524737 research-article2014
ISP0010.1177/0020764014524737International Journal of Social PsychiatryKwok
E CAMDEN SCHIZOPH
Article
Is there evidence that social class at birth increases risk of psychosis? A systematic review
International Journal of Social Psychiatry 2014, Vol. 60(8) 801–808 © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0020764014524737 isp.sagepub.com
Wingfai Kwok
Abstract Background: In the 1950s, researchers showed an association between low socio-economic status (SES) and psychosis. Two competing theories social causation and social drift were proposed to explain the findings. In the intervening years, contrasting evidence emerged as some studies showed no association between SES and schizophrenia. At present, the nature of the relationship is still unclear; currently, there are no reviews in the literature examining the association between social class at birth and psychosis. Aim: To search the literature to clarify the relationship between social class at birth, measured by paternal occupation at birth, and the risk of adult-onset psychosis. Methods: A systematic search of the literature using a combination of keywords in Group 1 together with the keywords in Group 2 was performed in October 2012 in the following online databases: (a) MEDLINE (1946–2012), (b) PubMed, (c) Embase (1980–2012), (d) PsycINFO (1806–2012) and (e) Web of Science (1899–2012). Reference lists were also hand searched. The search provided 3,240 studies; following screening of the titles and abstracts by inclusion and exclusion criteria and quality assessment of the full text, 14 studies were identified to be appropriate for the review. The keywords used for the search were as follows: Group 1 – social class, social status, socioeconomic, socio-economic, SES; Group 2 – psychosis, psychoses, schizophrenia. Results: Seven studies showed an association between low SES and psychosis. Four studies showed no association, and three studies showed an association with high SES. Conclusion: There is not enough evidence to support the association between social class and psychosis. While some findings showed an association between low social class and psychosis, there were a number of conflicting studies showing no association or a link with higher social class. Interestingly, the results followed a temporal pattern, as all the studies conducted after 2001 supported an association between low SES at birth and psychosis. Four of the six studies employed a prospective design with large sample populations, indicating the need for further investigation. Keywords Social class, socio-economic status, psychosis, schizophrenia
Introduction Socio-economic status and psychosis An individual’s rank in society, determined by the rewards and resources he or she earns is known as his or her socioeconomic status (SES). SES is primarily indexed by measures of income, occupation and educational attainment (Liberatos, Link, & Kelsey, 1988). In the literature, the terms social class and SES are used interchangeably, and in this article, they will be viewed as synonyms (Eaton & Harrison, 2001). An individual’s social class at birth is predominantly determined by measuring parental social class at birth, which is commonly defined by parental occupation on a 6-point scale (Table 1). Historically, paternal occupation is more frequently used to define individual
SES at birth compared to maternal occupation (Byrne et al., 2004; Werner, Malaspina, & Rabinowitz, 2007). In the mid-1950s, research in the United States and England showed a positive association between low SES and psychosis (Hollingshead & Redlich, 1958). For
School of Medicine, King’s College London, London, UK Corresponding author: Wingfai Kwok, School of Medicine, King’s College London, Guy’s Campus, London SE1 1UL, UK. Email: [email protected]
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International Journal of Social Psychiatry 60(8)
Table 1. The six categories of social class (Central Statistics Office, 1986). Social class scale Social class I – higher professional, higher managerial, proprietors and farmers with ≥200 acres (81 hectares) Social class II – lower professional, lower managerial and farmers with 100–199 acres Social class III – other non-manual and farmers with 50–99 acres Social class IV – skilled manual and farmers with 30–49 acres Social class V – semi-skilled manual and farmers with .05). Second, a case-control study (n = 352) of Irish adults showed no consistent trend in OR across the six paternal social classes (Mulvany et al., 2001). Furthermore, Corcoran et al. (2009) suggest that the association with psychotic illness is only present in the lowest paternal social class. A sample population (n = 88,829)
