Hawkins has claimed that a substantial fraction of the long-term optical variability of many high-redshift QSOs is caused by microlensing. This is an interesting hypothesis and, as his papers show, there are many reasons to believe it. However, it doesn't hold up to a quantitative analysis.
The links below point to one file containing the complete text, all figures and tables etc.
As are most of my publications this is available in two forms: