Complex sentence: Some studies have shown a correlation between national average IQ and atheism in society, although others have questioned whether any correlations are due to a complex range of social, economic and historical factors, which interact with religion and IQ in different ways.
Split into simpler sentences: Some studies have shown a correlation between national average IQ and atheism in society. However, others have questioned whether there is a causal link, or whether any correlations are due to a complex range of social, economic and historical factors, which interact with religion and IQ in different ways.