born between 1964 and 1976 were followed up between 22 and 34 years. Only individuals in the lowest social class at birth had a moderate increase in risk for schizophrenia regardless of the first four social classes used for comparison (RR = 1.4; 95% CI = 1.1–1.8, p = .002). Byrne et al. (2004) analysed a large case-control study (n = 200,294) of Danish adults. First, the researchers assessed the contribution of general background factors to risk of schizophrenia and found that an increased risk was associated with co-morbid psychiatric disorders, family history of schizophrenia and a history of attempted suicide. After controlling for these background factors, they found that an increased risk for schizophrenia was associated with paternal employment for less than 100% of the year (incidence rate ratio (IRR) = 1.21, 95% CI = 1.11– 1.33) when comparing other social classes to blue-collar workers (reference group). Interestingly, increased risk was also associated with maternal employment for less than 100% of the year (IRR = 1.21, 95% CI = 1.11–1.31) and being outside the workforce (IRR = 1.29, 95% CI = 1.17–1.42). Similar results were also reported by Wicks
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International Journal of Social Psychiatry 60(8)
et al. (2005) in a longitudinal study using nationally linked data of Swedish adults (n = 2,100,000) between 1970 and 1990. Individuals were followed up between 1987 and 2002, and the results showed that the HR for people with schizophrenia with unemployed parents in a single-parent household was 2.0 (95% CI = 1.9–2.2). The HR for individuals with other psychotic illnesses was 1.8 (95% CI = 1.7–2.0). In contrast to the previous studies, a longitudinal study by Mäkikyrö et al. (1997) found an association between high paternal SES at birth and adult-onset psychotic illness. A sample from the Northern Finland 1966 birth cohort (n = 11,017) was studied between the ages 16 and 27 years. The authors found that the incidence of schizophrenia was significantly higher (p < .05) in individuals of social class I (1.14%; 9/792) compared to social classes II–V (0.47%;48/10,225) at the age of 23 years. After this age, no new cases of psychotic illness was reported in social class I. Two other studies showed similar results. First, Wender et al. (1973) in an adoption study showed that there is a significant positive correlation (p < .05) between high adoptive paternal SES and risk for psychotic illness in offspring. Second, the OR was 0.49 (95% CI = 0.40–0.85, p < .0029) for people in low social classes compared to high social class in a small case-control study (n = 352; Mulvany et al., 2001), suggesting a slightly elevated risk for individuals in high social class. Furthermore, Timms (1998) found no association between low social class and psychotic illness in a prospective longitudinal study (n = 15,117). Curiously, individuals in the middle social classes had a non-significant increase in relative risk (RR = 1.60 95% CI = 0.95–2.72, p > .05) compared to the bottom social classes. Paternal SES is not associated with psychotic illness A pilot study by Goldberg et al. (1963) showed that in a small national sample (n = 369) of individuals with schizophrenia aged 25–34 years, there was an elevated number of observed patients in social class V than expected (observed = 90, expected = 34). Further comparison between these individuals and the general population aged 20–44 years showed very similar distributions of paternal social class, and thus suggests no association with psychotic illness. Similar results were reported by Hare et al. (1972) in a case-control study (n = 624) using a sample of British adults admitted into Bethlam-Maudsley hospital. The authors found that there was no significant difference between social classes of psychotic individuals and the general population. Wender et al. (1973) used an interesting study design to investigate the relationship between social class and risk of psychosis. The authors studied a sample of adopted individuals (n = 5,483) using a case-control design. The sample was drawn from adoption lists between 1924 and 1947 in Copenhagen, and paternal SES information was obtained for both biological and adoptee parents. The
sample population was then compared with the national psychiatric register in 1964. The authors showed there was no difference between the general population paternal SES and paternal SES of biological fathers (χ2 = 1.86, df = 2, p > .05) or adoptee fathers (χ2 = 0.75, df = 2, p > .05). Finally, a longitudinal study by Jones et al. (1994) analysed a British 1946 birth cohort (n = 5,362), with individuals followed up to age 43 years (1989). The results did not provide evidence that there is a significant association between low paternal social class and adult-onset schizophrenia (p > .05).
