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Chapter 9218heart rate: Unakafov et al., 2018; Wang et al., 2023), although heart rate variability cannot yet be estimated non-invasively (Madan et al., 2018). Thus, combining these measures with cognitive tasks can potentially help to determine whether cognitive biases are the result of positive or negative valence. In the end, this is an essential step for establishing a link between motivational state and cognitive biases.One further line of research that deserves attention in future research is measurement of individual preferences in primates. Currently employed cognitive tasks, including the ones described in this thesis, mostly rely on categorical designs, aiming to identify a preference or bias for one category over another across individuals. However, when considering mate choice, it is well-known that individuals can vary in their mate preferences (Jennions & Petrie, 1997). To capture such inter-individual variation in preferences, it will be necessary to develop suitable test designs. One example comes from systematic investigations of individual food preference in primates (Hopper et al., 2019; Huskisson et al., 2020, 2021). This line of research has shown that simple computerised paired presentation tasks can be used to identify individual preferences for food, and that these preferences are consistent across tasks. I suggest future studies should employ this approach beyond food preferences to gain a better insight into individual preferences for specific stimuli or, in the case of sexually selective cognition, specific potential mates. ConclusionRecent psychological research has suggested an important role for cognitive processes in human mate choice. In this dissertation, I have taken a closer look at these processes in humans, and investigated whether these are also present in Bornean orang-utans by running a set of comparative studies. The thesis has shown that attractiveness-based attentional biases are clearly present in humans, and that such biases might also be related to actual mate choice. When it comes to Bornean orang-utans, the findings are mixed: they did not show any biases in touchscreen tasks, but did have an attentional bias towards flanged males across two eye-tracking experiments. Furthermore, auditory signals may play a more important role in orang-utan compared to human mate choice. Even though the results of this thesis suggest that both humans and orang-utans have an Tom Roth.indd 218 08-01-2024 10:42