental inpatient care due to mental disorders,2 4 as well as parental non-suicidal death,4 have previously been identified as risk factors of suicidal behaviour in offspring. Knowledge of possible consequences of being on disability pension is limited.18 Receiving a disability pension seems to increase an individual’s own risk of suicide beyond the effects of his or her psychiatric hospitalisation and socioeconomic status.18 19 A recent study identified an increased suicide risk in offspring exposed to parental disability pension, but did not consider the nature of the parental diagnoses involved.6 In the present study, exposures to both parental psychiatric and somatic disability pension were associated with an increased risk of suicide and attempted suicide.
Concerning possible impacts of age at exposure to parental risk factors on the risk of suicidal behaviour in offspring, earlier studies found that the risk of bipolar disorder, which is an important risk factor of suicidal behaviour,20 was most pronounced when exposure to maternal suicide21 or parental loss22 occurred before the age of 10 years. A recent study analysed the impact of offspring’s age at exposure to parental suicide on the risk of completed suicide and identified an increased risk for offspring exposed under the age of 17 years compared to offspring exposed later in life.7 The present findings add that children exposed at the very youngest ages show an important vulnerability that may lead to suicidal behaviour later in life.
Interpretation
The present study was the first to identify an association between parental somatic disability pension and offspring’s risk of suicidal behaviour. The association was present for both sexes and remained after controlling for parental suicidal behaviour, parental socioeconomic conditions and marital status. This finding may be related to the genetic transmission of parental somatic disorders, or to uncontrolled co-morbidity of mental
disorders in this group, or to the psychosocial consequences of having a parent on disability pension.
The finding of more pronounced risks of attempted suicide in offspring of unmarried, non-cohabiting, divorced and widowed mothers stresses the importance of a specific awareness and support from the environment and healthcare system.23 Other socioeconomic indicators, specifically the parental socioeconomic index and parental education, did not significantly alter the effects of exposure to parental morbidity and mortality.
In accordance with the hypothesis of sensitive life periods,5 8 exposure to parental somatic disability pension, parental inpatient care due to mental disorders and psychiatric disability pension, seem to increase the susceptibility to attempted suicide and suicide in offspring most when exposure appears early in life.
Similarly, exposure to parental suicide may increase the susceptibility to completed suicide. Of note, the present risk estimates were only slightly attenuated by controlling for parental age at offspring’s exposure, which is a crude marker of parental age at onset of morbidity. In previous research, early-onset major depression in offspring, which is an important risk factor of suicidal behaviour, has been discussed to be caused by a stronger genetic disposition that may be reflected in early onset of major depression in parents.24
Compared to the risk pattern in completed suicide, the risk of attempted suicide in offspring exposed to parental suicidal behaviour was more pronounced for later exposure windows. This stronger effect in adolescence and young adulthood may underscore the importance of parental suicidal behaviours as triggering events for non-fatal suicidal behaviour in offspring. Of note, imitation has been discussed as an important factor in suicidal behaviour, particularly among young people.25 A triggering effect may also be reflected in the association of parental non-suicidal death and suicidal behaviour in offspring exposed after the age of 10 years.
The risk patterns of attempted suicides varied somewhat with suicide method. The risk patterns for violent attempt in offspring exposed to parental suicide resembled more the patterns estimated for completed suicide than for attempted suicide in general. Related research suggests that violent attempts constitute a further step in the suicidal process, and that medically serious attempted suicides and suicides are two overlapping populations that share common psychiatric diagnostic and history features.9 26 Future research is warranted to further scrutinise the present findings in this context.
CONCLUSION
The present findings comply with the ‘sensitive period’ hypothesis in life course epidemiology8 and add to our understanding of suicidal behaviour in the familial context during the life span. Parental disability pension, parental psychiatric inpatient care and parental suicide seem to increase the offspring’s susceptibility to suicide, particularly when they occur early in life. Parental non-suicidal death has most detrimental effects when occurring in adolescence or young adulthood. Early interventions in families with parental morbidity or suicidal behaviour seem necessary to prevent suicide in offspring. Evaluations of such interventions suggest possible preventive effects. An early intervention for families at psychosocial risk resulted in short-term improvements in motherechild relations.27 28 The six-week intervention focused on strengthening the mothers in their care giving skills and improving the motherechild relationship and interaction. Another study, which targeted early intervention through a five-year family counselling programme