Not all that many kids start reading anything outside of assigned schoolwork before 13 anyways.
I wonder if not reading fiction CAUSES lowered social understanding, rather than that being a bit socially blind leads to less fiction reading. That is, I suspect that most people read fiction and watch plays and so on to help them have insight into others' and their own emotions and motivations.
In the past 10 years I've made a concerted effort to read fiction, especially the classics. Its helped me a great deal understand the motivations and inner-lives of human beings -- something I wasn't great at, and still don't have a natural aptitude for.Fiction: It's an end unto itself AND helpful social conditioning! How any intellectually curious person could avoid 1) reading for pleasure in childhood and 2) the draw of narrative is utterly baffling. Is the idea that it didn't really happen, so it's not worth knowing? Fiction is just hypothesizing and testing with human personas as lab materials. You know if you're doing it right if it rings true with experience of actual humans.
As I see it, there are only 3 ways to understand humans (in descending order)
2) Statistically-valid surveys