Discussion The main strength of this study, to the author’s knowledge, is that it is the first systematic review to investigate the association between paternal SES at birth and risk of adult-onset psychosis. The results were variable; half the studies supported an association between low paternal SES and psychotic illness, and the other half showed either no association or association with high paternal social class. Four of the most recent studies supporting an association with low paternal SES at birth (Corcoran et al., 2009; Goldberg et al., 2011; Werner et al., 2007; Wicks et al., 2005) used a prospective longitudinal design with large sample populations (71,165–2,100,000) to reach the same conclusion, and thus possessed high validity. The three studies which showed no association were all undertaken between 1963 and 1973 (Goldberg et al., 1963; Hare et al., 1972; Wender et al., 1973). These studies were retrospective and used comparatively low sample populations (369–5,483), suggesting a low validity and weak statistical power. In contrast, three studies (Mäkikyrö et al., 1997; Mulvany et al., 2001; Timms, 1998) that supported an association with high SES at birth were performed between 1997 and 2001. Two studies had high validity and statistical power as large sample populations (11,017–15,117) were analysed in a longitudinal design (Mäkikyrö et al., 1997; Timms, 1998). After a review of the general trends, it cannot be concluded that there is an association between social class at birth and psychotic illness in later life due to variability of the individual study results. Further investigation into the temporal aspects of the findings may allow further insight into this relationship, as all of the studies conducted after 2001 supported the association between low SES and psychosis, four of which are of gold standard longitudinal design with large sample populations. If the total number of cases were to be tabulated from the 14 studies included in the review, the vast majority of cases would come from studies which support the relationship between low paternal SES and increased risk for psychosis in later life. From these results, it can be hypothesised that future observational studies will also support this association, but a cautious approach should be taken, as unfortunately, it was not
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Kwok possible to conduct a meta-analysis of the data due to study heterogeneity. The social causation theory suggests adversity and chronic strains associated with a low SES are causal factors of psychosis (Werner et al., 2007). First, parents of low SES may have fewer resources for their children leading to harsher residential environments, fewer educational opportunities and barriers to social networks leading to social isolation. Second, a body of evidence suggests that some patients with psychosis have the condition as a consequence of neurodevelopment deviance, where an insult to the developing brain leads to symptoms manifesting later on in life (Weinberger, 1987). Limited financial assets in low-SES households (Brown, Susser, Jandorf, & Bromet, 2000) may lead to insufficient prenatal and postnatal care, potentially causing brain insult due to obstetric complications, malnutrition and a low birth weight (Castle et al., 1993; Koenig, Kirkpatrick, & Lee, 2002). This causal relationship appears to only affect the lowest social classes. For example, both Harrison et al. (2001) and Corcoran et al. (2009) found the risk for schizophrenia was significant in only the bottom two social classes. Furthermore, there was no evidence for gradient of risk for schizophrenia (Mulvany et al., 2001). The associations between high SES at birth and psychosis may reflect underlying relations between education and social class (Mäkikyrö et al., 1997; Wender et al., 1973). Individuals of high educational attainment are more likely to be in a higher social class. Both Wiersma et al. (1983) and Byrne et al. (2004) have shown an increased risk of schizophrenia for individuals with high educational attainment. It has been hypothesised that the genetic predisposition for a high IQ may be associated with the genetic basis of schizophrenia (Aylward, Walker, & Bettes, 1984). Alternatively, a family history of psychiatric illness is a potential mechanism by which an individual’s risk for schizophrenia increases during later life (Werner et al., 2007). The parents may also suffer from psychosis and can potentially transmit a genetic risk to their offspring. Patients with schizophrenia experience widespread cognitive deficits which can severely impair social and vocational functioning (Heinrichs & Zakzanis, 1998), and there is evidence to suggest this cognitive decline can present before the onset of psychotic symptoms (Cosway et al., 2000). As the parent drifts into a lower SES due to their condition and associated social disabilities, the child is thus born into a lower social class (Mortensen et al., 1999). Future horizons. There are four main limitations of this study. First, there may be publication bias as only published studies were included in this review. Relevant studies may not have been found using the database and reference searches. Second, paternal psychopathology was not measured, and thus, confounding genetic effects could not be ruled out. Third, study heterogeneity made it
difficult to compare the studies. This is exacerbated by the limited number of studies in the literature. Fourth, only paternal occupation was used to define SES in this review and so may not be truly representative of social class. To overcome these limitations, a future review should analyse studies using large sample populations with a prospective longitudinal study design. Ideally, multiple markers of social class should be present together with a measure of parental psychopathology. Interestingly, Byrne et al. (2004) showed that after adjusting for general background and paternal SES, individuals with low SES in adulthood were at a higher risk of psychosis. This risk was associated with low levels of employment (Crude RR = 9.63, 95% CI = 8.86–10.46), wealth (Crude RR = 2.71, 95% CI = 2.53–2.91) and income (Crude RR = 20.77, 95% CI = 18.72–23.05). Additionally, individuals born in inner city London were at higher risk (OR = 2.1, 95% CI = 1.2–3.7) compared to being born elsewhere (Castle et al., 1993). Furthermore, after controlling for paternal SES, sex, year of birth and fathers’ age at birth, individuals in a low-SES area were still at increased risk compared to high-SES areas (OR = 1.26, 95% CI = 1.05–1.52, p < .05; Byrne et al., 2004). In light of these epidemiological findings, a review investigating association between individual SES and psychosis should be considered, as at present, there are no systematic reviews in the literature. At present, it cannot be concluded that paternal SES at birth is associated with psychosis. Acknowledgements The author thanks Robin Murray and Simona Stilo (Institute of Psychiatry, London) and the anonymous reviewers for their comments and suggestions on this manuscript. The author also thanks Siobhan Lynch and Simon Wessely (Institute of Psychiatry, London) for their continued support.
