In 1991, as the United States was emerging from a recession, Edward C. Johnson III, the chairman of Fidelity Investments, introduced what at the time was an unorthodox possibility: What if his company could facilitate charitable donations for its clients? The firm could help people get a tax benefit while making it easier for them to give to charities. By the end of the year, the company had obtained public-charity status for an organization called the Fidelity Charitable Gift Fund. Clients could create an account that would hold their donations to the fund and write off the contribution on their tax returns that year. Eventually, they would have to select one or more charities and direct Fidelity to funnel the money there, but they could do that at their leisure. Soon, Charles Schwab and the Vanguard Group had introduced similar services, which are known as donor-advised funds.

These funds aren’t what people usually think of when they think about charities—mentioning on Facebook that you donated to Vanguard Charitable won’t draw quite the same level of attention as a photo of you wearing pink in support of breast-cancer research—but they’re registered with the Internal Revenue Service as 501(c)(3) charitable organizations, just like Susan G. Komen. Recently, donor-advised funds have been collecting more money than many better-known organizations. On Sunday, the Chronicle of Philanthropy published its annual list of U.S. charities, ranked by the amount of private donations they received the previous year. Fidelity Charitable placed second, after United Way Worldwide and was, according to the Chronicle, on a path to displacing United Way in 2014. Schwab Charitable and Vanguard Charitable also made the top ten for 2013, along with the Salvation Army, Feeding America, Task Force for Global Health, Catholic Charities USA, the Silicon Valley Community Foundation, and American Red Cross.

The Chronicle report came out two days after Janet Yellen, the chairwoman of the Federal Reserve, gave a speech, in Boston, in which she decried the expanding gulf in wealth between the rich and the poor. (Last year, the poorest half of people in the U.S. owned only one per cent of the nation’s wealth.) “The extent of, and continuing increase in, inequality in the United States greatly concerns me,” Yellen said. Rob Reich, a political-science professor at Stanford University, told me that the widening wealth gap could be influencing where donors give. In 2007, the Center on Philanthropy at Indiana University partnered with Google to research giving patterns. Their findings showed that in 2005 people with an annual household income of less than a hundred thousand dollars tended to donate mostly to religious organizations and to groups, such as food banks, that help people meet their basic needs. By contrast, those whose household income was a million dollars or more gave disproportionately to health and education organizations, while those dedicated to basic needs received the smallest share. As more income gets concentrated among the rich, Reich said, it stands to reason that their chosen charities will benefit disproportionately. “If you’re a garden-variety social-service provider in a metropolitan area, you’re having a harder time raising money, because middle-class people haven’t seen their incomes grow,” he said.

Donor-advised funds tend to cater to affluent people; the National Philanthropic Trust found that in 2012 the average size of donors’ accounts was nearly two hundred and twenty-five thousand dollars. Given that people in this group have, as Yellen pointed out, seen their wealth expand at disproportionate rates in the past several years, it makes sense that donor-advised funds would also have become more prominent during that period. In an article accompanying its report, the Chronicle noted that last year the percentage gains in contributions to donor-advised funds “outstripped the percentage gains of nonprofits like United Way and the Salvation Army that depend largely on middle-class donors.”

While affluent people will often publicize their big contributions or permit the recipients to do so—which in turn allows research organizations and publications like the Chronicle to compile their figures—donor-advised funds don’t disclose many details about clients’ accounts. Fidelity Charitable says ninety-one per cent of its grants “include names and addresses” of the original donors “so the recipient organization can acknowledge the gift,” but donor-advised funds, including Fidelity Charitable, generally don’t publish overall data on where they funnel contributions or the amount given to each recipient charity.

The company has made some general information available, however. A Fidelity Charitable report published this year noted that thirty-four per cent of grant dollars—the funds it gives out to other nonprofits on behalf of clients—goes toward education, followed by sixteen per cent for religion and fourteen per cent for “society benefit.” Vanguard has said that from 2004 to 2013 thirty-five per cent of funds went to “human services,” while twenty-five per cent went to education and twelve per cent to religion. And Schwab Charitable has said that last year its biggest categories of funding were “education, health and human services, social benefit and religion.”

Stacy Palmer, the editor of the Chronicle, told me she would expect contributions through donor-advised funds to skew toward organizations that affluent people typically support, such as high-profile hospitals and universities, rather than, for instance, the local food bank—but, she noted, there’s no way to know for sure. Palmer said donor-advised funds could be inspiring people to donate who otherwise might not have had the time or wherewithal. Still, she said, the opaqueness of these kinds of contributions can be vexing, because it makes it more difficult for people like her to research philanthropic needs—which in turn can influence policy. During recessions, for instance, people invariably start to ask whether charities could handle an increased burden if public spending on social services were to decrease.

Some critics, including Ray Madoff, a professor at Boston College Law School, see another problem with donor-advised funds. While foundations are required each year to pay out at least five per cent of the assets they manage, no such rule exists for donor-advised funds. So people can contribute as much as they want—and see an immediate tax break—even if the money simply sits in their account, gathering interest, rather than being quickly granted to another nonprofit. The funds will have to be put to some philanthropic use in the long run, but Madoff and others argue that it’s unfair, and counter to the goal of letting people write off charitable contributions, for people to reap the tax benefits without offering any immediate benefit to the public.

Advocates tend to point out that donor-advised funds often give to charities far more than the five per cent that foundations are required to disburse—at Fidelity, Vanguard, and Schwab, the over-all figures have recently surpassed twenty per cent—so there’s no need for a law requiring them to give more. Critics note, in response, that, while this may be true in the aggregate, there may still may be individual donors who give little or none of what they have placed in their accounts, avoiding taxes while not doing any real good in the short term. “By granting the full deduction for money simply being set aside, the government is telling donors that they need not do anything else,” Madoff says. “This is a dangerous message. Charitable giving is difficult, and the path of least resistance is often just keeping the money in the account.”