Oakley reports on various studies done with his collaborators. From the article:
We found that fiction readers had substantially greater empathy as measured by the mind-in-the-eyes test, and also performed somewhat better on the interpersonal perception test than people who read predominantly non-fiction.
It's not just reading preferences of different personality types; they controlled for this. The effect is pretty immediate:
We found that people who read the Chekhov story underwent larger changes in personality than those who read the control text - although the types of changes varied from person to person. Results from the emotions questionnaire indicated that the personality changes were mediated by the emotions experienced while reading: a person's emotional state is known to influence their scores on personality tests.
This is why I liken fiction to a simulation that runs on the software of our minds. And it is a particularly useful simulation because negotiating the social world effectively is extremely tricky, requiring us to weigh up myriad interacting instances of cause and effect. Just as computer simulations can help us get to grips with complex problems such as flying a plane or forecasting the weather, so novels, stories and dramas can help us understand the complexities of social life.