Conditions of secular stagnation – low growth and low real interest rates – have counteracting effects on the cost of servicing the public debt. With sufficiently low interest rates relative to growth, governments can raise revenues by increasing the debt to GDP ratio. Mehrotra analyzes empirically and theoretically the tradeoffs involved with increased public debt. Using data from 1870 for advanced economies, interest rates on government debt are frequently less than GDP growth. However, despite current conditions of r < g, he finds a moderate probability of reversion to conditions with r > g over a 5 or 10 year horizon and substantial variability in r − g. Using a 56 period quantitative lifecycle model calibrated to the US, the author shows that slower population growth worsens the cost of servicing the debt, while slower productivity growth improves this cost. Despite r < g, the level of public debt that minimizes the cost of servicing the debt is lower than current levels.