Singing in synchrony, even if the song is an out-group anthem ("O Canada", when the subjects were USA residents), causes more trust and and greater feelings of being on the same team, as well as an increased willingness to cooperate in a public goods game.

Not that I disagree with the conclusion, but I would hazard that a lot of US residents consider Canada a part of 'us' - not the USA, but very friendly. Try substituting a national anthem for a country that typical US citizens aren't really aware of - Bhutan, say. For bonus points, use the anthem of the international communist party.