For more than a millennium, scholars have noticed a curious correlation: Atheists tend to be more intelligent than religious people.

It’s unclear why this trend persists, but researchers of a new study have an idea: Religion is an instinct, they say, and people who can rise above instincts are more intelligent than those who rely on them.

“Intelligence — in rationally solving problems — can be understood as involving overcoming instinct and being intellectually curious and thus open to non-instinctive possibilities,” study lead author Edward Dutton, a research fellow at the Ulster Institute for Social Research in the United Kingdom, said in a statement.