General descriptions of the relevant literature:A considerable amount of literature has been published on X
These studies
There is a large volume of published studies describing the role of
The first serious discussions and analyses of X emerged during the 1970s with
The generalisability of much published research on this issue is problematic
What we know about X is largely based upon empirical studies that investigate how
During the past 30 years much more information has become available on
In recent years, there has been an increasing amount of literature on
A large and growing body of literature has investigated
General reference to previous research/scholarly activity (usually more than one author)Many historians have argued that
Jones, 1987; Joh