Erin is a 5-year-old girl. Her parents brought her to see you because they have noticed that she never looks people in the eye when they are talking to her and she often seems not to notice that people are in her presence, even her playmates. She has no friends at school or church and her parents are worried. When someone is hurt, Erin doesn't express any empathy and she also doesn't seem to have any joy when others around her are excited about something. Her parents tell you she didn't use words until she was nearly 3-years-old, a significant delay in speech development. They have noticed that she often answers every question with "yes" regardless of the question or the appropriate response. It is as if she is simply responding rather than thinking about the question. Likewise, she seems to function best with pointless routines and she often engages in repetitive motor behaviors such as rubbing the back of her own head with her right hand. Her parents are most concerned that she doesn't play alone or with anyone else. How would you diagnose Erin? What differential diagnoses are possible