Recently, someone analyzed [computer science professors at top universities][1] and found that over half of the professors at the top 51 universities graduated from a top 10 university. [Others][2] have also brought this up. From my personal observations, most schools do hire graduates from better ranked schools. 


 1. Is this because of the competitive job market? *We have so many good applicants, we have to narrow it down some how!*
 2. Or is it simply that these schools produce the most PhDs? 
 3. Has this always been the case?
 4. How rare are exceptions to this? I know of a few people who graduated from a top 75 school and got hired at a top 50 school. But what about bigger gaps? The top 10 schools seem to just swap graduates, do they ever hire from a 50+ ranked school?


Although I am only familiar with the US and CS departments, I think this question can generalize to any field and country. 
  [1]: http://jeffhuang.com/computer_science_professors.html
  [2]: http://chronicle.com/article/What-Are-Low-Ranked-Graduate/136823/