The [Efficient Plots Hypothesis], as I imagine it, says that the ideal reader can’t know if the mood of a book is about to get sunnier or darker at any given point in the plot. This … [is] because the purpose of a narrative is to engross the reader. Engrossment proceeds through uncertainty. If you knew what was about to happen, you’d skim ahead or stop reading.

That is: at any moment in a story, the emotional trajectory is a random walk for the reader because anything else would be boring. And stories aren’t boring.

This could be tested empirically by asking readers if a book will get more positive or more negative over the next five pages, and by how much. In a pure EPH world, they’ll only be right about half the time.

…

If the EPH holds, then, it doesn’t suggest that fiction is truly arbitrary; rather, that it’s an elaborately constructed game between reader and writer, socially conditioned and in no way permanent. It would suggest that there are enough fundamental plots that at any point in a book you are unsure what plot you are in; and that plots tend to wear themselves out over time.

Read about it here.