Autism spectrum disorders (ASD) are currently diagnosed in the presence of 
impairments in social interaction and communication, and a restricted range of 
activities and interests. However, there is considerable variability in the 
behaviors of different individuals with an ASD diagnosis. The heterogeneity 
spans the entire range of IQ and language abilities, as well as other 
behavioral, communicative, and social functions. While any psychiatric condition 
is likely to incorporate a degree of heterogeneity, the variability in the 
nature and severity of behaviors observed in ASD is thought to exceed that of 
other disorders. The current paper aims to provide a model for future research 
into ASD subgroups. In doing so, we examined whether two proposed risk factors - 
low birth weight (LBW), and in utero exposure to selective serotonin reuptake 
inhibitors (SSRIs) - are associated with greater behavioral homogeneity. Using 
data from the Western Australian Autism Biological Registry, this study found 
that LBW and maternal SSRI use during pregnancy were associated with greater 
sleep disturbances and a greater number of gastrointestinal complaints in 
children with ASD, respectively. The findings from this "proof of principle" 
paper provide support for this "bottom-up" approach as a feasible method for 
creating homogenous groups.