Demographic trends pose a challenge to the U.S. asset management industry — and to adapt, firms will have to shift their focus from wealthy investors to the mass market, says Moody’s Investors Service in a new report.

Historically, industry assets have enjoyed organic growth amid rising populations and increasing incomes — but those trends are shifting, the rating agency said.

“The organic AUM growth outlook for the industry over the next 40 years will not match the experience of the past 40 years,” said Dean Ungar, vice president with Moody’s, in a release.

“Robust population growth was a driving factor of AUM growth in the 1980s and 1990s. The U.S. population is projected to grow over the next 40 years, but at significantly declining growth rates, especially among working age people, 25-64, which will be a formidable headwind on organic industry AUM growth,” Ungar added.

Not only are people are expected to live longer, consuming their accumulated retirement savings, but younger people in their peak earning years are expected to have less wealth per capita than in past generations, Moody’s said.

The rating agency noted that research by the U.S. Federal Reserve has found that millennials have “lower earnings, fewer assets and less wealth, but with the same or higher debt levels” than previous generations did at the same stage of their lives.

Taken together, the aging of the population, the stagnation in working-age people and lower wealth for future generations “will constrain AUM growth as it becomes more difficult to replenish aggregate AUM,” Moody’s said.

These trends in wealth accumulation “will pose a yet another challenge to asset managers,” the rating agency suggested.

In this environment, the key to growth for the industry will increasingly be to find ways to serve middle-class investors, Moody’s said.

To address the long-term prospect of weak organic AUM growth, “asset managers will need to develop products and distribution to profitably penetrate the lower wealth segments of the population,” Moody’s said.

“New business models designed to serve tech-savvy customers with lower levels of investable assets have already developed and are showing signs of market success.”