It has long been speculated that religious belief and intelligence are negatively correlated. In a review of 43 studies conducted by Bell (2002), it was found that all but four showed a negative correlation.
This particular study chose to evaluate IQ and religious belief internationally using four sources of evidence: (1) negative correlations between religious belief and intelligence; (2) lower rates of religiosity among the top intelligence tiers as compared to the general population; (3) a decline of religious belief as cognitive functions mature with age; (4) a decline of religiosity as general intelligence is increased among populations in the twentieth century.
This data is explained in a variety of ways, although most speculate that people of higher intelligence are more skeptical and prone to ask questions rather than accept faith blindly. The scientists conducting this experiment believe that religion is a social construct designed to explain the unexplainable and to serve as a justice system where previously there was none. But in the modern era, people are less reliant on religion and becoming more reliant on science.
There are few exceptions to the linear relationship between IQ and religious belief although a few should be noted. The two most outlying countries are Cuba and Vietnam which both have higher percentages of general disbelief (40 percent and 81 percent respectively) which would not be expected by analysis of their national IQs of 85 and 94 respectively. This anomaly was attributed to the fact that former or current communist countries are known to utilize strong atheist propaganda. Also, the United States is noted as being highly religious (only 10.5 percent are recorded as not believing in God), when compared to the national IQ of 98. This reflects a general trend among North West and Central European countries. One possible explanation is that Catholicism is highly prevalent among these countries, and predominately Catholic countries tend to show lower levels of disbelief than predominately Protestant countries. Another possible explanation could be the high levels of immigration to these areas from countries with high levels of religious belief. In fact, many of the immigrants from Europe to the United States immigrated in search of religious freedom. There have been a multitude of studies relating religiosity to a genetic factor or relating the transmittance of religious belief from parent to child, so it can be assumed that deeply religious immigrants passed on their religious traditions through generations.