The neural pathways of smell are the shortest pathways of all the senses into the brain. They also terminate in the most primitive part of the brain, the amygdala.
Many individuals with autism have sensory problems involving smell. They are often either hypersensitive to smells or have very low registration with smell.
There has been a study involving aromatherapy that tested the use of the following scents: neroli, eucalyptics, chamolile, sandalwood, and peppermint
The amygdala is implicated in autistic people who were known to have social deficits. The study was a functional magnetic resonance imaging (fMRI) study involving judging from the expressions of another person's eyes what that other person might be thinking or feeling. In this study, patients with autism or Aspergers did not activate the amygdala when making mentalistic inferences from the eyes, whilst people without autism did show amygdala activity. The amygdala is therefore proposed to be one of several neural regions that are abnormal in autism.
Additionally, the amygdala is implicated in anxiety and depression. There is increased blood flow to the
amygdala in social phobics. Children with autism have been found to have an increased chance at suffering from anxiety especially in situations involving obsessive compulsive disorder and separation anxiety. However, this is not true across the board. There is a lot of differences across the autistic spectrum. There are active but odd subtype that have extreme reactions to social situations and there are the aloof subtypes who have no reaction.
The amygdala is known to have a more selective role in detecting threats in the environment. However, the amygdala is not the sole cause of problems of social interaction in autism. Its dysfunction may play a role in the fear and anxiety which is often part of autism.
In the conference, Autism+, Dr. Elkhonon Goldberg has proposed that one of the key issues in autism is the integration between the amygdala and the frontal lobes.
I haven't found a connection yet between smell, social problems, and autism...but there are a lot of dots out there that look like they should be connected.