in MSM worldwide. The ability of MSM to be role versatile also predisposes them to large epidemics—removing this practice so as to mimic a heterosexual population in this regard reduced incidence by 19–55%, although we recognise that changing sexual practices on the scale needed to see these eff ects is an unrealistic prevention goal. This metric revealed major diff erences by setting, since role versatility is much more prevalent in US MSM than Peruvian MSM. The existence of high casual numbers of UAI partners for some men also facilitates HIV transmission, but to a lower extent than the specifi c biological factors of anal sex, and about as much as the existence of role versatility. Converting all casual UAI into the same amount of UAI within longterm main partnerships reduced HIV prevalence by 29–51%. Here too there were regional diff erences, with HIV prevalence dropping more in the high prevalence Peru scenario than the US one, since a higher proportion of UAI happens within the context of casual contacts in Peru than in the USA.