Funding This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
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study. Social Psychiatry and Psychiatric Epidemiology, 39, 87–96. Castle, D. J., Scott, K., Wessely, S., & Murray, R. M. (1993). Does social deprivation during gestation and early life predispose to later schizophrenia? Social Psychiatry and Psychiatric Epidemiology, 28, 1–4. Central Statistics Office. (1986). Census of population 1986: Classification of occupations. Dublin, Ireland: Stationery Office. Cooper, B. (1961). Social class and prognosis in schizophrenia. British Journal of Preventive & Social Medicine, 15, 17–41. Corcoran, C., Perrin, M., Harlap, S., & Deutsch, S. (2009). Effect of socioeconomic status and parents’ education at birth on risk of schizophrenia in offspring. Social Psychiatry and Psychiatric Epidemiology, 44, 265–271. Cosway, R., Byrne, M., Clafferty, R., Hodges, A., Grant, E., Abukmeil, S., … Lawrie, S. M. (2000). Neuropsychological change in young people at high risk for schizophrenia: Results from the first two neuropsychological assessments of the Edinburgh High Risk Study. Psychological Medicine, 30, 1111–1121. Dohrenwend, B. P., Levav, I., Shrout, P. E., Schwartz, S., Naveh, G., Link, B. G., … Stueve, A. (1992). Socioeconomic status and psychiatric disorders: The causation-selection issue. Science, 255, 946–952. Eaton, W., & Harrison, G. (2001). Life chances, life planning, and schizophrenia: A review and interpretation of research on social deprivation. International Journal of Mental Health, 30, 58–81. Eaton, W. W. (1985). Epidemiology of schizophrenia. Epidemiologic Reviews, 7, 105–126. Fearon, P., Kirkbride, J. B., Morgan, C., Dazzan, P., Morgan, K., Lloyd, T., … Murray, R. M. (2006). Incidence of schizophrenia and other psychoses in ethnic minority groups: Results from the MRC AESOP Study. Psychological Medicine, 36, 1541–1550. Goldberg, E. M., & Morrison, S. L. (1963). Schizophrenia and social class. British Journal of Psychiatry, 108, 785–802. Goldberg, S., Fruchter, E., Davidson, M., Reichenberg, A., Yoffe, R., & Weiser, M. (2011). The relationship between risk of hospitalisation for schizophrenia, SES, and cognitive functioning. Schizophrenia Bulletin, 37, 664–670. Hare, E. H., Price, J. S., & Slater, E. (1972). Parental social class and psychiatric patients. British Journal of Psychiatry, 121, 515–524. Harrison, G., Gunnell, D., Glazebrook, C., Page, K., & Kwiecinski, R. (2001). Association between schizophrenia and social inequality at birth: Case-control study. British Journal of Psychiatry, 179, 346–350. Heinrichs, R. W., & Zakzanis, K. K. (1998). Neurocognitive deficit in schizophrenia: A quantitative review of the evidence. Neuropsychology, 12, 426–445.
