The study, a Carnegie Mellon University experiment involving dummy résumés and social-media profiles, found that between 10% and a third of U.S. firms searched social networks for job applicants' information early in the hiring process. In those cases, candidates whose public Facebook  profiles indicated they were Muslim were less likely to be called for interviews than Christian applicants. The difference was particularly pronounced in parts of the country where more people identify themselves as conservative. In those places, Christian applicants got callbacks 17% of the time, compared with about 2% for Muslims.
The same experiment, conducted from February to July, found that online disclosures about job candidates' sexuality had no detectable impact on employers' early interest.
The research is the latest example of how people's digital trails can have far-reaching and unintended effects, particularly in the job market.
"There is so much information we reveal about ourselves online, sometimes in ways we do not even realize," said Alessandro Acquisti, an information-technology and public-policy professor at Carnegie Mellon and one of the study's authors. Even if people don't explicitly discuss sensitive information online or post embarrassing photos, employers can be influenced by other clues, the researchers said.
Quotes from a religious text could indicate a person's beliefs, while mentions of a baby registry might suggest a woman is pregnant or has children.
Discrimination in this part of the hiring process could be conscious or unconscious, meaning the employer could be influenced without realizing it, said Christina Fong, a senior research scientist at Carnegie Mellon who also worked on the experiment. Even before the social-networking era, studies showed that employers discriminate based on subtle cues, such as whether a name on a résumé was likely to be for an African-American.
More than a third of U.S. employers say they consult social-networking sites during hiring at least some of the time, according to a survey of nearly 1,000 human-resources workers released this year by EmployeeScreenIQ, a background-check firm. Only 7% said they always look at those sites.
"It's human nature to search. We want to fill in the blanks," said Rusty Rueff, a longtime human-resources executive now on the board of careers company Glassdoor Inc.
Most employers say they use social networks to find evidence of unprofessional behavior, such as complaints about previous employers or discussion of drug use. Many employment consultants advise job hunters not to share such obviously problematic details on social media.
But the new research suggests social-media profiles can contribute to more fundamental discrimination. The researchers focused their experiment on categories like religion and sexuality, which some federal and local laws prohibit companies from using in hiring decisions. "By and large, employers avoid asking questions about these traits in interviews. But now technology makes it easier to find that information," Mr. Acquisti said.
The Carnegie Mellon researchers sent out more than 4,000 fabricated résumés to private firms across the country that had more than 15 employees and were posting job openings online. The jobs included technical, managerial and analyst positions that required either several years of experience or a graduate degree.
Each résumé used one of four male names chosen for their uniqueness, meaning Web searches were almost guaranteed to lead viewers to carefully calibrated Facebook profiles linked to the names. One profile suggested the person was Christian, and another suggested he was Muslim. Two others indicated the person was either gay or straight.
Facebook declined to comment on the research. On its help site, the company tells people who want to limit what potential employers can find about them that they should protect the privacy of their photos, videos and status updates by choosing to share information with friends only.
Mr. Acquisti said the study isn't an indictment of Facebook but rather an example of the unintended consequences of information sharing online.
The Carnegie Mellon study involved only information that employers could access publicly. Researchers said they made sure the privacy levels of the profiles were normal for Facebook users so that the applicants didn't seem to be disclosing an unusually large amount of data about themselves.
The researchers also made professional profiles for the people on LinkedIn, as well as profiles for fake friends and colleagues, to create a more realistic online presence for the applicants. Those profiles didn't reveal such sensitive information. The names themselves were tested to make sure people didn't associate them with any particular race, religion or ethnicity.
Employment experts said the results show that businesses should be more careful about allowing people who make hiring decisions to look up candidates online.
"I advise employers that it's not a good idea to use social media as a screening tool," said James McDonald, a partner at Fisher & Phillips LLP who specializes in employment law. "You need to control the information you receive so you're only getting information that is legal for you to take into account."