Full Idea
At a very early stage loyalty to a group must have been reinforced by loyalty to a leader. In a large tribe the king or chief may be known the everybody even when individuals are strangers. This makes possible increase in the size of the group.
Gist of Idea
A monarch is known to everyone in the group, and can thus unite large groups
Source
Bertrand Russell (Authority and the Individual [1949], 1)
Book Reference
Russell,Bertrand: 'Authority and the Individual' [Unwin 1977], p.15
A Reaction
In this way humanity could move from hunter-gatherer groups to tribes or clans. In the UK even people who couldn't name the current Prime Minister are all fully aware of the monarch. In this way a merely constitutional monarch makes sense,