If friendship is a virtue, what are some of the less laudable cohesive bonds that link the Macphails to the Davidsons?

Both couples feel a sense of superiority to the other guests from the lower decks. Both couples, especially the missionaries, comment on the boorish behavior of lower class passengers. They judge the men who drink and gamble in the smoking rooms as well as the other parties that go on in the ship. There is a certain smugness that both couples consider part of their friendship.