Atheists are more intelligent than religious people according to dozens of studies.
Miron Zuckerman, Jordan Silberman and Judith A. Hall from the University of Rochester and the Northeastern University conducted a meta-analysis (that's a statistical analysis that combines the results of multiple scientific studies) of 63 studies that showed a significant negative association between intelligence and religiosity.
The association was strongest among university students and weakest in teenagers and children.
It was also stronger for religious beliefs than religious behaviour. In effect, people who believe religious teachings as opposed to those practising religions.
Religiosity was defined as "the degree of involvement in some or all facets of religion." This included beliefs in supernatural agents and "costly commitments to these agents" such as offering of property as a sacrifice. Another 'facet' is participating in communal rituals, such as going to church, and "lower existential anxieties such as death due to a belief in supernatural agents" (i.e. being less scared of death because you believe you're going to heaven).
It's not entirely clear why non-religious people are more intelligent - but the difference varies with age
At University the divide is the strongest.
It may be because more intelligent students are more likely to embrace atheism as a form of non-conformity. University tends to expose people to new ideas and influences, students tend to lose their beliefs or get more religious during this time, according to the study. These changes are often as a result of "the self-exploration that typifies emerging adulthood and that is often observed in students" as "the separation from home and the exposure to a context that encourages questioning may allow intelligence to impact religious beliefs". The study adds that
Using analytic (as opposed to intuitive) thinking, more intelligent college students may be more likely to eschew religion. If atheism is disapproved of at home, higher intelligence may facilitate resistance to conformity pressure.
Whereas later in life, more intelligent people are more likely to get and stay married which makes them less reliant on the attachment that the function of religion provides. More intelligent people are also more likely to have higher level jobs and spend more time in school, which leads to higher self-esteem and "encourages control of personal beliefs" according to the study.
Ageing, however, is more likely to increase awareness of mortality
The research has been going on for almost 80 years and has measured association with individuals of all ages.
Religious beliefs can help manage the terror of one’s impending death
According to the study, there is no evidence pertaining to the relation between intelligence and death anxiety. Although this logic suggests that "the negative relation between intelligence and religiosity might decline at the end of life, the relevant evidence we have indicates otherwise."
The highly intelligent members of the sample retained lower religiosity scores, relative to the general population, even in their golden years (age 75 to 91).