Using a large sample of countries and 60 years of data, the authors found a strong and intuitive link between demographic transitions and both GDP growth and capital market returns. Unlike previous researchers, who used ad hoc and restrictive demographic variables, the authors imposed a smooth and parsimonious polynomial curve across all age groups. They also performed robustness checks and produced forecasts for the coming decade, with all the necessary caveats.Demography is destiny. -Auguste Comte (1798-1857 emography is one of the rare social sciences in which forecasts-at least for the short run-have startlingly little uncertainty. Today's 40-year-olds are next year's 41-year-olds. We can count them; we know the likely mortality for 40-year-olds and the likely rate of immigration and emigration for this age cadre. Looking 10 years into the future, we can see some uncertainty in the number of people under 10 and in the number of people over 70 (depending on the progress of medical science), but surprisingly little uncertainty in the number of people aged 10-70-barring war, pestilence, or other catastrophes. As the Baby Boomers have aged, many researchers have studied past demographic data in an effort to extract indications of the future influence of Boomers on various aspects of the economy, from housing prices to consumer preferences to retirement plans.Although the genesis of our study has the same roots-curiosity about the potential impact of aging Baby Boomers-we decided to pursue a broader course of study that spans decades of data and dozens of countries. We concentrated on three areas in which demographic shifts might influence the economy: real per capita GDP growth, stock market returns, and bond market returns.We strove not to extend the theoretical relationships between demographic changes and financial markets or the economy but, rather, to apply new empirical techniques. First, we sought to extract more statistical significance by looking at data from many countries over many years. Second, instead of fitting regressions against broad and ad hoc demographic cohorts, we fit polynomials to the regression coefficients between demographic age groups and both GDP growth and stock and bond returns. The use of polynomials is intuitive because it satisfies two important criteria: parsimony (only a small number of parameters are required) and continuity across age groups (behavior should change reasonably smoothly from one age cadre to the next).Two core principles influenced our research design. First, we deemed models for GDP growth less interesting than models for real per capita PPPadjusted GDP growth. 1 All three of these modifiers of GDP growth are important. After all, in a country with 3 percent population growth, 3 percent GDP growth means no growth at all for the average citizen-hence, our reliance on per capita data. The same logic holds for 10 percent per capita GDP growth in a country with 10 percent inflationthus, our focus on real per capita GDP growth. The PPP adjustment creates a fairer ...