Risk of suicide in relation to income level in people admitted to hospital with mental illness: nested case-control study

People at higher risk of suicide, such as those who are socially and economically disadvantaged, are also at high risk of being admitted to hospital with a mental illness.¹,² In some cases it seems that mental illness is a factor on the causal pathway between social position and suicide.²,³ However, Mortensen and colleagues showed that the importance of socioeconomic variables as risk factors for suicide was reduced after adjustment was made for a history of mental illness.³ We present findings on 811 cases of suicide and 80 787 matched control subjects in a population based study which aimed to gain further insight into the association between social position and mental disorder.

Subjects, methods, and results

We used the Danish medical registers on vital statistics to establish a random, 5%, population based sample of 811 people who had committed suicide between 1982 and 1994. Up to 1983, suicide was defined as ICD-8 codes E950-959; for 1994, ICD-10 codes X60-X84 were applied. Each person who had committed suicide was matched with approximately 100 people of the same sex and year of birth who were alive on the date of the suicide. Information on dates of hospital admission and discharge and details of diagnoses was drawn from the Danish psychiatric central register, which has monitored all psychiatric inpatient facilities since 1969. Socioeconomic data on case and control subjects from two years before the suicide were added from the longitudinal labour market register. Detailed description of the registers can be found in Mortensen et al.³

The main variables included were annual gross income (wages, pensions, unemployment and social security benefits, and interest), grouped into fourths, and hospital admission status in relation to mental illness. Hospital admission status was categorised as follows: never admitted, currently admitted or first discharge within the present or preceding year, and first discharge before the preceding year. Trend variables were defined as variables taking the values 0, 1, 2, and 3 in the four income groups.

We also included socioeconomic and marital status in our analysis. Socioeconomic status was categorised as: fully employed, unemployed for 1%-20% of the year, unemployed for 21%-100% of the year, old age pensioner, disability pensioner, student, or recipient of social assistance, and there were three categories for marital status: cohabiting, single with children, and single without children. The psychiatric information gathered included the diagnosis (schizophrenia (ICD-8, 295), manic depressive psychosis (ICD-8, 296), reactive psychosis (ICD-8, 298)), and an indicator for more than one previous admission to hospital for mental illness. Data were analysed by conditional logistic regression.

In contrast to findings in the general population, the suicide risk for patients admitted to hospital with a mental illness fell significantly with decreasing income (P=0.0001). The table shows that, in comparison with the group with the highest income, the suicide risks for people recently discharged from hospital fell from 0.50 (95% confidence interval 0.25 to 0.97) in the second highest group, to 0.37 (0.18 to 0.77) in the third group, and 0.35 (0.17 to 0.69) in the lowest group. The table also shows that risk ratios for people whose first admission to hospital had occurred before the previous year showed a similar pattern. The unadjusted risk ratios in the general population fell gradually with income (table). Analogous risks, calculated by using the trend, were 2.30 (1.32³), 1.32 (=1.32²), 1.74 (=1.32¹), and 1 (=1.32⁰), respectively. No significant interactions were found between trends and the different diagnoses. In the adjusted analyses, a similar but less pronounced pattern was found in people who had never been admitted to hospital with a psychiatric disorder. The impacts of socioeconomic and marital status were as expected—that is, there were excess risks in single and unemployed people. Furthermore, an unadjusted analysis omitting these factors strengthened the results.

Comment

People with a history of mental illness and a high income are at greater risk of committing suicide than their lower income counterparts. Richer people with a mental disorder may be more suicidal before they are admitted to hospital or they may feel more stigmatised,⁴ vulnerable, and shameful⁵ about having a mental illness. In Denmark there are no private psychiatric hospitals or clinics. Perhaps treatment focuses on people from lower social classes as most patients are from this background, and perhaps patients from higher income groups are less likely to be admitted.

Table.

Risk ratios (95% confidence intervals) for suicide in relation to gross income and time since first discharge from hospital for a psychiatric disorder in 811 people who committed suicide and 80 787 control subjects

Gross income	Crude risk ratio*	Hospital admission status (adjusted risk ratio)†
		Never admitted	Currently in hospital or discharged in past year‡	Discharged >1 year ago
Highest fourth	1	1	1	1
Third	1.34 (1.09 to 1.65)	1.14 (0.86 to 1.51)	0.50 (0.25 to 0.97)	0.71 (0.47 to 1.07)
Second	1.78 (1.43 to 2.21)	1.26 (0.94 to 1.71)	0.37 (0.18 to 0.77)	0.55 (0.37 to 0.83)
Lowest fourth	2.27 (1.82 to 2.83)	1.35 (0.97 to 1.91)	0.35 (0.17 to 0.69)	0.45 (0.29 to 0.68)
Test of trend‡	P<0.0001	P=0.08	P=0.002	P<0.0001
Risk ratio for trend	1.32 (1.22 to 1.42)	1.10 (0.99 to 1.24)	0.70 (0.56 to 0.88)	0.77 (0.67 to 0.88)

^*Adjusted for age, time period, and sex by matching. 

^†Adjusted for socioeconomic and marital status; schizophrenia, manic depressive psychosis, and reactive psychosis; and an indicator for more than one previous admission by regression and for age, sex, and time period by matching. 

^‡The three trends in the adjusted model were significantly different (P<0.0001).