I just came across A User Study on Visualizing Directed Edges in Graphs by Danny Holten and Jarke J. van Wijk. The problem they look at in this paper is how to represented directed edges in visualization of graphs, e.g. to represent A is a part of B or A likes B, etc.
Interestingly, their results indicate that what I suspect might be the most commonly used approach (placing an arrow head at the end of the graph) is not the best. Rather, they suggest, using the tapered elements illustrated below is better.