The authors seem quick to attribute this to gestational factors. But there are two other more plausible explanations I can think of first. One is that women who are born in the spring face more looks-based pressure in school and are more likely to develop the disorder. Perhaps being the smallest/largest/medium kid in your class has something to do with it. Second, I would be surprised if the season of birth is completely random. Some parents undoubtedly plan when they want their kid to be born. How do we know that it isn’t parental characteristics that contribute to this? It’s plausible that gestational factors affect the development of this complex disorder, but the environmental/selection factors above are way more plausible.