As U.S. regulators and lawmakers push to see more privacy policies take hold in the mobile apps space, it appears app developers (and their lawyers) are taking note. Between September 2011 and June 2012 the Future of Privacy Forum think tank found a big spike in the percentage of top-selling apps sold for Apple and Google devices that incorporate privacy policies for user data, according to a new study.

Between Apple Inc. and Google Inc.’s respective platforms for apps, customers download more than 1 billion apps each month.

On Apple’s iOS App Store platform—which sells apps exclusively for the iPhone, iPad, and other Apple devices—the percentage of free apps that offer users some kind of privacy policy doubled, from 40 percent to 80 percent, compared to an FPF study conducted last year. The percentage of paid apps with privacy policies increased from 60 percent to 64 percent.

On the Google Play app platform—which distributes apps for a wide range of Android-based devices—the percentage of free apps with a privacy policy increased from 70 percent to 76 percent. The percentage of paid apps with privacy policies increased from 30 percent to 48 percent.

Amazon’s Kindle Fire Appstore is the new kind on the block, having launched in March 2011 to sell Android apps for Amazon’s proprietary devices. The study found that 48 percent of their free apps and paid apps alike come with a privacy policy.

Several app-focused regulatory developments occurred this year. In February, the Federal Trade Commission released a report that showed many apps aimed at children still lack privacy policies. That same month, the California Attorney General Kamala Harris reached an agreement with developers and platforms concerning compliance with the state’s Online Privacy Protection Act, which mandates that web companies inform California residents about how their personal identifying information is collected.

Facebook recently signed onto that agreement, as well, joining Apple, Google, Amazon, Microsoft, Research in Motion, and Hewlett-Packard.

“This study concludes that app developers have begun to heed the call for privacy policies,” the report states.

The report examined the 150 most popular apps across the Apple, Google, and Amazon platforms. Half of the apps were free and half were paid.

The researchers looked in a few different places to see if the app had a privacy policy, including on the listing page of the particular app store and within the app itself. If the researchers couldn’t identify a privacy policy in either of those locations, they searched the developer’s website for a privacy policy that covered mobile apps.

All in all, the survey found that 61.3 percent of the top apps have a privacy policy. Free apps were more likely to have one than paid apps: 69.3 percent of free apps have a privacy policy vs. 53.3 percent of paid apps.

What accounts for that difference? “Free apps are more likely to have been required by an ad network to disclose tracking practices,” the authors say.

In particular, the study highlights the progress made toward compliance with the California law. As part of the California AG’s agreement with the major app sellers, those providers agreed to “make it easier for consumers to find and review an app’s privacy policy before downloading it,” according to the report. The companies can do that by creating:

Either (a) an optional data field for a hyperlink to the app’s privacy policy or a statement describing the app’s privacy practices or (b) an optional data field for the text of the app’s privacy policy or a statement describing the app’s privacy practices . . .