Such studies merely measure the degree of modernization and not intelligence.
These types of studies don’t square with other studies that show religious people are also more educated.
If I came up with a study that said people with pink or chocolate skin color were less intelligent I would be considered a racist.
The sociologist C. Wright Mills once wrote that groups exercise social control by linguistic devices, imputing good or bad motives or lack of intelligence to those groups they wish to cognitively dominate. If my group of atheists are deemed more intelligent then we can dominate the university and can banish those who are religious. If my in-group of atheists are more intelligent then we should formulate all social policies to control the masses of backward people. Atheists are more intelligent all right: they want everyone to believe they are more intelligent so they can be their masters. I’m sorry to say many libertarians fall into the same sort of thinking in my experience.
Google:”Over-education Linked to Poor Mental Health”