The number of Americans engaged in charitable giving has trended downward since the beginning of the 21st century, a recent study by The Chronicle of Philanthropy found.

From 2000 to 2014, the latest year for which data are available, the share of Americans donating to nonprofit organizations fell by some 11 percentage points, from 66.2% to 55.5%.

The trend showed up across several demographics. Among donors 51 to 60 years old — prime years for charitable giving — only 58% made a donation in 2014, down 20 points from 2000 and the biggest drop of any age group.

Giving decreased by 10% for those with less than $50,000 of annual income and by 5% for those with more than $150,000. The study found that some 87% of America’s biggest earners gave to charity in 2014, while fewer than four in 10 at the opposite end of the earnings spectrum made contributions.

Giving by donors with less than a high school education up to those with some college fell by 16% to 17%, with a smaller drop-off for those with college and graduate degrees.

Donations by single men and cohabiting couples fell by twice the rate for women and married couples.

Finally, donors who reported a religious affiliation gave 11% less in 2014 than in 2000, while unaffiliated donors gave 7% less.

The Chronicle said no definite explanation exists for the decline in giving. However, it suggested several possible contributing factors. For one, wealth is increasingly concentrated in fewer hands, while the middle class struggles after years of stagnant wages.

Generation X is smaller than both the baby boom and the millennial generations, which means that the number of people turning 50 and entering their prime giving years is down and will remain low for at least the next decade.

The Chronicle also noted that many nonprofits have a poor track record cultivating donors of color at a time when the country is growing more racially diverse.

According to the study, research has shown that religiously affiliated people are likelier than those who are unaffiliated to contribute to both religious and secular charities. However, the percentage of Americans who attend worship services or claim affiliation is in decline.

Pew Research has chronicled how the U.S. public is becoming less religious.

Another factor contributing to the decline in giving is the fact that Americans now have more ways to support a cause than earlier generations did. Susan Raymond, chief innovation officer at Edmundite Missions, a social-service charity, cited one example in an interview with The Chronicle.

Raymond said folks who care about the environment can do a variety of things today besides make donations, such as drive hybrid cars, install solar panels on their homes, buy eco-friendly products or invest in companies that produce clean energy.

“You are not less committed to the environment because of that,” she said. “You, in fact, may be more committed.”

In addition, the study noted that crowdfunding, whereby money is often earmarked for individuals rather than charities, is becoming increasingly popular, and competing with nonprofits for donations.

— Related on ThinkAdvisor: