In the theory of distribution in economics, equality assessments have generally been formulated on the basis of the distribution of some individual achievement – health, income, literacy, opportunity, and so on – across the population. Thus, equality is a multifaceted phenomenon. Equality becomes an issue of scrutiny in multiple dimensions, and it is not reducible to equality in one sphere only (Sen, 2009). Although a great many problems have been addressed and resolved, new problems are being posed and analyzed. (For a recent discussion on equality from different perspectives, see Piketty, 2022.)

The choice of a specific dimension of wellbeing is certainly a matter of value judgment. In this book we will consider income as the only dimension of human wellbeing because the main body of the book, Chapters 4–8, will study income taxation and subsidy allocation from different standpoints, covering a wide range of topics. Since people also care about other dimensions, our focused analysis does not express the view that they are less important for human welfare.

Social policy makers often become concerned with low equality because low equality means high income gaps between the rich and the poor. This may give rise to social disorder and lead to civil wars. Following the publication of Russett's (1964) seminal article, many researchers have attempted to determine the relationship between inequality and conflict. Nonetheless, it has sometimes been argued in the literature that the relationship between inequality and conflict is a debatable issue.