Not an argument, just answering the question. Also, > If it was so difficult to demonstrate a correlation then so many studies wouldn't have been able to do j

Not an argument, just answering the question. Also, > If it was so difficult to demonstrate a correlation then so many studies wouldn't have been able to do just that. employs a fallacy, assuming the conclusion aka begging the question. But that's ok, because I'm really not arguing any position.