WARNING: Major spoilers for Run ahead
In Run, Diane Sherman (Sarah Paulson) is shown to be an overbearing mother who has dedicated her entire life to caring for her ill teenage daughter, Chloe (Kiera Allen), however, her own secret illness isn't addressed by the movie at all. There are several reasons why Diane's illness was left out, and despite it being a very rare condition, it's no less relevant to the larger conversation prompted by the 2020 psychological thriller.
Director Aneesh Chaganty burst onto the scene with 2018's Joker or James McAvoy's character in M. Night Shyamalan's Split, have been especially harshly scrutinized. However, Run was ultimately made better by not focusing on Diane's illness, opting instead to explore it through the details of how she interacts with her daughter, and providing strong context clues that what she's suffering from.
Though never confirmed, Diane very likely would be formally diagnosed with Munchausen Syndrome by proxy (MSP), a relatively rare disorder. The Cleveland Clinic cites that only 1,000 cases of child abuse out of 2.5 million yearly occurrences are possibly related to MSP, and while there are several speculated causes—including major trauma or stress that leads to a psychological break—the exact cause is unknown. In Run, it can be presumed that Diane has developed the deep-seated need for attention and the feeling of being needed by someone she loves due to the loss of her own, newborn child after a complicated birth. This was shown in the very beginning of the movie, but later revealed that Chloe was not, in fact, Diane's sick child — that infant died soon after being born, and Diane stole a different baby (Chloe) from the hospital. However, more interesting than the actual diagnosis is why Chaganty chose not to explore it. Likely, it's to do with how it would have detracted from Diane's character by adding a sympathetic note where one wasn't needed.
Audiences who are well-versed with thrillers may recognize Diane's secret illness from The Sixth Sense, where it was also featured in the background of the movie's overall narrative. The ghost girl who was seen vomiting was later revealed to have been poisoned by her mother, who also had Munchausen by proxy — or likely did. It's a tragic outcome for any child, and showcases the severity of Chloe's situation, which is something that, unfortunately, real children have experienced at the hands of their parents. However, while this sort of diagnosis is often depicted as villainous—and rightfully so—it's important to that the person suffering from MSP is ill, and needs treatment. Though the end of Run does see Diane tucked away in a psychiatric hospital where she is now a victim of Chloe's, and still given the green and white pills (which were revealed to be dog medicine) to keep her from ever being able to be a danger to Chloe and her family again.
Given the way MSP is depicted in the instances where it is shown, it's very likely that Chaganty and Sev Ohanian, his co-writer, didn't want to give the audience any reason to empathize with Diane or her actions. While the abuse of Chloe is done by an obviously unwell and very mentally unstable person, Diane is meant to be the story's villain. If MSP became a focal point of Run's story, it would possibly showcase how the people who experience trauma, like Diane did when she lost her newborn baby, can go on to make harmful decisions as a twisted way of showing their love for their victims. Movies have been criticized on numerous occasions for glamorizing abuse, and Run ultimately works better with a solid villain who isn't tainted by her own mental health issues, even if they're part of her day to day life and better explain why she hurts Chloe the way she does.
