First we looked at games in which each person had an either-or choice.
One problem was the familiar multi-person prisoners' dilemma: everyone made the same choice, and it was the wrong one.
Next we saw examples in which some people made one choice while their colleagues made another, but the proportions were not optimal from the standpoint of the group as a whole.
This happened because one of the choices involved greater spillovers, i.e., effects on others, that the choosers failed to take into account.
Then we had situations in which either extream-everyone choosing one thing or everyone choosing the other-was an equilibrium.