In some neuro-developmental disorders, the combined effect of intellectual disability and atypicalities of social cognition may put individuals at increased vulnerability in their social environment. The neuro-developmental disorders Williams syndrome, characterised by "hypersociability", and autism spectrum disorders, characterised by "social withdrawal", are at two extremes of atypical social functioning in humans. In this article, we use Williams syndrome and autism spectrum disorders as exemplars to demonstrate how atypicalities of social cognition may contribute to social vulnerability in these populations. The lives of individuals with both these disorders are marred by an increased risk of social isolation, bullying, unsteady relationships, employment difficulties and abuse. While different behavioural interventions have been tried to improve social functioning in these populations, there has been great variability in their success. Finally, we discuss different issues regarding social independence of these individuals; including employment, safety and decision making. (Contains 3 tables, 2 figures and 1 footnote.)