Testosterone is a hormone that plays an important role in human and nonhuman animals’ sexual and competitive behaviour. In this study, we firstly examined how acute increases in testosterone in men influence how their personality is seen and evaluated by others. We found that female observers rate men as even more dominant in an aroused “competitive state”, when their testosterone levels increased more from (unaroused) baseline levels. Secondly, we investigated whether men’s testosterone levels and their personality characteristics predict their number of female sexual partners over a period of 18 months – put differently, do men’s testosterone levels and personality provide information on how successfully they acquire sexual partners? Preliminary findings show that not men’s testosterone levels, but instead how dominant they are being perceived by female observers (based on the men’s video-recordings), predicts how many sexual partners they reported to have had over 1.5 years.