Think you're gathering precious data by forcing customers to complete a sign-in form before buying? You may be losing something more valuable: the buyers themselves.

A new report from Portland-based software startup Janrain and Blue Research says 75 percent of consumers who are asked to register with a site are bothered by it and change their behavior as a result.

The report—called "Consumer Perceptions of Online Registration and Social Sign-In"—surveyed 657 online shoppers during the 2010 holiday season. It's being billed as the first of its kind to study consumer attitudes about using social identities across the Web and the impact this has on purchase behavior and brand loyalty. (Click here to download the report—ironically, you'll have to sign in.)

What does "change their behavior" mean, exactly? Over half (54 percent) may leave the site or not return. Nearly 20 percent go to a different site, if possible. More than 3 in 4 say they give incorrect information or leave forms incomplete. And 45 percent say they ditch the website if they forget password or logon information, instead of answering security questions or re-setting their password.

What's the solution? The study reveals 66 percent of consumers favor social sign-in, a.k.a. the ability to sign in to a website using Facebook, Google or Twitter. (Janrain's business includes a software plugin that makes it easy for websites to offer multiple login methods.) What's more, these customers are more likely to return to sites, spend larger dollar amounts on the site, and have more favorable views about the brand.

Nearly half of those surveyed say companies that offer social sign-in are more up-to-date and innovative than those that don't. Over half (55 percent) say they're more likely to return to a site that automatically recognizes them, and just about half (48 percent) are more likely to buy something on the site. Two in five (41 percent) of those surveyed preferred a social sign-in to using a guest account (35 percent) or creating a new account (24 percent).

Other findings: A third (35 percent) say social network posts encouraged them to buy a product.

"The findings of the survey clearly show that consumers are frustrated with the traditional online registration process and will favor brands that make it easy for them to be recognized," says Paul Abel, Blue Research's managing partner. "The rapid growth of social media has dramatically impacted consumers' expectations of websites and this shift in behavior can mean measurable benefits for businesses prepared to capitalize on the trend."