BackgroundFor more than a century, the idea of particular personality/behavioural characteristics being associated with people with Down's syndrome has been explored, but with inconclusive results.MethodThe Disability Assessment Schedule was used to ascertain the behavioural profiles of 360 adults with Down's syndrome and 1829 adults with learning disabilities of other aetiologies, who were the whole identified population within a defined geographical area. Comparison was made between the two total groups and additionally for the subgroups aged < 35 years and aged ⩾35 years. Comparison was also made with regards to cluster analysis findings.ResultsDespite an equal age and developmental quotient, the Down's syndrome group were less likely to demonstrate maladaptive behaviours. The behaviour characteristics of the adults with Down's syndrome remained constant in the younger and older age groups. Cluster analysis demonstrated adults with Down's syndrome to have an increased prevalence in cluster groupings with lower rates of maladaptive behaviours.ConclusionsThis study confirms there to be a behaviour phenotype among adults with Down's syndrome. The reasons for this (e.g. genetic/psychological/social) require further research. Such research may establish a better understanding of the aetiologies of maladaptive behaviours among people with learning disabilities in general.