If you cannot work out by yourself why an academic article would include that the historical range of the subject under discussion is all over the place, then I am not sure that any attempt of leading you to it is going to help. Perhaps just have a bit more of a think about it. Have you read any of the referenced papers?

If the range is that large either some or all of the studies were poorly done or too different to compare.

It seems that much of what passes for social sciences isn't really science.

So, you do understand why they included the wider historical review then. At least you have got exactly the point they were trying to make by including it in the first place. The literature here historically has been shit and all over the place. Which is the kind of point you might want to make if you are putting forward some other papers based on more recent studies that both opens out the field and narrows the error bars somewhat.