Background. Based on prevalence studies and the few incidence studies of pervasive developmental disorders (PDDs) the prevalence and incidence of these disorders have been claimed to be increasing.

Method. The annual and age-specific prevalence and incidence rates of childhood autism, atypical autism, Asperger's disorder, and pervasive developmental disorder not otherwise specified (PDD-NOS) in Denmark during the period 1971–2000 in children younger than 10 years were estimated using data from the Danish Psychiatric Central Register.

Results. A total of 2·4 million children younger than 10 years were followed and 2061 cases with the PDDs studied were identified. Generally, the prevalence and incidence rates of the PDDs studied were stable until the early 1990s after which an increase in the occurrence of all disorders was seen, until 2000. The annual incidence rate per 10000 children younger than 10 years was 2·0 for childhood autism, 0·7 for atypical autism, 1·4 for Asperger's disorder, and 3·0 for PDD-NOS in 2000. We calculated a ‘corrected’ prevalence of childhood autism at 11·8, atypical autism at 3·3, Asperger's disorder at 4·7, and PDD-NOS at 14·6 per 10000 children younger than 10 years on 1 January 1 2001.

Conclusions. We found that the estimated prevalences of the PDDs studied were probably underestimated. Furthermore, the increasing prevalence and incidence rates during the 1990s may well be explained by changes in the registration procedures and more awareness of the disorders, although a true increase in the incidence cannot be ruled out.

Background. For people with intellectual disabilities there is a difficult balance to be struck between empowering people to claim their sexual rights and protecting them from abuse. Hypothetically, services should be guided by whether a particular person with intellectual disabilities has the capacity to consent to sexual relationships. However, there has been little agreement on how to define such capacity. This study examines the issue of capacity to consent to sexual relationships using a functional approach.

Method. Adults with intellectual disabilities (n=60) and young people presumed in law able to consent (n=60) were assessed for their sexual knowledge and vulnerability to abuse.

Results. Adults with intellectual disabilities were significantly less knowledgeable about almost all aspects of sex and appeared significantly more vulnerable to abuse, having difficulty at times distinguishing abusive from consenting relationships. Nevertheless, some adults with intellectual disabilities scored highly on all measures, especially if they had relatively high IQs and had had sex education.

Conclusions. The reasons for the poorer knowledge and increased vulnerability of people with intellectual disabilities are discussed and it is recommended that they should have on-going access to sex education. Implications of the findings for definitions of capacity to consent to sexual relationships are considered.