Religion boosts health in the US. Yet, the most religious countries are the sickest. This paradox can be explained in terms of stress management.
Evidently, the health benefits of religion emerge here because religious people are better at managing stress. Why do residents of the most religious countries -- that are also the poorest -- not enjoy a health boost? It seems that any benefit of religion in a poor country is drowned out by a heavy disease load.
The Puzzle
Why do comparisons within a developed country yield such wildly different conclusions from comparisons between countries?
Religion evolved as an emotional coping mechanism as I argue in my book Why Atheism will Replace Religion and a widely-read blog post of the same title.
In poor countries that have major health problems, sickness motivates people to go to church as do the many other disadvantages of living in a less developed country, including random acts of violence and perpetual political instability. Residents go to church to deal with the stress of such existential insecurity.
Although, religion may be effective as a form of emotional coping or security blanket, it cannot relieve the practical problems of raging epidemics, inadequate healthcare, dirty drinking water, and failure to inoculate for tuberculosis, measles, polio and other epidemic diseases. Most people experience chronic health problems whether they are religious or not because the diseases are endemic.
The health drawbacks of under development swamp the impact of religious beliefs and practices. If a person suffers from malaria, or contracts HIV/AIDS, for example, the mere fact that religion relieves stress is unlikely to prolong their lives substantially.
In a developed country, where health and life expectancy are much better, the health benefits of religion become more visible because more illnesses are stress-related. A person living in a developed country escapes many of the health scourges of less developed places, including infectious and parasitic diseases such as water-borne diarrhea, cholera, malaria, liver flukes, and sleeping sickness.
Basic improvements, such as clean drinking water, and improved sanitation reduce the impact of many of these plagues. Inoculation programs also take a huge bite out of diseases with some, such as smallpox and tuberculosis, brought to the verge of extinction in developed countries. Even those infectious diseases that stay around are treatable thanks to antibiotics and other potent medicines that are routine in developed countries but hard to get in poor countries.
These improvements in medicine and public health brought dramatic increases in life expectancy in developed countries. Now, health outcomes are much more determined by the person himself, or herself, rather than by the environment.
We Make Ourselves Sick
The major causes of illness in the developed world, are behavioral, many stress-related.
The biggest single cause of chronic health problems is obesity that causes secondary diabetes and is a major risk factor for heart disease, liver disease, and kidney failure.
Experts in health behavior have begun to appreciate that stress plays a major role in obesity and its associated diseases ("the metabolic syndrome"). So overeating is partly a response to stress. Stress-induced snacking often involves high-energy foods, such as potato chips, cheese, and chocolates that cause obesity according to animal experiments.
Stress plays a big role in other major health problems also. Clinical depression may be caused by severe and prolonged psychological stress and depression is a risk factor for heart disease.
Indeed, if the problem of anxiety could by solved by putting something analogous to fluoride in the water supply, most psychologists and psychiatrists would be out of work and we would lead longer, healthier lives.
The Puzzle Solved
Stress plays a key role in most of the leading causes of sickness, and early mortality in developed countries. Stress-related illnesses may have similar consequences in less developed countries but these are less noticeable because other causes of disease - particularly epidemics and parasites - are so much more important.
These ideas can help explain why religion boosts health in developed countries but is associated with illness in developing countries.
Religiously active people in developed countries probably enjoy at least a modest health advantage over those in their community who stay away from church. This is mostly irrelevant in very unhealthy nations where most people suffer from chronic illnesses for much of their brief lives - and also resort to religious beliefs and practices to look for peace of mind. Health is so much worse in developing countries that religion cannot move the needle in country comparisons.
Conversely, in wealthier countries where scientific medicine controls somatic diseases, the level of psychological stress has a bigger impact on variation in health outcomes so that better stress management improves health to a measurable extent.
This means that religious people stand to gain if religiosity helps them to control stress. Research finds that religious rituals do indeed have this effect as do secular equivalents like meditation and Western yoga.