Americans are the most generous people in the world: Charitable giving in the United States ran to about 2.1% of GDP in 2015, and giving by Americans has held steadily at roughly 2% of national income for many decades.

But will this always be so?

A new report published by the Indiana University–Purdue University Indianapolis’s Women’s Philanthropy Institute suggests that total giving by Americans may not be so robust in the future.

The report looks at philanthropic giving by young adults—those between the ages of 25 and 47—and it compares the giving of the so-called “Silent Generation” (born during the Depression and World War II years of 1928–1945) to the giving of the Gen X and Millennial generations (born after 1964).

The report found the average amount given annually by members of the Silent Generation as young adults was $624 in today’s dollars; the average amount given annually by those in the Gen X and Millennial generations as young adults has been dramatically lower, $430.

Moreover, the report found a marked drop in the percent of Gen Xers and Millennials who gave “large amounts” in their young adult years as compared to those in the Silent Generation. (The report’s author’s set $600 annually in today’s dollars as giving a “large amount.”)

Of course, giving varies over the course of a lifetime, with the largest gifts coming late in life after the accumulation of a lifetime of earnings and wealth. Gen Xers and Millennials will likely give more and more as they age.

However, there are also important generational effects, as different generations of citizens are shaped by different experiences. Robert Putnam, Charles Murray, and other social scientists and commentators have noted that—on many measures of civic engagement from willingness to volunteer for military service to membership in clubs and civic organizations—Gen Xers and Millennials are less engaged than members of the Greatest Generation (born before 1928) and Silent Generation. The new Women’s Philanthropy Institute report adds to the evidence of declining civic engagement by finding that these younger generations give less generously to charitable causes.

Of course, the decline in charitable giving among more recent generations is not necessarily evidence that these younger Americans are inherently more stingy. Gen Xers have been burdened by the collapse in housing prices and year-after-year declines in real median household income in the wake of the Great Recession (even with the bump in real median household income reported in 2016, median household income remains below what it was in 2007, and well below its 1999 peak). Millennials began their careers in the toughest job market in decades, and many are weighed down by student debt. In contrast, the Greatest Generation and Silent Generation prospered during decades of nearly uninterrupted economic growth.

One of the most interesting findings in the new report is, while giving by young adults has declined on average, giving by unmarried young women has ticked slightly upward: Gen X and Millennial single women gave an average of $244 annually, modestly more than their Silent Generation counterparts, who gave only $216 annually. But while Gen X and Millennial unmarried young women gave more than unmarried young women of an earlier generation, women gave less than men in every generation included in the report.

The overall picture of declining giving during the young adult years is a worrying trend, because if lower levels of giving as young adults translates into lower levels of giving as older adults as Gen Xers and Millennials age, it will surely be harder to maintain the robust tradition of American philanthropy in the decades ahead.