For some behavioral symptoms, the problem is more what behavior is absent than what is present. For example, one common sign of autism in young children is a lack of joint attention. Joint attention is the ability to share a common focus with another person, for example, playing with the same object together, or pointing to an object that both people are looking at.

One of the best ways to illustrate a lack of joint attention is by comparing two children in the same situation: one who is typically developing, and one who has been diagnosed with autism spectrum disorder. Video can be a powerful aide in demonstrating this.

In this video, Dr. Christopher Smith, of Mount Sinai School of Medicine, narrates a video comparison of twins during a routine play session with mom. One twin, Nathan, is on the autism spectrum, while the other, Benjamin, is typically developing. Compared to Benjamin's enjoyment and easy engagement in the session, Nathan's enjoyment in the interaction is dubious, characterized by minimal sustained eye contact, smiling or social reciprocity.