The association between income inequality and SWB is complex and highly dependent on methodological variations across studies. The findings of this review do not support a link between income inequality and SWB in general. Subgroup analyses revealed that the association between income inequality and SWB is significantly influenced by the country economic development. The association between income inequality and SWB is significantly negative in developed countries (pooled r = − 0.06, 95% CI − 0.10 to − 0.02) but significantly positive in developing countries (pooled r = 0.16, 95% CI 0.09–0.23).

Nevertheless, the association between income inequality and SWB was not influenced by (a) the measure used to assess SWB (i.e. happiness and life satisfaction), (b) geographic region (i.e. studies conducted in the USA versus studies conducted in the European countries) or (c) the way income inequality was operationalised (i.e. exogenous Gini vs. endogenous Gini).

How to interpret the exploratory findings?

Our findings suggest that the direction of the association between income inequality and SWB differs between developed and developing countries. Differences in different preferences for income inequality might explain this finding. For example, the evolutionary modernisation theory [11, 58] hypothesises differences in tolerance for income inequality as economies move from developing to developed countries. According to this theory [11, 58], people in developing countries might perceive income inequality as an economic opportunity or incentive to work, innovate and develop new technologies and therefore as a more core determinant of their well-being compared to developed countries. In contrast, technology, economic growth and innovation might be taken for granted in developed countries, meaning that income inequality may be perceived as a treat rather than a challenge [11, 59]. Moreover, our findings do support the “tunnel” effect theory suggesting that the rise of income inequality may signal future mobility and an increase of SWB [60]. The “tunnel” effect theory supports the idea that people in developing countries may tolerate income inequality by observing other people’s increasingly rapid progression and interpreting this evolution as a sign that their turn will come soon [60, 61]. A study conducted in Poland found that when an increase of income inequality was associated with growth and when it was perceived to change rapidly, people were more satisfied with their lives [39]. For example, Berg has suggested that “income inequality is not necessary harmful to well-being. Beja added that people may accept income inequality when they see the possibilities to rise above their current position” ([12], p. 153).

Research and social policy implications

The main contribution of this systematic review and meta-analysis is that the country level of development influences the link between income inequality and SWB: income inequality is more likely to be a contributor to SWB in citizens of developing countries than in developed countries. Reducing income inequality could be a potentially fruitful approach for governments and policy makers of developed countries as a means of improving the SWB of their citizens [11, 12]. The inverse association of SWB with income inequality in developing countries suggests that income inequality is more likely to be seen as job opportunities for innovation in these countries. However, this review was only based on cross-sectional studies and no causal inferences are allowed; longitudinal studies are needed prior to forming any causal links. The association between income inequality and SWB was not influenced by the measure used to assess SWB, geographic region or the way income inequality was operationalised. Our findings are in line with previous research conducted in OECD countries suggesting no association between income inequality and SWB [9] “the best evidence that we have to date is that redistribution beyond the minimum for advanced societies does not enhance subjective well-being/quality of life” ([9], p. 1107). Nevertheless, further studies are needed to understand the circumstances in which income inequality reduces SWB [3, 4, 62] versus the circumstances in which income inequality is not necessarily harmful to SWB [6, 12]. For example, extraordinary circumstances such as the great recession may affect how inequality is associated to subjective well-being. This gap in knowledge is critical because some government and policy makers still ask whether people care about income inequality and if income inequality affects SWB. At present, the evidence base is weak and cannot support strongly such decisions. Most importantly, the present systematic review highlights the need to produce a higher-quality evidence base to support social and political decisions relating to income inequality and SWB, both with respect to identifying (a) what are the consequences of income inequality and (b) what are the antecedents of SWB.

Strengths and limitations

This review has several strengths. First, the search was conducted according to PRISMA published guidance [27]. Consistent with the Cochrane guidance [16], the search strategy comprised a thorough literature review, screening of reference lists and contacting authors for additional information. Second, this is the first systematic review that investigated the association between income inequality and SWB, and therefore the findings of this review have the potential to inform the literature in this area.

Nevertheless, it is important to recognise few key limitations of this review. First, the preponderance of cross-sectional studies means that it was impossible to establish a temporal or causal relationship between income inequality and SWB. Second, the poor reporting of data in combination with the use of different analytic approaches precluded any firm conclusions about the direction and strength of the association between income inequality and SWB. Future studies are encouraged to concentrate on establishing an initial correlation between income inequality and SWB before embarking on multivariate analyses. Third, this study investigated the relationship between income inequality and SWB. Nevertheless, previous studies investigating people’s quality of life have reported a link between inequality, SWB and health status [5, 9]. Further studies are needed to systematically investigate the association between income inequality, SWB and health status [1, 2]. Finally, the majority of studies included in this review were conducted in developed countries (N = 14) and only five studies were conducted in developing countries. This is problematic in terms of the representativeness for the purpose of global decision-making. More studies are needed to be performed in developing countries. Due to limitations in the available data, we were unable to compare Latin America to Europe or the USA because only one Latin America country had data amenable to meta-analysis. Social and political history may affect the association between income inequality and SWB because Inglehart et al. report that, with the same level of wealth, Latin America is happier than their counterparts in Ex-Communist nations [59]. We strongly encourage more methodologically sound investigations to examine the association between income inequality and SWB and to elucidate current gaps and inconsistencies.

Conclusion

In conclusion, this is the first systematic synthesis of the literature regarding the link between income inequality and SWB. The main finding of this review is that the association between income inequality and SWB is complex. More rigorous investigations are needed to elucidate the link between income inequality and SWB, and to identify what are the antecedents and consequences of income inequality and SWB taking into account the country development level.