Because they're getting wealthier faster than the economy grows, they come to command ever-larger shares of the economy, so even when the pie gets bigger, their slices gets bigger still, and the remainder that we all share isn't just proportionally smaller - it's *actually* smaller. We don't just have less relative to the rich - we have less relative to our parents.
For Piketty, this is an iron law of markets, born out by analysis of hundreds of years' worth of capital flows.
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