From introduction:

The quality of life in a country can be conceptualized as a vector of several components, e.g., GDP per capita, equality of opportunity – as distinct from that of income, the potential of individuals’ self-realization, the extent of fear and intimidation faced by the ordinary citizens. I will focus here on the economic component, but I will also mention the others.

GDP per capita is an average and it may mask very different distributions of individual income. I will leave this issue aside, as I will focus on large differences in this indicator across countries or across time in different countries. We can then see that every group in a much richer country can enjoy a higher standard of living than a corresponding group in a poorer country. And the poorer groups in a richer country may have a higher income per capita than the richer groups in a poorer country.