We argue that the economy of the industrialized world, taken as a whole, is currently—and for the foreseeable future will remain—highly prone to secular stagnation. But for extraordinary fiscal policies, real interest rates would have fallen much more and be far below their current slightly negative level, current and prospective inflation would be further short of the 2 percent target levels, and past and future economic recoveries would be even more sluggish. We start by arguing that, contrary to current practice, neutral real interest rates are best estimated for the bloc of all industrial economies, given capital mobility between them and the relatively limited fluctuations in their aggregated current account. We show, using standard econometric procedures and looking at direct market indicators of prospective real rates, that neutral real interest rates have declined by at least 300 basis points over the last generation. We argue that these secular movements are in larger part a reflection of changes in saving and investment propensities rather than the safety and liquidity properties of Treasury instruments. We highlight the observation that, ceteris paribus, levels of government debt, the extent of pay-as-you-go old-age pensions, and the insurance value of government health care programs have all operated to raise neutral real rates. Using estimates drawn from the literature—as well as two general equilibrium models emphasizing, respectively, life-cycle heterogeneity and individual uncertainty—we suggest that the "private sector neutral real rate" may have declined by as much as 700 basis points since the 1970s. The extent of the substantial shifts in private saving and investment propensities over time has been obscured by the impact of this decline in real rates. Our diagnosis necessitates radical revisions in the conventional wisdom about monetary policy frameworks, the role of fiscal policy in macroeconomic stabilization, and the appropriate level of budget deficits, as well as social insurance and regulatory policies. To that end, much more of creative economic research is required on the causes, consequences, and policy implications of the pervasive private sector excess saving problem.