From Wikipedia:

There has been much confusion and debate over a lyric in the chorus of the song. This is regarding whether or not Brandon Flowers is saying “denser” or “dancer”, when posing the question “Are we human or are we denser/dancer?” Though “denser” seemingly makes more sense in the context of the lyrics, Flowers’ pronunciation of the word clearly sounds like “dancer” (IPA: [dænsəɹ]) over “denser” ([dɛnsəɹ]). “Denser” is only an oblique Rhyme with the preceding lyrics, “…looking for the answer.” This has lead several critics to believe that Flowers is in fact saying dancer, and therefore questioning what the meaning of the lyric is. Entertainment Weekly‘s Pop Watch section, called the lyrics the “silliest lyrics of the week” under the assumption that Flowers was saying “dancer” in the chorus. They were puzzled by this, stating “most dancers are generally human.”^[3] On the October 4, 2008 episode of Saturday Night Live the Killers performed “Human” and Flowers appeared to sing both “dancer” and “denser.” Some have said that the lyrics do make sense because “dancer” might be appealing to the “marionette” sense of the word, in which case the song is asking whether we are human and make our own independent decisions or whether we just dance in a show made by, and perhaps even made for, someone else. The Killers have yet to make any official statement regarding what the correct lyric is.