Income inequality and the prevalence of mental illness: a preliminary international analysis

The World Health Organisation's world mental health survey initiative has recently provided comparable cross‐national estimates of the prevalence of any, and serious, mental illness.¹ In an exploratory study, we estimated the relations of two important economic factors—living standards and income inequality—to mental illness in developed countries.

Methods and results

Prevalence of mental illness in the WMH survey initiative is derived from face to face interviews using the WMH version of the WHO composite international diagnostic interview (WMH‐CIDI), a fully structured, lay administered psychiatric diagnostic interview. Data have so far been reported for eight developed countries (USA, France, Netherlands, Belgium, Spain, Germany, Italy, and Japan). Surveys were based on household probability samples, with an average response rate of 70%.¹ Living standards were measured by gross national income (GNI) per capita from the online World Bank World Development Indicators database. Income inequality was measured as the ratio of the income share of the richest 20% to the income share of the poorest 20% in the 1990s by the United Nations development programme human development indicators, 2005 (table 1). We computed Pearson correlations between the prevalence of any mental illness and income inequality and GNI per capita, and also between serious mental illness and the two economic measures.

Table 1 Prevalence of mental illness, living standards, and income inequality in eight developed countries.

Country	Any mental illness% (95% CI)	Serious mental illness% (95% CI)	GNI per capita US$, purchasing power parity	Income inequality (top 20:bottom 20 ratio)
Belgium	12.0 (9.6, 14.3)	2.4 (1.2, 3.5)	28130	4.5
France	18.4 (15.2, 21.6)	2.7 (1.1, 4.3)	27040	5.6
Germany	9.1 (7.2, 10.9)	1.2 (0.6, 1.7)	26980	4.3
Italy	8.2 (6.7, 9.7)	1.0 (0.4, 1.7)	26170	6.5
Japan	8.8 (6.2, 11.4)	1.5 (0.7, 2.2)	27380	3.4
Netherlands	14.8 (12.0, 17.7)	2.3 (1.1, 3.5)	28350	5.5
Spain	9.2 (7.8, 10.7)	1.0 (0.7, 1.3)	21210	5.4
USA	26.3 (24.6, 27.9)	7.7 (7.0, 8.4)	36110	8.4

There are strong, positive linear associations of GNI per capita with any mental illness (r = 0.80, p value = 0.02), and with serious mental illness (r = 0.89, p value <0.01). There is also a strong (r = 0.73) and significant (p value = 0.04) linear correlation between the prevalence of any mental illness and income inequality (fig 1) and between serious mental illness and income inequality (r = 0.74, p value = 0.03). Using different measures of income inequality (10:10 ratio, Gini coefficient) does not substantially affect these results.

Figure 1 Relation between income inequality and prevalence of mental illness in eight developed countries.

Comment

Income inequality has been linked to physical morbidity, mortality, and such psychosocial outcomes as violence.² This preliminary analysis suggests that higher national levels of income inequality are linked to a higher prevalence of mental illness and, in contrast with studies of physical morbidity and mortality, as countries get richer rates of mental illness increase. Within countries, markers of socioeconomic disadvantage (low education, unemployment, and deprivation)³ and low levels of social capital⁴ have been associated with mental illness.⁵ As comparable data for more countries become available, it will be possible to estimate the independent, ecological associations between mental health, inequality and income levels.