Clinical observations and first person accounts of living with autism suggest that individuals with autism are highly motivated to engage in special interests, and that these interests remain important throughout life. Previous research assessing special interests has mainly focused on parental reports of children with autism spectrum conditions (ASC). To better understand the significance of and motivations for engaging in special interests it is essential to use self‐report ratings. This paper aims to systematically explore the motivations for engagement in special interests, and whether these differ in adults with ASC, first‐degree relatives and general population controls. The Special Interest Motivation Scale (SIMS) was developed to assess motivation to engage in special interests. The internal structure of this scale was evaluated using factor analysis, and mean scores on the SIMS factors were subsequently compared across individuals with autism, parents and general population controls. Factor analysis indicated a 20‐item SIMS containing five factors assessing Personal life values and goals; Intrinsic interest and knowledge; Prestige; Engagement and “flow” and Achievement. Individuals with autism were more motivated by Intrinsic interest and knowledge and by Engagement and flow than controls. The 20‐item SIMS is a quick to administer measure that provides a reliable description of motivation to engage in special interests. This study indicates that individuals with ASC are highly motivated to engage in their special interest, and are more motivated than controls by intrinsic motivational factors, some of which are associated with positive affect. This has implications for research and clinical practice. Autism Res 2016, 9: 677–688. © 2015 International Society for Autism Research, Wiley Periodicals, Inc.