Automatic Duplicate Management determines the master record by the following criteria.  It goes down the list for importance and once one is true it will set the Master record:
  1. Member ID -is not blank.
  2. Login Date- most recent.
  3. Last modified date- most recent.
  4. Cons ID - is a higher number.
Ex: One record has a member id, but the last login date was 1/1/15 and the other is 8/13/14 (with member id).  The master record in this scenario would be the record with the member id even though the last login date is older.