While the study does not examine why firstborns and only children are more likely to receive these diagnoses, it does identify birth order as a potential marker of risk (among many) for anxiety and depression. The finding "adds to our understanding of kids' risk factors," Caleb Cox, head of research and data science at Epic Research, told HuffPost.

"Anxiety and depression don't have a single cause, so understanding what factors are likely to impact a child can help both parents and clinicians to make sure that they are helping the child with their specific needs," Cox continued. He noted that there are plenty of second- or later-born children who struggle with anxiety and depression as well, "so it's important for parents to make sure their kids are getting the help that they need, regardless of the child's birth order."