Hjern, A., Wicks, S., & Dalman, C. (2004). Social adversity contributes to high morbidity in psychoses in immigrants- a national cohort study in two generations of Swedish residents. Psychological Medicine, 34, 1025–1033. Hollingshead, A. B., & Redlich, F. C. (1958). Social class and mental illness: Community study. Hoboken, NJ: John Wiley & Sons, Inc. Jones, P., Rodgers, B., Murray, R. M., & Marmot, M. (1994). Child develop-mental risk factors for adult schizophrenia in the British 1946 birth cohort. Lancet, 344, 1398– 1402. Kirkbride, J. B., Barker, D., Yang, M., Jones, P. B., & Coid, J. W. (2008). Psychoses, ethnicity and socio-economic status. British Journal of Psychiatry, 193, 18–24. Koenig, J. I., Kirkpatrick, B., & Lee, P. (2002). Glucocorticoid hormones and early brain development in schizophrenia. Neuropsychopharmacology, 27, 309–318. Liberatos, P., Link, B. G, & Kelsey, J. L. (1988). The measurement of social class in epidemiology. Epidemiologic Reviews, 10, 87–121. Mäkikyrö, T., Isohanni, M., Moring, J., Oja, H., Hakko, H., Jones, P., & Rantakallio, P. (1997). Is a child’s risk of early onset schizophrenia increased in the highest social class? Schizophrenia Research, 23, 245–252. Mortensen, P. B., Pedersen, C. B., Westergaard, T., Wohlfahrt, J., Ewald, H., Mors, O., … Melbye, M. (1999). Effects of family history and place and season of birth on the risk of schizophrenia. New England Journal of Medicine, 340, 603–608. Mulvany, F., O’Callaghan, E., Takei, N., Byrne, M., Fearon, P., & Larkin, C. (2001). Effect of social class at birth on risk and presentation of schizophrenia: Case-control study. BMJ, 323, 1398–1401. Timms, D. (1998). Gender, social mobility and psychiatric diagnoses. Social Science & Medicine, 46, 1235–1247. Weinberger, D. R. (1987). Implications of normal brain development for the pathogenesis of schizophrenia. Archives of General Psychiatry, 44, 660–669. Wender, P., Rosenthal, D., Kety, S., Schulsinger, F., & Welner, J. (1973). Social class and psychopathology in adoptees. Archives of General Psychiatry, 28, 318–325. Werner, S., Malaspina, D., & Rabinowitz, J. (2007). Socioeconomic status at birth is associated with risk of schizophrenia: Population-based multilevel study. Schizophrenia Bulletin, 33, 1373–1378. Wicks, S., Hjern, A., Gunnell, D., Lewis, G., & Dalman, C. (2005). Social adversity in childhood and the risk of developing psychosis: A national cohort study. American Journal of Psychiatry, 162, 1652–1657. Wiersma, D., Giel, R., De Jong, A., & Slooff, C. J. (1983). Social class and schizophrenia in a Dutch Cohort. Psychological Medicine, 13, 141–150.
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ISP0010.1177/0020764014524737International Journal of Social PsychiatryKwok
E CAMDEN SCHIZOPH
Article
Is there evidence that social class at birth increases risk of psychosis? A systematic review
International Journal of Social Psychiatry 2014, Vol. 60(8) 801–808 © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0020764014524737 isp.sagepub.com
Wingfai Kwok
Abstract Background: In the 1950s, researchers showed an association between low socio-economic status (SES) and psychosis. Two competing theories social causation and social drift were proposed to explain the findings. In the intervening years, contrasting evidence emerged as some studies showed no association between SES and schizophrenia. At present, the nature of the relationship is still unclear; currently, there are no reviews in the literature examining the association between social class at birth and psychosis. Aim: To search the literature to clarify the relationship between social class at birth, measured by paternal occupation at birth, and the risk of adult-onset psychosis. Methods: A systematic search of the literature using a combination of keywords in Group 1 together with the keywords in Group 2 was performed in October 2012 in the following online databases: (a) MEDLINE (1946–2012), (b) PubMed, (c) Embase (1980–2012), (d) PsycINFO (1806–2012) and (e) Web of Science (1899–2012). Reference lists were also hand searched. The search provided 3,240 studies; following screening of the titles and abstracts by inclusion and exclusion criteria and quality assessment of the full text, 14 studies were identified to be appropriate for the review. The keywords used for the search were as follows: Group 1 – social class, social status, socioeconomic, socio-economic, SES; Group 2 – psychosis, psychoses, schizophrenia. Results: Seven studies showed an association between low SES and psychosis. Four studies showed no association, and three studies showed an association with high SES. Conclusion: There is not enough evidence to support the association between social class and psychosis. While some findings showed an association between low social class and psychosis, there were a number of conflicting studies showing no association or a link with higher social class. Interestingly, the results followed a temporal pattern, as all the studies conducted after 2001 supported an association between low SES at birth and psychosis. Four of the six studies employed a prospective design with large sample populations, indicating the need for further investigation. Keywords Social class, socio-economic status, psychosis, schizophrenia
