Can Biology Explain Why Love Fades and Infidelity Rises?

Living things often become less responsive to a repeated stimulus over time, a process called habituation. To regain sensitivity, the stimulus must either be stopped for a while or gradually made stronger. This biological process helps us adjust to both good and bad stimuli. For example, when entering a very smelly place, the strong odor is initially unpleasant. However, within minutes, we notice the smell less because our sense of smell adapts. From an evolutionary view, this desensitization to repeated stimuli within the central nervous system (CNS) helps filter out constant, unimportant signals. This allows the brain to focus on newer or more critical information. For instance, in a noisy crowd, the brain ignores ongoing background sounds to pay attention to sudden or relevant noises.

At the social level, things we enjoy often become less exciting after we experience them repeatedly. Even the best entertainment, places, restaurants, or hotels can start to feel ordinary after several visits. To feel the same level of pleasure again, we often need to try something new. However, this process may also be a hidden reason for problems in our social relationships with others, such as an intimate friend or a colleague.¹

Dopamine and the reward system in the brain

Dopamine is a neurotransmitter that primarily functions within the brain's reward system, which consists mainly of the ventral tegmental area (VTA) in the brainstem and the nucleus accumbens. These two components also closely interact with the limbic system, which is a collection of brain structures that play an essential role in regulating emotions, memory, motivation, and instinctive behaviors.

In his book, The Molecule of More, Daniel Lieberman argues that dopamine is the main neurotransmitter in the reward system, driving desire, motivation, pursuit, and imagination focused on the future, what we could have, achieve, or experience. It contrasts sharply with "here-and-now" neurochemicals such as serotonin, oxytocin, endorphins, and endocannabinoids that mediate pleasure in the present moment. Dopamine plays roles in important aspects of human behaviors such as the pursuit of goals, creativity and genius, sexual arousal, love, and in the etiology of addictive behaviors and schizophrenia.

Dopamine increases when we face a new and attractive potential partner, triggering intense lust and excitement. This is the "falling in love" phase. At this stage, the focus is not yet on the individual themselves, but rather on the anticipation of rewards, such as heartwarming romantic relationships, sex, and the excitement of new experiences they bring. Dopamine fuels imagination and fantasy about new partnerships. This dopamine-driven fantasy frequently conflicts with the realities of a long-term relationship and sex life, leading to dissatisfaction and a desire to seek novelty elsewhere in an attempt to rekindle the dopamine high. Dopamine drives us to seek new goals for the future, not the satisfaction of possession. This fall in dopamine levels in the brain explains why passionate love often transitions into a mild and less excitable phase. The mission of dopamine, as Lieberman argued, is not a "here and now" neurotransmitter like serotonin or oxytocin, but rather is a driver for what we do not possess.¹

When the dopamine falls

Although not everyone reacts in the same way, many individuals may respond similarly when there is a decline in dopamine levels or a decrease in sensitivity to dopamine due to the habit of having multiple partners. For some, the drop in dopamine makes them feel sad or empty, almost like quitting an addiction. In support of this claim, a study indicated that patients with Parkinson’s disease who have lost dopaminergic neurons exhibit lower levels of cheating behavior when faced with opportunities for dishonest gain.²

As dopamine receptors become less responsive due to repeated stimulation, the brain may seek novelty to restore the same dopamine high, potentially contributing to behaviors like infidelity as a way to regain lost intensity. This desire for new thrills, which dopamine affects, is a major cause of cheating for some people. A decrease in dopamine levels or its sensitivity following overstimulation is known as anhedonia, which causes depression and a sense of indifference. Individuals experiencing anhedonia may attempt to regain pleasure through risky behaviors.

It seems that some genetic variations in dopamine signaling potentiate cheating behaviors in someone with lower levels of dopamine. For instance, genetic differences in dopamine receptor D4 (DRD4) may be linked with infidelity and unfaithfulness, possibly because this genetic makeup enhances the brain's response to new and exciting experiences.³ Moreover, there is evidence that a decrease in an enzyme in the brain of some people that regulates dopamine levels, known as monoamine oxidase A, may lead to extramarital affairs.⁴

While dopamine may drive us to seek new love experiences, other neurochemical molecules like oxytocin and vasopressin are more directly associated with long-term bonding and monogamy. For instance, research has shown that differences in the vasopressin receptor gene in some people may influence pair-bonding and marital satisfaction, particularly in women. Additionally, these genetic differences may contribute to the likelihood of infidelity.

Infidelity Essential Reads

Why Do Single Women Prefer Affairs with Married Men?

Rebuilding After Infidelity and Betrayal

Overall, although some believe humans are not naturally monogamous creatures, and that both men and women tend to seek new relationships after a long-term partnership, it seems not everyone behaves the same way in this situation. Even if many feel the urge to look for new partners, cultural pressures learned from childhood likely have a strong effect on human behavior in maintaining marriages. These pressures make people stick to social norms despite their biological urges.

These findings do not try to justify cheating in relationships; however, they provide intriguing insights into how genetics and biology influence human behavior. This perspective should make us fundamentally rethink our moral and legal judgments. These findings invite a more nuanced view of infidelity, not as a purely moral failing, but as behavior potentially influenced by neurobiological and genetic factors, similar to how we now understand addiction. However, it can also be reinforced by maintaining intimacy through open communication, sharing new romantic experiences, and attending to each other's emotional and physical needs.