Causality is a different question, but I would argue that, for many reasons, countries benefit from global competition and trade differently, leading to increases in interstate variation. Within some countries, economic development lowers inequality, while in others it sharply increases it.
|Measures of variation of GDP at PPP|
|Measures of variation of per capita GDP at PPP|
Next are Gini coefficients for various groups. The data here comes from the UNI-WIDER Income Inequality Database. The Gini coefficient is a measure of income inequality where 100 is perfectly unequal (one person gets all the income) and 1 is perfectly equal. The fidelity of this data is much more suspect and far more difficult for researchers to gather, so it is to be taken with a grain of salt. The groupings I used were key European countries and the U.S. in the first figure; Japan, Malaysia, and the Asian Tigers in the second; key countries in the Americas in the third; and late developers in Asia in the fourth. These charts are busier, but a few takeaways are immediately apparent. The U.S. is diverging from Europe, with a much higher level if inequality that approaches Latin American levels. Japan, South Korea, and Taiwan have converged toward European levels of inequality, while Singapore and Hong Kong are diverging toward higher inequality than their Asian cohort. The Latin American countries are converging in the 50-60 range, which is the highest of the groups I have plotted, and the U.S. is rapidly approaching this range. The developing Asian states have lower inequality than Latin America, while China has the most rapidly growing inequality in this group. India's inequality is relatively low among this cohort.