兩性關係中常存在著被背叛的風險，例如戀愛過程中對方的劈腿與機會性遺棄（遇見更好對象時移情別戀），又例如婚姻生活中的外遇與機會性遺棄（為更好的新歡而離婚）。在兩性關係中被背叛，不只會經歷情緒上的傷痛，從遠古時的演化角度而言，更是個體生存與繁衍上的危機。本文的主旨在討論愛情（love）這種非理性的情緒，是如何被演化出來解決這種被背叛的危機。首先，本文引用心理學家Herrnstein的「配合法則」（報酬的吸引力與延遲實現的時間成反比）來分析兩性關係中「背叛者的誘惑」的來源與型態。以此論誰在某些常見的情境下，真愛者比背叛者更有利於子孫的繁衍。其次，本文以真愛者與背叛者的演化賽局模擬分析，何以愛情與背叛皆會在演化中共同留存。

In an intimate relationship, there often exists a risk of sexual betrayal, such as infidelity in dating or in marriage, or abandoning partner when the better one appears. Being betrayed, from evolutionary point of view, is an adaptive problem which reduce the reproductive success. This paper explain how love, a kind moral emotion, play as a part of adaptation to resolve the adaptive problem. Firstly, Herrnstein's matching law is used to explain how love help to resist the speciously attractive rewards of sexual betrayal, which come from the cognitive biases of the different time-discounting benefit and cost when consider a sexual betrayal. Secondly, following Frank's evolutionary game model of moral sentiments, the paper find that the numbers of infidelity and fidelity will both share a proportion in a population after the evolutionary process reach a ”break-even point”.