World personal income distribution evolution measured by purchasing power parity exchange rates

TL;DR

This study analyzes global income distribution changes from 1988 to 2018 using purchasing power parity exchange rates, employing dual distributions to better capture data, revealing a transition from bimodal to unimodal patterns due to middle-class growth.

Contribution

It introduces a dual-distribution approach combining log-normal and gamma functions to more accurately model global income distribution evolution.

Findings

Global income distribution shifted from bimodal to unimodal.

Dual-distribution models fit data more accurately than single distributions.

Middle-class growth in populous countries influenced distribution shape.

Abstract

The evolution of global income distribution from 1988 to 2018 is analyzed using purchasing power parity exchange rates and well-established statistical distributions. This research proposes the use of two separate distributions to more accurately represent the overall data, rather than relying on a single distribution. The global income distribution was fitted to log-normal and gamma functions, which are standard tools in econophysics. Despite limitations in data completeness during the early years, the available information covered the vast majority of the world's population. Probability density function (PDF) curves enabled the identification of key peaks in the distribution, while complementary cumulative distribution function (CCDF) curves highlighted general trends in inequality. Initially, the global income distribution exhibited a bimodal pattern; however, the growth of middle classes in highly populated countries such as China and India has driven the transition to a unimodal distribution in recent years. While single-function fits with gamma or log-normal distributions provided reasonable accuracy, the bimodal approach constructed as a sum of log-normal distributions yielded near-perfect fits.