So far as I can remember, there is not one word in the Gospels in praise of intelligence.
—Bertrand Russell (1872 - 1970)
Over 70 years of research on IQ and religiosity has shown a consistent pattern. IQ and religiosity are negatively correlated. No study to date has shown that religion enhances or improves intelligence, but almost all studies show that, on average, the higher the level of religiousity, the lower the IQ. This is not to say that all people who are religious are unintelligent, but that religion seems to suppress intelligence. IQ differences can be as much as 5 points or 1/2 of a standard deviation lower among highly religious groups. My own guess is that this difference is due to the inhibitions and suppression of curious and questioning behavior in religious followers. If questioning is prohibited from early childhood, it probably has a decided effect on development or critical thinking skills which are associated with higher intelligence.
With respect to mental health, the picture is even more interesting. Nationally, the more religious a region is the higher the incidence of sexual abuse and child abuse. The best predictor of child abuse in a home is alcohol use BUT the second best predictor is religiosity of the parents. Finally, the best predictor of divorce in the US is religiosity. The highest level of divorce is among Baptists and similar groups. George Barna, a admittedly evangelical researcher, has reported consistently that divorce is highest among Baptists and fundamentalist evangelical groups and that divorce occurs most often after one or both spouses are “saved.” He also found that agnostics and atheists have among the lowest divorce rates.
Yet another indicator might be the incidence of teen pregnancy which is positively related to religiosity. The least religious teens do not get pregnant as often and they also have a lower incidence of STD’s. Finally, all religious groups are represented in prison population about equal to their frequency in the general population with the big exception of atheists and the non-religious. This group is significantly under represented in the prison population.
If we agree that divorce, criminal behavior ,teen pregnancy and child abuse are proxy indicators of mental health, then “Houston we have a problem.” Religion is not associated with high levels of mental health.
See links below for various reports
A few references on IQ and religion:
Nyborg, Helmuth (2008-03). "The intelligence–religiosity nexus: A representative study of white adolescent Americans". doi:doi:10.1016/j.intell.2008.08.003. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W4M-4TFV93D-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=db2ee09bae0195cc1ecbd026da77245c. Retrieved on 2008-10-17.
Lynn, Richard; John Harvey and Helmuth Nyborg. "Average intelligence predicts atheism rates across 137 nations". Elsevier Inc. doi:10.1016/j.intell.2008.03.004. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W4M-4SD1KNR-1&_user=10&_coverDate=04%2F29%2F2008&_alid=759868596&_rdoc=1&_fmt=high&_orig=search&_cdi=6546&_sort=d&_docanchor=&view=c&_ct=1&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=bdb3ca48b21fdb2959f6f8ce4b6001de. Retrieved on 2008-06-27.
Divorce and child abuse: