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General introduction251traits and attention. While we know that secondary sexual characteristics affect cognitive processes such as attention in humans (Garza & Byrd-Craven, 2023; Yang et al., 2015), this topic has been virtually unexplored in great apes. However, to develop a full understanding of the evolutionary underpinnings of such attentional biases, it is important to test a wide range of species (Smith et al., 2018), ideally with different mating systems (Petersen & Higham, 2020). Because orang-utans are a suitable model organism for this topic, part of this thesis investigates whether Bornean orangutans show cognitive biases towards flanged males. Figure 2. Pictures of flanged males of each orang-utan species. Left picture: courtesy of Ronald van der Beek. Middle picture: Tom Roth. Right picture: Tim Laman (redistributed under CC BY 4.0).Dissertation outlineThis dissertation consists of one review chapter and six empirical research chapters that investigate the influence of attractiveness or attractive traits on cognitive mechanisms that underly mate choice in either humans or orangutans. The thesis is divided into three sections and a general discussion. Section 1 (Chapter 2) presents a literature review summarizing experimental paradigms that are relevant to the thesis. Section 2 (Chapters 3-5) provides evidence for biased visual attention towards attractive conspecifics in humans. Section 3 (Chapters 6-8) investigates visual attention and preference towards flanged males in Bornean orang-utans (Pongo pygmaeus). Chapter 9 concludes this thesis with a general discussion. I want to emphasize that, while the chapters are Tom Roth.indd 25 08-01-2024 10:41