By Nathan Gershfeld, co-host of the Beat Your Genes Podcast
One of our listeners recently sent in a question that I think a lot of people quietly wonder about but rarely say out loud. She had been happily married for over 15 years, had no evidence her husband had ever been unfaithful, and yet she had a persistent sense that it was possible. After reading David Buss’s research on the prevalence of affairs in men in happy marriages, she wanted to know: how common are affairs really? And if they happen as often as the data suggests, why does it seem like nobody in your social circle ever talks about it?
I brought this question to Dr. Doug Lisle in episode 360 of the Beat Your Genes Podcast. His answer was one of the most clear-eyed and grounded explanations of infidelity I have heard, and it challenged several assumptions I think most people walk around with.
The Most Likely Number Is Zero. Then One.
The first thing Dr. Lisle did was push back on the idea that affairs are rampant. He was careful about the data — he acknowledged he was working from memory rather than citing current figures — but his overall reading was that affairs are serious, consequential, and relatively rare rather than casual and frequent.
His key framing: the most likely number of affairs in a person’s lifetime is zero. After that, the most likely number is one.
He also identified a major reason why male infidelity statistics have historically looked higher than female statistics: a significant portion of what men were reporting as “affairs” turned out to be visits to prostitutes. Once those are separated out, the male and female numbers come much closer together, and the picture of human sexuality looks considerably less wild than headline statistics suggest.
What Dr. Lisle Calls the Love Instinct
Dr. Lisle introduced a concept he said he had coined himself — the love instinct — which he described as an umbrella of instinctual mechanisms involved in mate acquisition, selection, and retention. This is central to understanding why affairs happen the way they do.
His core argument is that most affairs are not casual flings. They are serious business. The love instinct has been activated with a new person, and what you are actually watching in most affairs is a genuine vetting of a potential partner change. The typical affair lasts six months to a year. It either dissolves the marriage or it doesn’t — usually it doesn’t — but either way it is not a trivial event for the people involved.
The love instinct, Dr. Lisle explained, was designed by nature to cast protection over the enormous biological investment the female makes in having a child. It evolved to secure male protection and provision for several years. It was specifically not designed to last forever — and understanding that, he argued, is essential to understanding both why affairs happen and why relationships that once felt electric eventually feel flat.
Why the Modern Workplace Is a Perfect Storm
One of the most interesting insights from this episode was Dr. Lisle’s observation about why the modern workplace creates such fertile conditions for affairs.
He pointed out that throughout human evolutionary history, there was essentially no social configuration where males and females spent all day together in close proximity. Males would go out and hunt, females would work together, and they would come together at the end of the day. The workplace as we know it — where a woman and a man who is not her partner spend eight hours a day together, helping each other, being available, building shared context — is completely novel in evolutionary terms.
Her Stone Age brain, as Dr. Lisle put it, reads that daily availability as loyalty. The person at work is always there, always responsive, always showing up. Meanwhile, her actual partner is elsewhere most of the day. The brain doesn’t automatically adjust for the fact that we now know the difference. It responds to the sensory data in front of it.
This isn’t a moral failing. It is the love instinct operating exactly as designed in an environment it was never designed for.
Relationships Are Genetic
The second listener question in episode 360 came from a woman who had been through a painful on-again off-again relationship. She had one child, multiple miscarriages, and had ultimately been left as a single mother. Her friends were encouraging her to go to therapy to work on being too disagreeable. She was asking whether the problem was her, or the match.
Dr. Lisle’s answer introduced another concept central to his thinking: relationships are genetic.
What he means by this is not that your specific relationships are predetermined, but that your personality — and therefore how you evaluate and react to another person — is genetically fixed. Your personality was essentially set by the time you were four months in the womb. It does not meaningfully change based on experience, environment, or therapy.
