Why do people stay together in monogamous relationships? Love? Fear? Habit? Ethics? Integrity? Desperation? In this essay I will consider a rather surprising answer that comes from mathematics. It turns out that cooperative behaviour, such as mutually-faithful marriages, can be given a firm basis in a mathematical theory known as game theory. I will suggest that faithfulness in relationships is fully accounted for by narrow self interest in the appropriate game theory setting. This is a

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... ng answer because faithful behaviour is usually thought to involve love, ethics, and caring about the well being of your partner. It seems that the game-theory account of faithfulness has no need for such romantic notions. I will consider the philosophical upshot of the game-theoretic answer and see if it really does deliver what is required. Does the game-theoretic answer miss what is important about faithful relationships or does it help us get to the heart of the matter? Before we start looking at lasting, faithful relationships, though, let's get a feel for how mathematics might be employed to help in matters of the heart. Let's first consider how mathematics might shed light on dating to find a suitable partner.