There have also been studies of pairings, both voluntary (married couples) and forced (student roommates), to see which remained together and which split up. Again, the evidence seems to favour similarity rather than complementarity as an omen of a successful relationship, though there is a complication: where marriage is concerned, once the field has been narrowed down to potential mates who come from similar backgrounds and share a broad range of attitudes and values, a degree of complementarity seems to become desirable. 

has been narrowed，这里为什么要用现在完成时态？