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General introduction191immediate attention, voluntary attention and preferences were modulated by ratings of attractiveness (humans) or presence of secondary sexual characteristics (orang-utans). Overall, the aim of this thesis is two-fold: to investigate to what extent attractiveness affects human cognition by employing different test paradigms, and to explore whether we can extend these paradigms to a closely-related great ape species. Such a comparison between related species will not only result in deeper understanding of the evolutionary emergence of cognitive phenomena (Panksepp et al., 2002; Smith et al., 2018), such as mate-related cognitive biases, and how they relate to socio-ecological differences between species, but the results of this thesis could also inspire practical applications, such as test paradigms that can identify individual mate preferences of zoo-housed animals. To start, I first provide an introduction to several concepts that are key to this thesis, after which I will provide an overview of the main themes discussed in this thesis.Sexual selection and the evolution of attractivenessFrom a biological perspective, evolved mate preferences are likely the result of sexual selection. In The Descent of Man (1871), Charles Darwin proposed his theory of sexual selection to explain the evolution of exaggerated secondary sexual characteristics, which are common in the animal kingdom. Darwin defined sexual selection as selection on “the advantage which certain individuals have over others of the same sex and species solely in respect of reproduction” (Darwin, 1871, p. 256). Sexual selection can generally be divided into two types: intersexual selection, where traits make an individual more attractive to the opposite sex, and intrasexual selection, where traits are useful in competition between members of the same sex for access to mates. The sex that exerts these different types of sexual selection depends strongly on parental investment (Trivers, 1972), with females typically investing more energy in reproduction, leading to male-male competition and female choice. However, in species with biparental care, such as humans, mutual mate choice might be more common (Courtiol et al., 2016), because both sexes invest valuable energy in caring for their offspring.Darwin’s ideas about sexual selection remain remarkably relevant to current research (Jones & Ratterman, 2009). More specifically, his work has inspired many Tom Roth.indd 19 08-01-2024 10:41