Not one region of the world has seen its economic growth rate return to levels seen before the Great Recession almost 10 years ago. While there are a multitude of economic theories why the current recovery has been the weakest one in postwar history, it has been characterized by high indebtedness (but not at the consumer level), growing income inequality around the world, and an excess of caution stemming from the financial crisis. One factor that has been consistently overlooked is the dramatic slowdown in the growth of the global labor force.

Traditionally, a country’s (or a region’s or the world’s) potential growth rate is calculated by adding the rate at which the labor force is expanding to the rate of productivity increases. We’ll put aside measurements of productivity for the moment, given widespread disagreement on how best to measure it, and focus on the labor force, defined as those between the ages of 15 and 64.

Between 1960 and 2005, the global labor force grew at an average rate of 1.8% per year. However, since 2005, the rate has declined to 1.1% per year, and is headed downward as fertility rates decline worldwide. For 30 years, the working-age population in the U.S. grew much faster than its major industrial rivals—twice as fast as France and the U.K., hence the outperformance of the U.S. economy in decades past. Today, the U.S. labor force is now growing at 0.5% per year, down from 1.7% (1960-2005). For a developed economy, the U.S. labor force growth rate looks excellent. Countries such as Germany, China, Italy and Japan all face rapidly shrinking labor forces.

Why does it matter? For much of the postwar era, global population grew at two percent per year. Add productivity gains and you got healthy global economic growth rates. Around 1990, population growth started dropping and now hovers near one percent. While that one percent difference may seem small, it equates to the difference of 1.4 million workers. Global fertility rates have been falling at the same time that medical advances have increased life expectancies from 50 years in 1969 to 70 years today. Because it takes 15-25 years for babies to mature into working-age adults, the economic impact of falling fertility rates is only starting to become visible.

History tells us that when a country’s working age population grows, so does its economy. A recent study found 56 instances where a country was able to sustain economic growth of at least 6% for a decade. The average working-age population growth rate for those countries was 2.7%, and in no cases was it negative. As recently as the 1980s, 17 of the 20 largest emerging economies had working-age growth rates above two percent. Today, only two of them do: Nigeria and Saudi Arabia.

In a world with fewer younger people, economic growth will be harder to come by. However, other trends such as automation may help ameliorate the threats to economic growth caused by depopulation. As the ranks of working-age humans thin, smart machines may be the answer for a diminishing labor force.

These demographic trends are also a factor in our national conversation around immigration. Among immigrants to the U.S. are 30% of all American Nobel Prize winners and the founders of 90 of our Fortune 500 companies. Adding companies founded by the children of immigrants brings the number closer to 200. As such, from our view in the Mile High City, we track this conversation closely given the influence it may have on our economy, the markets, and client portfolios over the long term.