This is a claim he makes with full awareness of how radical it sounds. He described it as one of the most tenacious delusions in all of psychology — the belief that personality is shaped by experience. He argued, drawing on researchers like Kevin Mitchell and Robert Plomin, that decades of systematic research have failed to find meaningful evidence of environmental influence on personality. Not 10 percent wrong. Not 37 percent wrong. The whole hypothesis, he said, was wrong.
I should say here that this is genuinely controversial among psychologists. But it’s the framework Dr. Lisle has built his entire clinical approach around, and it has significant practical implications for how he advises people on relationships.
What This Means If You Have Been in a Difficult Relationship
For the listener asking whether she needed therapy to become less disagreeable, Dr. Lisle’s answer was direct: no amount of therapy is going to change your personality, and nor should you want it to. What you can change is your circumstances — specifically, who you choose to be with.
The conflict she experienced with her partner was not a bug. It was information. It was evidence, present from early in the relationship, that the match was not right. The red flags were on the field from the beginning. She stayed because the value proposition had something in it she needed — in her case, a child. But she was never going to hammer silver into gold.
His advice for moving forward was simple and direct: be yourself as quickly and as authentically as is socially appropriate. The right match will respond to who you actually are. Someone who is put off by your personality is not someone the relationship was ever going to work with. You are not going to successfully maintain an Oscar-winning performance of a different person indefinitely, and you should not try.
The question of whether she would be less disagreeable with the right partner, he said, is actually the right question. Not because she would suppress who she is — but because genuine compatibility means there is less to be disagreeable about. The friction disappears not because you changed, but because the match is right.
Listen to the Full Episode
This post covers the main threads from episode 360 but there is considerably more in the full conversation, including Dr. Lisle’s thoughts on what he called tenacious delusions in psychology, his analysis of John Gottman’s research, and a fascinating discussion of why the love instinct is optimistic by design. The episode runs over an hour and is worth the full listen.
Listen to Episode 360: Affairs, Flares, and Fantasy Matches
If this resonated with you, the Beat Your Genes Podcast has over 350 episodes applying evolutionary psychology to everyday questions about relationships, motivation, and happiness.
Frequently Asked Questions
How common are affairs according to Dr. Lisle? Dr. Lisle’s reading of the research is that affairs are serious and relatively rare rather than casual and frequent. He emphasizes that the most likely number of affairs in a person’s lifetime is zero, and the second most likely number is one. A significant portion of what has historically appeared as high male infidelity rates turns out to be visits to prostitutes, which substantially changes the picture when separated out.
What does Dr. Lisle mean by the love instinct? The love instinct is a term Dr. Lisle coined to describe the umbrella of instinctual mechanisms involved in human mate acquisition, selection, and retention. He describes it as an instinct that turns on under specific conditions, drives serious vetting of a potential partner, and is specifically designed to secure protection and provision during the high-investment period of having a child. It was not designed to remain active forever.
Why do affairs happen in happy marriages? Dr. Lisle points to the modern workplace as a particularly potent environment for the love instinct to activate with someone other than a spouse. Daily proximity, mutual availability, and repeat exposure create conditions that the Stone Age brain reads as loyalty and desirability — conditions that never existed in human evolutionary history.
What does “relationships are genetic” mean? Dr. Lisle argues that personality is genetically fixed and does not meaningfully change based on experience or environment. Since how you evaluate and react to another person flows from your personality, your characteristic response to a given partner is also essentially genetic. This is why the conflicts present early in a relationship tend to persist — neither person’s underlying personality changes.
Should you go to therapy to become less disagreeable in relationships? Dr. Lisle’s answer is no — not because therapy has no value, but because the goal of changing your personality is a false one. Your personality is not going to change. What you can do is find a match where your personality generates less conflict naturally, because the compatibility is genuine rather than forced.
The Beat Your Genes Podcast features evolutionary psychologist Dr. Doug Lisle, PhD, who trained at Stanford and the University of Virginia and has over 30 years of clinical experience. Nathan Gershfeld, D.C. is the co-host of the Beat Your Genes Podcast and founder of Fasting Escape.