Research into behaviours associated with specific syndromes, such as Cornelia de Lange syndrome (CdLS), has neglected to explore the parents' perspective, particularly the potential impact of the notion of behavioural phenotypes on parents' causal explanations. Given the research focus on self-injurious behaviour (SIB) in CdLS, the present study examined parental concern across four topographies of challenging behaviour, causal explanations for these behaviours and optimism for change. As part of a larger study, a questionnaire survey of 86 parents of children and adults with CdLS was conducted. Quantitative data on parental concern and optimism with regard to behaviour problems were collected. Causal explanations for behaviour problems were examined by subjecting open-ended responses to a content analysis. Parents were as concerned about physical aggression and disruptive behaviours as they were about SIB. The majority of parents had deconstructed how CdLS might be associated with SIB in terms of other factors associated with CdLS, such as degree of intellectual disability. Parents did not believe that CdLS influenced SIB more than other challenging behaviours and their beliefs did not effect optimism regarding future change in the behaviour. Despite the focus of research on SIB in CdLS, parents of children and adults with CdLS are also concerned about other challenging behaviours. There was no evidence that a deterministic perspective had been adopted by parents and causal explanations were unrelated to optimism for future change.