Introduction

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Psychiatric disorders have been consistently shown to be more common among people in lower social classes. ^{1 2} Schizophrenia is the mental illness most strongly linked to class, with working class people being about five times more likely to be diagnosed with schizophrenia than other groups.³

Many people who develop schizophrenia do not achieve or maintain the social class they were born into. By the time they have contact with psychiatric services, patients have often moved into a lower social class. ^{4 5} However, this does not exclude the possibility that low social class increases the risk of later schizophrenia. ^{6 7} Evidence is accumulating that the origins of the disorder lie in early life, and various environmental factors have been shown to be associated with an increased risk of later schizophrenia.⁸ These factors include obstetric complications, ^{9 10} prenatal infections,¹¹ and nutritional deprivation,¹² all of which are more common among people in lower social classes.^13-15 However, it remains unclear whether people born into lower social classes are at increased risk of schizophrenia. ^{1 16 17}

Social class at birth could also modify the course of the disorder. There is often a considerable delay between the onset of psychotic symptoms and adequate treatment. People who have a longer duration of untreated psychosis have been shown to have a poorer outcome, ^{18 19} and social class influences the age at which people with other illnesses present to healthcare services.²⁰ Using a case-control design, we investigated whether social class of origin influenced the risk of schizophrenia and the age at which patients first presented to psychiatric services for treatment.

	Participants and methods

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The sample was drawn from patients referred to Cluain Mhuire Family Centre, a geographically defined community based psychiatric service for a population of 165 000 people. Cases were limited to patients who were subsequently admitted to Saint John of God Hospital for the first time with schizophrenia (diagnosis based on ICD-9, international classification of diseases, 9th edition). We selected a consecutive case series of patients (n=629) discharged from their first admission to hospital between January 1984 and May 1993. We compiled clinical and demographic data for each patient within the catchment area from case notes and computerised records. We defined age at first contact with psychiatric service as any inpatient or outpatient contact with a mental health professional. The age at first diagnosis was defined as the date at which schizophrenia was diagnosed.

We searched the birth registration records of the 629 patients with the help of the General Register Office and recorded details of their father's occupation at the time of their birth. We were unable to locate the records for 140 patients, and a further 64 patients were excluded because they were not born in Ireland (19 people), they had an incorrect date of birth (4), details of fathers' occupation were not recorded (2), or their maiden name was missing (39 married women).

We recorded the paternal occupation of the next registered same sex birth from the birth registration district of the patient and used that child as a matched control. Paternal occupational data were compiled in accordance with the Census of Population Classification of Occupations, which consists of six categories (box).²¹ Farmers are assigned to a social class category on the basis of farm acreage, but this was not included on the birth registration record. We were therefore unable to assign social class categories to 31 cases and 51 controls. We were also unable to assign social class codes to fathers described on the birth certificate as unemployed (2 cases, 2 controls), student (1 case, 1 control), or pensioner (1 control). The final sample consisted of 391 cases and 370 controls, of which 352 were matched pairs.

Power
With 352 case-control pairs, this study had a power of 87% to detect a difference of 10% in the proportions of discordant pairs (classes I-III v IV-VI) with respect to social class of origin provided that the proportion of discordant pairs was 37% at a two sided 5% significance level. We had sufficient patients in social classes I and VI to detect a difference between the two classes of 8.5 years in the age of first contact (80% power), 8.2 years in the age of first ever admission (78% power), and 10 years in the age of first admission with schizophrenia (88% power) at a two sided 1% significance level.

Statistical analysis
We investigated the distribution of social class backgrounds between cases and controls using the Wilcoxon matched pairs signed rank test. We then examined the proportions of discordant pairs for social class using McNemar's test, in which a binary social class variable (social class I-III=high, social class IV-VI=low) was created. In addition, we used a model of conditional logistic regression to fit the six social class categories as an explanatory variable. Next, we carried out analysis of variance to examine any difference in the age at first contact with psychiatric services, age at first ever admission, and age at first admission with a diagnosis of schizophrenia among the six social class groups. Finally, we fitted a regression model to quantify differences in ages at first use of the psychiatric services among the six social class of groups. Except for the conditional logistic regression analysis (STATA programme), we used the SPSS statistical package throughout.

	Results

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Social class as risk factor for schizophrenia
There was no overall significant difference between cases and controls in the distribution of social class (Wilcoxon rank sum z=1.3912, P=0.16). Within the 352 matched pairs, the patients and controls came from the same social class in 77 pairs; patients were from a higher social class in 149 pairs and from a lower social class in 126 pairs. The odds ratio of developing the disorder associated with social class (low v high) was 0.59 (95% confidence interval 0.40 to 0.85, P<0.0029), indicating that people from low social classes have a reduced risk of schizophrenia compared with those from high social classes.

Relation between social class and age at first presentation
There was an overall significant difference in the age at first contact among the six social classes (F=2.95, df=5, 345; P=0.01). The mean age at first contact with psychiatric services was 30.1 years in the whole sample (men 28.6 years; women 32.2 years). Patients in social class I were youngest at first contact (25.3 years) and those in social class VI the oldest (34.4 years). Similarly, there was an overall significant difference in the age at first ever admission (F=2.55, df=5, 345; P=0.03). The mean age at first ever admission to a psychiatric hospital was 29.6 years (men 28.2 years; women 31.6 years). Patients in social class I were youngest at first ever admission (24.8 years) and those in social class VI oldest (33.1 years). We also found a significant difference in the age at first admission with schizophrenia (F=3.92, df=5, 345; P=0.002); the mean age at first admission with schizophrenia was 33.3 years (men 31.4 years; women 36.0 years), with patients in social class I being the youngest (28.0 years) and those in social class VI the oldest (38.8 years).

	Discussion

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We found no link between social class at birth and risk of schizophrenia. However, people in lower social classes tended to present later than those in higher social classes. Men also presented at a younger age than women in the same social class.

Validity of data
We located 68% of the birth registration records and had matched pair data for 56% of records. This is a similar proportion to that identified by Goldberg and Morrison (55%), but lower than that in the study by Hare and colleagues (75%), both of which used similar methods. ^{1 17} If the rate of mandatory birth registration of people from lower social groups were lower than that from higher social groups, the study would have systematic bias. We could find no evidence to support systematic bias, but an undetected bias would greatly affect our conclusions.

Comparison with other studies
Our data contrast with the findings of Croudace and colleagues, who reported an increased risk of schizophrenia among people from lower social classes.⁷ A study of a northern Finland 1966 birth cohort, in which 11 017 people alive at the age of 16 years were followed up, found similar results to ours.²⁴ The two studies are not directly comparable, however, because our case-control study is sensitive to systematic bias and their cohort study did not follow up participants beyond age 27 years.

Effect of age on presentation
We found more than an eight year difference in age at first presentation to psychiatric services between the lowest and highest social classes. Although it is possible that schizophrenia has a different age at onset across social classes, this is unlikely. Similarly, it is unlikely that family doctors are selectively not referring patients from lower social classes. Use of health services differs between the social classes. For example, people in the lowest social class are least likely to seek early antenatal care; middle class parents are more likely to seek medical attention for their children; and working class adults are likely to be more ill than are middle class adults before seeking help.²⁰ Schizophrenia is probably subject to the same social class effects.