Dear Editor,

The correlation (r = 0.73) between income inequality and prevalence of mental illness reported by Pickett, James and Wilkinson (2006) was an intriguing finding, but we should be extremely cautious interpreting it.

First, it was admittedly only a preliminary analysis and hence the number of data points (countries) was small (n = 8). Consequently, the correlation estimate will lack precision and this is reflected in the confidence intervals for the correlation estimate, 95% CI [0.03, 0.95]. Hence, the correlation estimate could be substantially lower.

Second, referring back to the raw data that originated from the UNDP Human Development Report (2005) and Demyttenaere et al. (2004) I discovered that data from the four developing countries (Mexico, Columbia, Ukraine and Nigeria) was excluding from the original analysis. If we recomputed the correlational analysis using the original data supplemented by the data from the four developing countries we obtain a correlation of 0.07 (95% CI [-0.52, 0.62]).

Third, assuming that there are good reasons for excluding the developing countries, a high correlation between income inequality and prevalence of mental illness tells only part of the story. What is equally important is the unstandardized slope of the linear regression line (i.e., simple effect size). This is because, though income inequality and prevalence of mental illness may be highly correlated, the magnitude that the prevalence of mental illness increases in relation to the income inequality may be negligibly small. The unstandardized slope of the linear regression line for the original data from Pickett et al (2006) is 3 (95% CI [0.12, 5.87]). Therefore, as the income equality, measured in terms of the ratio of the top 20% to the bottom 20% incomes increases by 1 the prevalence of mental illness increases by 3%. But what we have to bear in mind is that a ratio increase of 1 actually represents a 100% increase in the difference between the top 20% and the bottom 20% incomes. Therefore, the prevalence of mental illness increases by only 3% when the difference between the top 20% and the bottom 20% incomes increases by 100%. Or in other words 1% increase in the difference between the top 20% and the bottom 20% incomes is associated with a 0.03% increase in the prevalence of mental illness. Hence, the impact of income equality on the prevalence of mental illness appears to be, as one would expect, relatively small.

References

Demyttenaere K, Bruffaerts R, Posada-Villa J, et al. (2004). Prevalence, severity, and unmet need for treatment of mental disorders in the World Health Organization world mental health surveys. JAMA, 291, 2581 -90.

Pickett, K.E., James, O.W., & Wilkinson R.G. (2006). Income inequality and the prevalence of mental illness: a preliminary international analysis. Journal of Epidemiology & Community Health, 60, 646-647.

United Nations Development Programme. (2005). Human Development Report. New York: Oxford University Press. Retrieved from: http://hdr.undp.org/en/media/HDR05_complete.pdf

Conflict of Interest:

None declared

Dear Editor

We appreciate Huisman and Avendano’s interest in our research letter on income inequality and the prevalence of mental illness. [1] They point out that the correlations we report between income inequality and mental illness are driven by the position of the United States as an outlier, with a very high prevalence of mental illness and very high levels of income inequality. [2]

As we pointed out in our report, our study was exploratory and intended to stimulate further research, rather than a definitive examination of this question. We felt it important to restrict our report to those eight countries with strictly comparable prevalence estimates provided by WHO. [3] However, we have also examined this relationship including a further four countries (Canada, Australia, Singapore and the UK) with prevalence estimates from population-based surveys of adults (data available upon request). In these analyses the correlation between income inequality and mental illness is r=0.63, p value=0.03, and this correlation is unaffected by the exclusion of the USA (r=0.63, p value=0.09). In fact, if we limit our analyses to the four English- speaking nations for which we have data (USA, UK, Canada and Australia), the correlation is even stronger (r=0.99, p=0.003), suggesting that within a larger framework of nations the USA may be representative of a particular relationship between social structure and mental health, rather than an outlier.

So, the USA might be an outlier with respect to continental Western Europe, and within Western Europe there may be insufficient variation in income inequality to explore its relation to health. Nevertheless, within a larger grouping of nations, the positive correlation of income inequality and prevalence of mental health suggests interesting avenues for further research. Rather than contradicting the notion that income inequality might be an important determinant of health, these preliminary results support our recent finding of overwhelming evidence in support of this hypothesis among studies conducted at the level of nations. [4]

References

1.Pickett, K.E., O.W. James, and R.G. Wilkinson, Income inequality and the prevalence of mental illness: a preliminary international analysis. J Epidemiol Community Health, 2006. 60(7): p. 646-7.

2.Huisman, M. and M. Avendano, Income inequality and the prevalence of mental illness: the “outlier” US drives the association. J Epidemiol Community Health Online, 2006(26 September).

3.Demyttenaere, K., et al., Prevalence, severity, and unmet need for treatment of mental disorders in the World Health Organization World Mental Health Surveys. Jama, 2004. 291(21): p. 2581-90.

4.Wilkinson, R.G. and K.E. Pickett, Income inequality and population health: A review and explanation of the evidence. Soc Sci Med, 2006. 62(7): p. 1768-84.

Dear editor,

In their report on income inequality and the prevalence of mental illness based on data from several European countries and the US, Pickett, James and Wilkinson conclude that higher national levels of income inequality are linked to higher prevalence of mental illness [1]. They base their conclusion on an observed correlation of 0.73 between income inequality (the ratio of the top to the bottom 20% of the income distribution) and the national prevalence of mental illness of these eight countries.

We find it difficult to agree with this conclusion. The high correlation between income inequality and the prevalence of mental illness is clearly driven by the “outlier” US. Recalculations of the same data (See Table 1 in their paper) indicate that leaving the US out of the analysis results in a non-significant correlation of 0.24 of income inequality with prevalence of any mental illness. When considering only severe mental illness instead of any mental illness, removing the US gives a correlation of –0.03! This clearly suggests that at least within Western Europe, there is no association between income inequality and the prevalence of mental illness.

What should then be the conclusion from these results? This exploratory study does show that the US is an outlier, both in terms of income inequality and the prevalence of mental illness. Indeed, this seems to be in line with other findings, suggesting that the US is a ‘special case’ with respect to the correlation of income inequality with health [2][3]. These extreme values may point at the impact of broader healthcare and income distribution policies in the US, which clearly contrast with more egalitarian policies in Western European countries. Further research on the political and macro-economic determinants of population mental health is therefore warranted. But given that even a country with large income inequality such as Italy can have a low prevalence of mental illness [1], these findings suggest that the country-level determinants of mental illness do not lie primarily with income inequality, but elsewhere.

Martijn Huisman, Mauricio Avendano

Department of Public Health, Erasmus MC, University Medical Center Rotterdam, P.O. Box 2040, 3000 CA Rotterdam, The Netherlands

Conflict of interest: None

References

1) Pickett KE, James OW, Wilkinson RG. Income inequality and the prevalence of mental illness: a preliminary international analysis. J Epidemiol Community Health 2006;60:646-647.

2) Lynch J, Davey Smith G, Hillemeier M, Shaw M, Raghunathan T, Kaplan G. Income inequality, the psychosocial environment, and health: comparisons of wealthy nations. Lancet 2001;358:194-200.

3) Mackenbach JP. Income inequality and population health. Br Med J 2002;324:1-2.