i'm in social sciences. i believe there are more single-author papers around than multiple-authored ones, and single-authoring is the standard (it is different in disciplines such as psychology and economics which sometimes are counted as social sciences, too).
yes, my relationship to my supervisor is definitely good enough for discussing such issues - if they'd ever come up. i think i would never downright refuse her, but mainly because she would never insist if i had good reasons against it.