More of the nation’s largest companies are reining in political spending, avoiding making direct contributions to candidates, banning independent expenditures and putting oversight mechanisms in place, according to a study of corporate behavior released this week by a nonprofit group that studies corporate governance.

The study, commissioned by the Investor Responsibility Research Center Institute and conducted by the Sustainable Investments Institute, tracked changes in the political spending practices and policies during the past year of Standard & Poor’s 500 companies.

Among the study’s key findings: Large companies are twice as likely to report that they will not write checks to candidates and political parties from the corporate treasury, with 64 saying their policies expressly prohibit direct contributions; the number of companies that say they don’t spend money on influencing the government and elections has grown to 57 from 40 the year before; and 77 companies report that they will not use independent expenditures going forward.

The companies have also focused more on the oversight and disclosure of the money that is spent. Oversight is now an explicit responsibility of 31 percent of the companies’ boards, up from 23 percent in 2010, and nearly two-thirds of those surveyed now identify the officers who make political spending decisions.

“While many people assume that strong disclosure and governance practices will reduce corporate political spending, the data show that’s far from a foregone conclusion. Indeed, on a revenue-adjusted basis, companies with greater board involvement actually spend more,” said IRRCI Executive Director Jon Lukomnik in a release.

Of the $1.1 billion that the companies collectively spent on political activity, for example, $915 million came from the top 40 percent when measured by revenue. Of the total figure, $979 million went to lobbying the federal government, $112 million went to state-level candidates, parties and ballot initiatives and $31 million went to federal political committees, the researchers concluded.

Lukomnik said that the correlation between company size and spending could be due in part to heavily regulated companies spending disproportionately compared with those in industries that are not.

The report authors cautioned that the study provided an incomplete look at corporate political spending, given that two-thirds of companies in the S&P 500 appear to spend money from their treasuries on political activity without disclosing it to investors. Only 26 companies acknowledge working with the type of nonprofit group that has become a key player in elections by paying for issue ads.

“There’s a small but growing number of firms shying away from exercising their new right to fund ads that support or attack candidates, but that leaves hanging a question mark for nearly four-fifths of S&P 500 companies. Also, we found only 26 companies in the whole index mention anything about the 501(c)(4) ‘social welfare’ groups that are playing a key role in funding issue ads,” said Heidi Welsh, SI2’s executive director and the report’s main author.