Men and women, like most mammals, are influenced by a biological process that can reduce interest in monogamous sex over time.
“The recent period has brought a lot of stress, and stress immediately affects our relationships. It creates tension, increases aggression and reduces empathy, and empathy is very important for a good relationship,” said Dr. Liat Yakir, a biologist who studies the biology of emotions.
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In the stage of falling in love, all the 'good' hormones are released, and we feel a high with every touch and interaction from the other person
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Yakir returned to the “Sex Appeal” podcast to explain how biology and hormones shape relationships, and why many people experience a decline in sexual excitement over time, sometimes leading them to seek novelty elsewhere. The phenomenon is known as the “Coolidge effect,” a concept used to explain sexual habituation in animals and humans.
In the early stages of attraction, hormones such as estrogen and testosterone play a central role, drawing people to partners who appear energetic, confident and fertile. Dopamine is also released due to the novelty of meeting someone new.
As relationships progress into the stage of falling in love, additional chemicals come into play. Adrenaline creates the familiar “butterflies,” while serotonin contributes to excitement triggered by attention, touch and compliments.
“Another key substance is PEA, phenethylamine, a neurotransmitter linked to infatuation, arousal and euphoria,” Yakir said. “It affects dopamine and norepinephrine and can create a sense of addiction. Unfortunately, it is also one of the first to decline as the relationship develops.”
According to Yakir, the infatuation stage can last anywhere from a few hours to two years. Over time, however, the intensity fades.
“The first rule of a good relationship is to stop chasing the butterflies and understand that when they fade, it’s simply part of how our system works,” she said.
As familiarity grows, dopamine and adrenaline levels drop because the stimulus becomes predictable.
“The more we have sex, the more we know what to expect. It becomes almost like a protocol,” she said.
This decline in excitement is what scientists call the Coolidge effect, a natural reduction in arousal when exposed repeatedly to the same partner.
“The brain constantly seeks new stimuli to boost dopamine,” Yakir said. “That’s why people may suddenly feel attracted to someone new at work or in their environment.”
From an evolutionary perspective, Yakir noted, most animals are not monogamous. “Females tend to choose the dominant male, and when he loses status, they move on. It’s not one partner for life,” she said.
Early research on the Coolidge effect, conducted in the 1950s at the University of California, Berkeley, showed that male mice exhibited declining sexual interest with the same partner, but renewed interest when introduced to a new one. Later studies suggested the effect exists in females as well, though often to a lesser degree.
Modern evidence can also be seen in human behavior, Yakir said, including patterns observed in pornography consumption, where repeated exposure to the same stimuli reduces arousal, prompting a search for novelty.
Still, Yakir stressed that biology does not determine outcomes.
“Yes, desire tends to decline over time, but that doesn’t mean we are doomed to replace partners every few years,” she said. “What actually creates long-term happiness is emotional security, feeling seen, valued and cared for.”
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It is very natural, and even biological, for your sex life to become less exciting over time
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She emphasized that stress is one of the most significant factors affecting sexual desire. Elevated cortisol levels, particularly during demanding life stages such as raising young children, can suppress libido.
“Children bring a lot of love, but also stress and lack of sleep, and sleep is critical,” she said. “If you’re exhausted and overwhelmed, adding pressure about sex won’t increase desire.”
Instead, she recommends focusing on emotional regulation and connection.
“The goal is balance, increasing the hormones linked to pleasure while reducing stress hormones,” she said.
Simple practices, such as shared moments of calm, physical closeness and daily expressions of appreciation, can help rebuild intimacy.
Citing relationship expert Dr. John Gottman, Yakir noted that couples should aim for a ratio of five positive interactions for every negative one.
“At the end of the day, it’s all chemistry,” she said.