More than 3.2 million Americans contributed to federal candidates in the 2016 elections, but fewer than 16,000 of them provided half the donations— a sign of the increasing concentration of donor activity in the United States, according to a new report.

The analysis released Friday by the Bipartisan Policy Center mapped the growing influence of rich political contributors and independent political groups in the seven years since federal court decisions unleashed a new era of big-money spending.

Super PACs spent $1.1 billion in the 2016 elections, nearly 17 times more than such independent political committees put into federal races in 2010, the first year they came into existence, the report found.

"The system has completely transformed," said Robert Bauer, a Democratic election law attorney who authored the report with GOP campaign-finance lawyer Benjamin Ginsberg and Stanford Law School professor Nathaniel Persily.

Together, they outlined the findings of a panel of 16 political scientists on the legal, political and technological shifts in the role of money on federal campaigns in the past 15 years. The research was primarily funded by the William and Flora Hewlett Foundation and the Democracy Fund.

Although the Supreme Court's 2010 Citizens United v. Federal Election Commission decision was expected to unleash a wave of independent corporate political spending, companies have largely held back from such activity, they wrote.

"All the available evidence suggests that business corporations, especially publicly traded corporations, have not taken advantage of their newly established political spending rights post-Citizens United to the extent that many predicted," according to the report.

In 2012, corporations spent about $75 million from their treasuries on federal elections, roughly 1 percent of the overall spending that cycle, according to the report. In 2016, just 10 companies made independent expenditures, spending "relatively minuscule amounts," the report said.

The reason, researchers wrote, may be that corporations are finding that lobbying on public policy is a more efficient use of their money than taking public political stances that may anger shareholders or customers.

Unions have taken greater advantage of the new latitude they have under the Citizens United decision, giving more to super PACs in the past two election cycles than business corporations and trade groups.

But it has been wealthy individuals and corporate leaders who have driven the largest share of donations to super PACs, accounting for 64 percent of the contributions to such groups in recent elections.

As super PACs emerged as central players in the 2016 elections, the role played by nonprofit organizations set up under section 501(c)(4) of the tax code receded. Such groups are allowed to engage in limited political activity without disclosing their donors.

Spending by 501(c)(4) groups rose from $5 million in 2006 to $309 million in 2012, the report found. But in 2016, it dropped by more than half, to $147 million.

"It remains to be seen whether it is true, as conventional wisdom holds, that the 2016 election represents an aberration, due in large part to the unique features of that presidential contest and the fact that many prominent 501(c)(4) organizations decided to sit the election out," the researchers wrote.