Introduction Socio-economic status and psychosis An individual’s rank in society, determined by the rewards and resources he or she earns is known as his or her socioeconomic status (SES). SES is primarily indexed by measures of income, occupation and educational attainment (Liberatos, Link, & Kelsey, 1988). In the literature, the terms social class and SES are used interchangeably, and in this article, they will be viewed as synonyms (Eaton & Harrison, 2001). An individual’s social class at birth is predominantly determined by measuring parental social class at birth, which is commonly defined by parental occupation on a 6-point scale (Table 1). Historically, paternal occupation is more frequently used to define individual
SES at birth compared to maternal occupation (Byrne et al., 2004; Werner, Malaspina, & Rabinowitz, 2007). In the mid-1950s, research in the United States and England showed a positive association between low SES and psychosis (Hollingshead & Redlich, 1958). For
School of Medicine, King’s College London, London, UK Corresponding author: Wingfai Kwok, School of Medicine, King’s College London, Guy’s Campus, London SE1 1UL, UK. Email: [email protected]
Downloaded from isp.sagepub.com at UCSF LIBRARY & CKM on March 27, 2015
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International Journal of Social Psychiatry 60(8)
Table 1. The six categories of social class (Central Statistics Office, 1986). Social class scale Social class I – higher professional, higher managerial, proprietors and farmers with ≥200 acres (81 hectares) Social class II – lower professional, lower managerial and farmers with 100–199 acres Social class III – other non-manual and farmers with 50–99 acres Social class IV – skilled manual and farmers with 30–49 acres Social class V – semi-skilled manual and farmers with .05). Second, a case-control study (n = 352) of Irish adults showed no consistent trend in OR across the six paternal social classes (Mulvany et al., 2001). Furthermore, Corcoran et al. (2009) suggest that the association with psychotic illness is only present in the lowest paternal social class. A sample population (n = 88,829)
born between 1964 and 1976 were followed up between 22 and 34 years. Only individuals in the lowest social class at birth had a moderate increase in risk for schizophrenia regardless of the first four social classes used for comparison (RR = 1.4; 95% CI = 1.1–1.8, p = .002). Byrne et al. (2004) analysed a large case-control study (n = 200,294) of Danish adults. First, the researchers assessed the contribution of general background factors to risk of schizophrenia and found that an increased risk was associated with co-morbid psychiatric disorders, family history of schizophrenia and a history of attempted suicide. After controlling for these background factors, they found that an increased risk for schizophrenia was associated with paternal employment for less than 100% of the year (incidence rate ratio (IRR) = 1.21, 95% CI = 1.11– 1.33) when comparing other social classes to blue-collar workers (reference group). Interestingly, increased risk was also associated with maternal employment for less than 100% of the year (IRR = 1.21, 95% CI = 1.11–1.31) and being outside the workforce (IRR = 1.29, 95% CI = 1.17–1.42). Similar results were also reported by Wicks
Downloaded from isp.sagepub.com at UCSF LIBRARY & CKM on March 27, 2015
806
International Journal of Social Psychiatry 60(8)
et al. (2005) in a longitudinal study using nationally linked data of Swedish adults (n = 2,100,000) between 1970 and 1990. Individuals were followed up between 1987 and 2002, and the results showed that the HR for people with schizophrenia with unemployed parents in a single-parent household was 2.0 (95% CI = 1.9–2.2). The HR for individuals with other psychotic illnesses was 1.8 (95% CI = 1.7–2.0). In contrast to the previous studies, a longitudinal study by Mäkikyrö et al. (1997) found an association between high paternal SES at birth and adult-onset psychotic illness. A sample from the Northern Finland 1966 birth cohort (n = 11,017) was studied between the ages 16 and 27 years. The authors found that the incidence of schizophrenia was significantly higher (p < .05) in individuals of social class I (1.14%; 9/792) compared to social classes II–V (0.47%;48/10,225) at the age of 23 years. After this age, no new cases of psychotic illness was reported in social class I. Two other studies showed similar results. First, Wender et al. (1973) in an adoption study showed that there is a