My position would be that Atheists would likely have the best mental health of all, explanation at the bottom of the page.
Courtesy of Psychology Today:
Much research indicates that religious people as a group tend to have better mental health than the ‘nones’ as a group. This is manifest in various indicators, including lower rates of depression, anxiety, suicide, self-harm, and substance use among the religious.
The protective mental health effects of religiosity have been attributed to various factors. These include social support in religious congregations, a sense of purpose and meaning offered by religions, and moral codes commanding certain behaviors (e.g. abstinence) within religions. These are discussed in the short video below with Dr. Eric Jarvis, a leading authority on religion, atheism and mental health.
However, the studies leading to these conclusions often collapse a variety of different groups (e.g., agnostics, lapsed, unaffiliated, weak atheists, strong atheists) into a single category of ‘nones,’ comparing these to a single category of ‘religious.’ This binary ‘lumping’ approach loses granular-level information about the many specific sub-groups within the ‘nones.’
New research has set out to examine the broad mental health differences in the sub-categories constituting the ‘nones.’ Interestingly, a growing number of studies suggest that people possessing strong religious beliefs and convinced atheists tend to share similarly positive mental health. The worst mental health is observed in those with more ambiguous, confused and weaker religious or spiritual beliefs.
For example, a just-published study by Dr. Joseph Baker at East Tennessee State University indicates that atheists have the best mental health among the ‘nones,’ similar to that of the highly-religious. In contrast, ‘non-affiliated theists’ had the poorest mental health.
The conclusion by the author of this paper is that the “certainty of belief” rather than the belief itself is what assures positive mental health, however, I would push back on that assertion.
First off Atheism is the lack of faith, not just another faith.
And second, the suggestion that people of great faith have better mental health has irritated me for decades.
The simple reason is that though they may be less likely to become depressed or to end their lives prematurely, there is nothing mentally healthy about believing that invisible gods, angels, or saints are watching over you, or even communicating with you.
For instance, if somebody told you that there was a voice telling them to move to France, even if you agreed that it was a good idea, you should worry about what else a voice inside this person’s head might suggest next.
For that reason, I have always believed that a person with a healthy amount of skepticism about magical creatures and invisible deities is likely to be in a far healthier mental state than a person who believes evil is the result of demonic influence and that all good comes from unquestioning faith.
As for happiness being used as a measure of mental health, I have certainly seen my share of gleeful people in a manic state of mind who seem to exude great positivity but they are by no means mentally healthy, and in fact, should be monitored closely.
Remember, just because millions of people think a thing exists without the benefit of evidence is no proof that thing actually exists, nor is it proof of their mental stability.