  We study a class of games in which a finite number of agents each controls a
quantity of flow to be routed through a network, and are able to split their
own flow between multiple paths through the network. Recent work on this model
has contrasted the social cost of Nash equilibria with the best possible social
cost. Here we show that additional costs are incurred in situations where a
selfish ``leader'' agent allocates his flow, and then commits to that choice so
that other agents are compelled to minimise their own cost based on the first
agent's choice. We find that even in simple networks, the leader can often
improve his own cost at the expense of increased social cost. Focusing on the
2-player case, we give upper and lower bounds on the worst-case additional cost
incurred.
