How do I make this SQL more efficient?

I am an Oracle neophyte and I need some advice on how to improve some sql that I have written to do some data clean up. The problem is that one of our primary tables (1M+ rows) has multiple rows with the same key and different genders indicated. I want to derive the 'majority' gender occurrences and then update the rows with the derived gender. The following sql works but runs for 4 hours. I have included interim result sets for one sample key as comments. Can anyone suggest alternatives that would be more efficient?

TIA update table1 z

   set gender =

( select max ( a.gender )  as  majority_gender  -- MAJORITY_GENDER
                                                -- -
                                                -- F

--
--
    from ( select keycolumn_1                   --                       G COUNT


                , keycolumn_2                   -- ------- ---------- -- - -----

                , keycolumn_3                   -- 8016209 05-02-1993 02 F     6

                , gender                        -- 8016209 05-02-1993 02 M     2

                , count ( gender ) as count

             from table1
            where keycolumn_1 is not null
              and keycolumn_2 is not null
            group
               by keycolumn_1


                , keycolumn_2

                , keycolumn_3

                , gender

         ) a
       , ( select max ( gender_count ) as count --      COUNT
             from ( select keycolumn_1          -- ----------
                         , keycolumn_2          --          6
                         , keycolumn_3
                         , gender
                         , count ( gender )  gender_count
                      from table1
                     where keycolumn_1 is not null
                       and keycolumn_2 is not null
                     group
                        by keycolumn_1
                         , keycolumn_2
                         , keycolumn_3
                         , gender
                   )
            group
               by keycolumn_1


                , keycolumn_2

                , keycolumn_3

         ) b
   where a.count = b.count
     and z.keycolumn_1 = a.keycolumn_1
     and z.keycolumn_2 = a.keycolumn_2
     and z.keycolumn_3 = a.keycolumn_3
   group
      by a.keycolumn_1
       , a.keycolumn_2
       , a.keycolumn_3
       , a.count
)
;