From the 2005/2006 Distinguished Lecturer Series.  Abstract: This presentation focuses on studies nested in our population-based screening for Autism Spectrum Disorders (ASD) - Aspects of psychiatric diagnoses. These studies evaluated the reliability and stability of psychiatric diagnoses, head size, attachment, and joint-attention. The associations between the current DSM-IV algorithm for autism and clinical diagnoses in very young children with and without ASD, and between the algorithm proposed for PDD-NOS and clinical diagnoses, were analyzed. Stability of diagnoses was evaluated through follow-up assessments, two years after the first assessment. Head size data was also collected on our sample of very young children with ASD, children with mental handicap without ASD and control children, and multilevel modelling employed to disentangle the effect of ASD and of mental handicap on head size. Attachment was investigated by the Strange Situation Procedure in very young children with autism, with broad ASD, non-ASD disorders, and normal children. Cortisol responses and heart rate variability during the Strange Situation Procedure were also examined.