Thomas Piketty's book, "Capital in the Twenty-First Century", has a bit to say on this. It argues that historically, and increasingly so today, the return on c

Thomas Piketty's book, "Capital in the Twenty-First Century", has a bit to say on this. It argues that historically, and increasingly so today, the return on capital exceeds the return on labour. This means wealth is a positive feedback cycle: wealth earns more wealth, at the expense of those who only have their labour. Once someone hits a net worth threshold it snowballs from there. Agree with it or not, the book is quite readable.