Using Fuzzy Logic to Model Character Affinities for Story Plot Generation

In the book The Thirty-Six Dramatic Situations [4] Polti explores the assertion made by Gozzi (author of Turandot) saying that there can only be thirty-six tragic situations. At the end of the book, he begins his conclusions by saying that, to obtain the nuances of the situations, the first thing he did was to “enumerate the ties of friendship or kinship between the characters”. A century before that, Goethe had already proposed his theory of elective affinities to depict human relations, specially marriages, and he showed how affinities between characters can be represented by a topological chart [6]. This evidences that the affinity between characters is an important factor to take into account when generating stories, and one that can help us to maintain the necessary narrative tension to keep the reader interested in the story. One of the most relevant research works on the subject is Thespian [5]. The authors describe the use of affinity to model social interaction which affects how characters can behave towards each other. Affinity is affected by other factors, such as social obligations and characters goals. We have developed a storytelling system where we use a numeric representation that allows us to use common arithmetic operators to modify the degree of affinity between characters [3]. The main limitation of this approach is that it is difficult to calibrate the model and interpret what is happening in the simulation. To overcome the limit, we have opted for a representation similar to the fuzzy concepts proposed in [7], an approach that has already been used by other authors to model cognitive architectures [1, 2]. We have modeled four levels of affinity according to four different affinity kinds: foe (no affinity), indifferent (slight affinity), friend (medium affinity) and mate (high affinity). These four levels of affinity overlap on their limits, which allows for relationships not to change constantly when moving around the limits of two different levels. An additional aspect of affinity is the lack of symmetry. Given two characters, their mutual affinity is likely to have different values and it may even be situated in different levels, with the exception of mates: character A considers character B as its mate only if character B considers character A as its mate, too. However, if they are not mates, character A may think character B is a friend, whilecharacter B may think character A is a foe. There are two ways in which the affinity value can change. The first one is by lack of interaction, in which case the affinity value moves towards the indifferent level. The second