significant positive correlation (p < .05) between high adoptive paternal SES and risk for psychotic illness in offspring. Second, the OR was 0.49 (95% CI = 0.40–0.85, p < .0029) for people in low social classes compared to high social class in a small case-control study (n = 352; Mulvany et al., 2001), suggesting a slightly elevated risk for individuals in high social class. Furthermore, Timms (1998) found no association between low social class and psychotic illness in a prospective longitudinal study (n = 15,117). Curiously, individuals in the middle social classes had a non-significant increase in relative risk (RR = 1.60 95% CI = 0.95–2.72, p > .05) compared to the bottom social classes. Paternal SES is not associated with psychotic illness A pilot study by Goldberg et al. (1963) showed that in a small national sample (n = 369) of individuals with schizophrenia aged 25–34 years, there was an elevated number of observed patients in social class V than expected (observed = 90, expected = 34). Further comparison between these individuals and the general population aged 20–44 years showed very similar distributions of paternal social class, and thus suggests no association with psychotic illness. Similar results were reported by Hare et al. (1972) in a case-control study (n = 624) using a sample of British adults admitted into Bethlam-Maudsley hospital. The authors found that there was no significant difference between social classes of psychotic individuals and the general population. Wender et al. (1973) used an interesting study design to investigate the relationship between social class and risk of psychosis. The authors studied a sample of adopted individuals (n = 5,483) using a case-control design. The sample was drawn from adoption lists between 1924 and 1947 in Copenhagen, and paternal SES information was obtained for both biological and adoptee parents. The
sample population was then compared with the national psychiatric register in 1964. The authors showed there was no difference between the general population paternal SES and paternal SES of biological fathers (χ2 = 1.86, df = 2, p > .05) or adoptee fathers (χ2 = 0.75, df = 2, p > .05). Finally, a longitudinal study by Jones et al. (1994) analysed a British 1946 birth cohort (n = 5,362), with individuals followed up to age 43 years (1989). The results did not provide evidence that there is a significant association between low paternal social class and adult-onset schizophrenia (p > .05).
Discussion The main strength of this study, to the author’s knowledge, is that it is the first systematic review to investigate the association between paternal SES at birth and risk of adult-onset psychosis. The results were variable; half the studies supported an association between low paternal SES and psychotic illness, and the other half showed either no association or association with high paternal social class. Four of the most recent studies supporting an association with low paternal SES at birth (Corcoran et al., 2009; Goldberg et al., 2011; Werner et al., 2007; Wicks et al., 2005) used a prospective longitudinal design with large sample populations (71,165–2,100,000) to reach the same conclusion, and thus possessed high validity. The three studies which showed no association were all undertaken between 1963 and 1973 (Goldberg et al., 1963; Hare et al., 1972; Wender et al., 1973). These studies were retrospective and used comparatively low sample populations (369–5,483), suggesting a low validity and weak statistical power. In contrast, three studies (Mäkikyrö et al., 1997; Mulvany et al., 2001; Timms, 1998) that supported an association with high SES at birth were performed between 1997 and 2001. Two studies had high validity and statistical power as large sample populations (11,017–15,117) were analysed in a longitudinal design (Mäkikyrö et al., 1997; Timms, 1998). After a review of the general trends, it cannot be concluded that there is an association between social class at birth and psychotic illness in later life due to variability of the individual study results. Further investigation into the temporal aspects of the findings may allow further insight into this relationship, as all of the studies conducted after 2001 supported the association between low SES and psychosis, four of which are of gold standard longitudinal design with large sample populations. If the total number of cases were to be tabulated from the 14 studies included in the review, the vast majority of cases would come from studies which support the relationship between low paternal SES and increased risk for psychosis in later life. From these results, it can be hypothesised that future observational studies will also support this association, but a cautious approach should be taken, as unfortunately, it was not
