These statistics are included in ‘Glass Half Full: The Decline and Rebirth of the Legal Profession’ by Tennessee University law professor Benjamin Barton. Professor Barton is concerned at the trend, citing the fall of Dewey & LeBoeuf as a practice which was culled partly because of its practice of making lateral hires. 

Sharing Dewey's fate?

Overpaying lateral hires is one danger that the author cites. Another is that the recruitment of lawyers at partner level dilutes the culture of the firms. Professor Barton warns that other firms could go the way of Dewey. Source: Bench & Bar of Minnesota