What the researchers found was clear evidence of discrimination against the Muslim candidate among employers in more conservative states. The researchers also found no evidence of discrimination against the gay candidate relative to the straight candidate. Results from the online test were consistent with those from the field experiment.

"We find more extensive bias against the Muslim candidate than the gay candidate, and significantly more bias among employers in Republican states than employers in non-Republican states," said researchers Alessandro Acquisti, associate professor of information technology and public policy at CMU's H. John Heinz III College, and Christina Fong, senior research scientist at CMU's Dietrich College of Humanities and Social Sciences.

According to their findings, "In more Republican-leaning states, only 2 percent of applications by the Muslim candidate received interview invitations compared to 17 percent for the Christian candidate."

There were no differences in callbacks received by the Christian and Muslim candidates in the 10 states with the lowest proportion of Mitt Romney voters in the 2012 election.

As the researchers pointed out, job seekers nowadays typically reveal personal information online that is not easily detectable in job interviews, and which often is illegal for employers to request or use in the hiring process.

But employers can and do search for this information online, raising concerns about job discrimination.

The Equal Employment Opportunity Commission has issued warnings to employers against this but the trend isn't expected to slow.

Companies admit to searching blogs or online profiles for evidence of unprofessional traits and behaviors.

"(But) so much more can be gleaned about a prospective hire from her online presences. A tweet can reveal her place of worship. A blog post can imply her sexual orientation. A photo on LinkedIn can show her race. A comment on Facebook or an image on a social media profile can suggest her family status," the researchers said.

The researchers had no way of knowing how many employers actually did a search of social media before deciding whether to interview a candidate, but they estimated that between 10 percent and 33 percent of the employers did so.  

Although Muslims make up less than 2 percent of the U.S. population, they accounted for about one-quarter of the 3,386 religious discrimination claims filed with the EEOC in 2011. 

In 2009, the EEOC received 1,490 complaints from Muslims, the fifth consecutive year the number of complaints rose. In 2011, the number of cases had  reached 4,151.

Under federal law, employers and other covered entities are barred from discriminating against an employee or job applicant because of the person's race, color, religion, sex or national origin.

Nearly half of Muslim Americans pointed to either negative views about Muslims (29 percent) or discrimination and prejudice (20 percent) as the most pressing issues facing their community in a 2011 Pew Research Center survey.