Assumptions about human mating psychology can range from a lay person’s entertaining intuitions to ideas that are taken for granted by mating researchers but that, upon closer examination, rest on precarious theoretical and empirical foundations. Dealbreakers and short-term mate preferences are two topics that most people feel like they understand, but the research investigating these topics is limited to a handful of studies, leaving some important questions untested. Here, my aim is to offer some clarity about dealbreakers and mate preferences through the precision of computational models and the power of a large, cross-cultural dataset. In chapter one I ask two questions. First, what are dealbreakers? And second, how do dealbreakers influence attraction and mate choice? I use an evolutionary perspective to generate hypotheses about how dealbreakers might be used by the mind. I propose that dealbreakers can either be disqualifiers or preferences. By disqualifiers, I mean traits that cause us to eliminate people as potential mates. By preferences, I mean traits that influence how attractive we find a potential mate. I use agent-based modeling, a method where computer simulated agents serve as avatars for real life participants and interact in a simulated mating market, to test between these two possibilities. Here, I found evidence that many of the traits we consider to be dealbreakers, such as smoking status, height, and religion, are not used by the mind as disqualifiers, rather they act like preferences and are integrated into overall assessments of mate value. A person’s sex, on the other hand, acts like a disqualifier. If a person is not our preferred sex, we do not consider them to be a potential mate. In chapter two I examine patterns of short-term mate preferences in over 50,000 participants from 56 countries around the world. I test whether men and women have different short-term preferences, and if short-term preferences vary across countries due to disease prevalence, gender equality levels, and sex ratio. Additionally, I examine patterns of long-term mate preferences, to see if previous findings replicate. Overall, I found that both men and women, on average, prefer short-term sexual partners who are kind, healthy, and attractive, but women had higher ideal preferences for short-term mates than men did. There was one exception to that pattern— both men and women preferred the same level of physical attractiveness in a short-term mate, on average. In chapter three I explore whether short-term mate preferences and long-term mate preferences are different from each other, indicating distinct short-term and long-term mating strategies. I used the same cross-cultural data from chapter two and a machine learning technique to explore whether individuals have different short-term and long-term mate preferences. I found that there are differences between short-term and long-term preferences, on average. Physical attractiveness preferences are higher, while kindness and resources preferences are lower, for short-term mates compared to long-term mates. However, I found that most participants (59-80%) preferred the same type of ideal mate for short-term and long-term relationships. Overall, this dissertation helps to bolster the theoretical and empirical foundations of dealbreakers and mate preferences research.