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Kwok possible to conduct a meta-analysis of the data due to study heterogeneity. The social causation theory suggests adversity and chronic strains associated with a low SES are causal factors of psychosis (Werner et al., 2007). First, parents of low SES may have fewer resources for their children leading to harsher residential environments, fewer educational opportunities and barriers to social networks leading to social isolation. Second, a body of evidence suggests that some patients with psychosis have the condition as a consequence of neurodevelopment deviance, where an insult to the developing brain leads to symptoms manifesting later on in life (Weinberger, 1987). Limited financial assets in low-SES households (Brown, Susser, Jandorf, & Bromet, 2000) may lead to insufficient prenatal and postnatal care, potentially causing brain insult due to obstetric complications, malnutrition and a low birth weight (Castle et al., 1993; Koenig, Kirkpatrick, & Lee, 2002). This causal relationship appears to only affect the lowest social classes. For example, both Harrison et al. (2001) and Corcoran et al. (2009) found the risk for schizophrenia was significant in only the bottom two social classes. Furthermore, there was no evidence for gradient of risk for schizophrenia (Mulvany et al., 2001). The associations between high SES at birth and psychosis may reflect underlying relations between education and social class (Mäkikyrö et al., 1997; Wender et al., 1973). Individuals of high educational attainment are more likely to be in a higher social class. Both Wiersma et al. (1983) and Byrne et al. (2004) have shown an increased risk of schizophrenia for individuals with high educational attainment. It has been hypothesised that the genetic predisposition for a high IQ may be associated with the genetic basis of schizophrenia (Aylward, Walker, & Bettes, 1984). Alternatively, a family history of psychiatric illness is a potential mechanism by which an individual’s risk for schizophrenia increases during later life (Werner et al., 2007). The parents may also suffer from psychosis and can potentially transmit a genetic risk to their offspring. Patients with schizophrenia experience widespread cognitive deficits which can severely impair social and vocational functioning (Heinrichs & Zakzanis, 1998), and there is evidence to suggest this cognitive decline can present before the onset of psychotic symptoms (Cosway et al., 2000). As the parent drifts into a lower SES due to their condition and associated social disabilities, the child is thus born into a lower social class (Mortensen et al., 1999). Future horizons. There are four main limitations of this study. First, there may be publication bias as only published studies were included in this review. Relevant studies may not have been found using the database and reference searches. Second, paternal psychopathology was not measured, and thus, confounding genetic effects could not be ruled out. Third, study heterogeneity made it
difficult to compare the studies. This is exacerbated by the limited number of studies in the literature. Fourth, only paternal occupation was used to define SES in this review and so may not be truly representative of social class. To overcome these limitations, a future review should analyse studies using large sample populations with a prospective longitudinal study design. Ideally, multiple markers of social class should be present together with a measure of parental psychopathology. Interestingly, Byrne et al. (2004) showed that after adjusting for general background and paternal SES, individuals with low SES in adulthood were at a higher risk of psychosis. This risk was associated with low levels of employment (Crude RR = 9.63, 95% CI = 8.86–10.46), wealth (Crude RR = 2.71, 95% CI = 2.53–2.91) and income (Crude RR = 20.77, 95% CI = 18.72–23.05). Additionally, individuals born in inner city London were at higher risk (OR = 2.1, 95% CI = 1.2–3.7) compared to being born elsewhere (Castle et al., 1993). Furthermore, after controlling for paternal SES, sex, year of birth and fathers’ age at birth, individuals in a low-SES area were still at increased risk compared to high-SES areas (OR = 1.26, 95% CI = 1.05–1.52, p < .05; Byrne et al., 2004). In light of these epidemiological findings, a review investigating association between individual SES and psychosis should be considered, as at present, there are no systematic reviews in the literature. At present, it cannot be concluded that paternal SES at birth is associated with psychosis. Acknowledgements The author thanks Robin Murray and Simona Stilo (Institute of Psychiatry, London) and the anonymous reviewers for their comments and suggestions on this manuscript. The author also thanks Siobhan Lynch and Simon Wessely (Institute of Psychiatry, London) for their continued support.
Funding This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
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