The question of how cooperative groups can evolve and be maintained is fundamental to understanding the evolution of social behaviour in general, and the major transitions in particular. Here, we show how selection on an individual trait for group size preference can increase variance in fitness at the group-level, thereby leading to an increase in cooperation through stronger group selection. We are thus able to show conditions under which a population can evolve from an initial state with low cooperation and only weak group selection, to one where group selection is a highly effective force.