Pretty much since 1976 (the passage of Hart-Scott-Rodino Act) US antitrust authorities have been reviewing mergers, challenging only 1.8 per cent of the total number of above-threshold notifications (IIRC I read it in a book by Bob Pitofsky).
What are the studies that control for changes in technology, capital availability and perhaps other factors and quantify the impact of merger control on economy-wide market structure/concentration measures?