Though the number of people who identify as atheists has risen in recent years, atheists still are viewed as untrustworthy or frightening by many Americans. Now, new research might offer an explanation, writes LiveScience’s Stephanie Pappas — atheists can trigger death-related thoughts, which can cause people to cling even more tightly to their religious values.

Pappas reports on a new study that shows atheists are seen as an “existential threat,” a threat that fuels anti-atheist sentiment. Researchers interviewed a group of 202 students from diverse religious backgrounds. One group was given questions about death like “Please describe the emotions that the thought of your own death arouses in you” and “Write down, as specifically as you can, what you think will happen physically when you die,” while the other was asked about extreme pain.

Then, the researchers asked all participants about their attitudes towards both Quakers and atheists. While people seemed to distrust atheists across the board, the group who had been reminded of their own mortality was much more negative.

In another experiment, they asked participants to think about atheism, extreme pain, or death first, then fill in word fragments that could be construed as either neutral or death-related words (for example, D_ _D, which could be interpreted as words like “deed” or “dead”). Participants asked to think about atheists first were more likely to choose death-related words than people prompted to think about pain — and on par with people asked to think about death first.

The study’s authors, who named their study “What if They’re Right?”, suggest that the “mere existence” of people who go against mainstream cultural values is “fundamentally threatening.” Though they acknowledge that results could differ if the experiment were tried in a less secular society, they view their findings as evidence that “the mere contemplation of atheism can arouse intimations of morality.”

Corey Cook, a social psychologist who lead the study, tells Pappas that he was surprised by the results. He suggests that his research can be used to help figure out how to change public perceptions of atheists and reduce prejudice.