Introduction to the special issue on multiple perpetrator sexual offending

The termmultiple perpetrator rape (MPR) was proposed by Horvath and Kelly (2009, p. 94) as an overarching term when referring to a “sexual assault which involves two or more perpetrators”. Although it is believed that MPR poses a significant problem internationally, the exact incidence and prevalence is difficult to determine due to differences in definitions and study designs utilised (Harkins & Dixon, 2010). Furthermore, it is not possible to distinguish between lone and MPR in the majority of official records of rates of sexual offending (Hauffe & Porter, 2009). From the existing studies it is possible to conclude that internationally approximately nine per cent to 33 per cent of sexual assaults are committed by multiple perpetrators (Australian Bureau of Statistics, 2004; Curran & Millie, 2003; Franklin, 2004; Jewkes, Sikweyiya, Morrell, & Dunkle, 2009; Kelly, Lovett, & Regan, 2005; Swart, Gilchrist, Butchart, Seedat, & Martin, 2000). Not only is MPR internationally prevalent, it also seems to occur in many different contexts. Harkins and Dixon (2010) identify several subcategories of MPR that they divide into two main categories: MPR of peers and adults, and MPR against children. Compared to research conducted on lone sexual offending, MPR is a relatively under-researched area with significant gaps in academic knowledge (Harkins & Dixon, 2010; Horvath & Woodhams, 2013). However, it is encouraging to see that in the last decade there has been a surge in interest in MPR that has resulted in the publication of a number of articles (Adolfsson, Stromwall, & Landstrom, 2017; Alleyne, Gannon, Ó Ciardha, & Wood, 2014; Bamford, Chou, & Browne, 2016; Chambers, Horvath, & Kelly, 2010, 2013; da Silva, Woodhams, & Harkins, 2014, 2015, 2018; Morgan, Brittain, & Welch, 2012; Woodhams, Cooke, Harkins, & da Silva, 2012; ‘t Hart-Kerkhoffs, Vermeiren, Jansen, & Doreleijers, 2011) and even the first book dedicated to MPSO (Horvath & Woodhams, 2013). Much of the research to date has focused on the characteristics of MPR offences, perpetrators, and victims (da Silva et al., 2014). Other areas of investigation, which are still in the early stages of exploration, include research on differences within MPR, for example, group sizes (da Silva et al., 2014; Park & Kim, 2016); identification of leaders and followers (Porter, 2013; Porter & Alison, 2001; Woodhams et al., 2012); the development of a MPR proclivity scale (Alleyne et al., 2014); blame attribution in MPR (Adolfsson et al., 2017), and motivations of MPR perpetrators (da Silva, Woodhams, & Harkins, 2018). The goal of this Special Issue is to build on these emerging topic areas in MPR, and to highlight new research in the area. The four articles included in the Special Issue examine different aspects and contexts of MPR. Porter reports on the results of a refinement to the Scale of Influence initially created in 2001 with Alison which has been used in other studies of leadership in MPR (Woodhams et al., 2012). She demonstrates how the refined measure identifies a group member as the leader in the vast majority of her sample of MPRs. Further, she explores the various hierarchical structures that define the MPRs in her sample, identifying several different structures. Having a reliable means for identifying group leaders is important for advancing our understanding of how leaders might differ from other group members, and understanding the various hierarchical structures within MPRs has implications for the study of dynamics within such groups.