As an evolutionary biologist and sex scientist, I have devoted most of my professional life to researching romantic and sexual relationships. The range of wants and needs I am fortunate enough to study is bountiful, beautiful, and sometimes mind-boggling.
It’s also work that has taken me to many interesting places – such as a legal brothel in Pahrump, in the middle of the desert, an hour outside Las Vegas.
I’d gone there – with colleagues – to ask questions in the name of science. Over lunch a few months earlier, my colleagues and I had hatched a plan to conduct interviews with the employees on their daily lives and the habits and preferences of their clients.
The building, which looked like a converted one-storey motel, had two entrances: one to a restaurant and bar, the other to GIRLS! GIRLS! GIRLS! We chose door number two, showed our IDs to a cheerful middle-aged woman who introduced herself as the house manager, and filed in.
In the vestibule was a large poster, propped up on an easel, with a menu of the sexual acts on offer.
The less expensive options all seemed fairly straightforward: everything from sexual intercourse to breast massage and dressing up.
Then my attention was drawn to the ‘specials’ where, on offer, was a service that started at an astonishing $20,000.
It was clearly not for the faint of heart. But what was it?
‘Oh, that’s the full Girlfriend Experience. A great choice for someone who wants something… personal,’ explained our guide. ‘Sex isn’t necessarily a part of it, but you’ll get a hell of a cuddle.’
We were struck silent.
While we are wired to be socially monogamous we are not necessarily wired to be sexually monogamous
Dr Justin Garcia is an evolutionary biologist and sex scientist
So here was the most expensive sex act money could buy in a brothel, and it didn’t necessarily involve sex.
What people were buying – that precious, rare and elusive ‘white whale’ they obsessively pursued – was intimacy.
Intimacy is a nebulous scientific concept. It is that pleasurable and comforting feeling associated with any close connection that grows between humans.
It’s making eye contact across the table at a dinner party and knowing exactly what the other person is thinking.
It’s feeling safe enough to lower your emotional armour and expose your deepest insecurities.
Intimacy is at the core of every successful romantic relationship, yet so few of us understand its importance, and how it has impacted the evolution of our species, perhaps because it lives in the shadow of that other primal urge: our sex drive.
And when those two powerful forces clash or fall out of step – that’s when our relationship problems begin.
Intimacy versus sex paradox
It’s easy to grasp how the human sex drive evolved: it’s there to motivate mating, which is, of course, essential for the survival of our species.
Yet so is our motivation to seek out intimacy and love – pair bonding – which is a much stronger drive than we have been led to believe, and much less widely understood.
As my late friend and collaborator, the biological anthropologist Helen Fisher, would say: ‘Everywhere in the world people pine for love, live for love, kill for love, and die for love.’
Evolutionary scientists believe our ancestors started pair bonding around the same time that we started walking upright, about 4.4 million years ago.
Coupling up allowed us to share resources as hunters and gatherers, protect females during pregnancies and also divide up infant caring duties, thereby shortening the intervals between births, as one parent no longer had to wait until a first baby was more or less self-sufficient to care for another.
It meant that a couple who mated and cared for each other in sync became a recipe for the human race’s success.
At this intersection of our existential need for love and sex lies a fundamentally intractable paradox, however. While we are wired to be socially monogamous – that is, form intense pair bonds with other humans, usually one at a time, and sometimes lifelong – we are not necessarily wired to be sexually monogamous.
When our sexual desire and monogamy are in sync, we feel the kind of love and passion that poets dream of. When they’re at odds, we often find ourselves unhappy and unfulfilled and more likely to test those pair bonds.
Many of the mistakes or bad choices that we make in relationships, both big and small, arise from this fundamental tension between our evolutionary desire for sex and our biological need for intimacy.
So, we’re left with one hell of a question: how can we reconcile our competing desires to enjoy a deeper and more satisfying form of romantic love?
Anyone fancy a throuple?
One way some people have reconciled the problems created when our sex drives and need for love become out of step, is to try consensual non monogamy – or ‘open relationships’, as they’re commonly called.
Therapists have traditionally cautioned against this path, regarding those relationships as unstable or unsustainable.
However, the newest body of scientific literature argues that people who live and love this way don’t necessarily fare worse psychologically or emotionally than monogamous ones.
And a series of studies have deduced that while consensually open relationships might not work for everyone, or even for most people, there are many people for whom they do work perfectly well.
Evolutionarily, though, these exceptions might prove the rule. Open relationships also introduce new tensions, the strain of which I’ve come to believe is more than most people can or want to handle.
In general, our brains don’t appear particularly well suited to processing intimacy with more than one partner at a time. One of the prevailing rationales for consensual non-monogamy is that some among us simply have too much love to give to just one person, but the reality is the opposite: most people don’t have the biological, psychological, and social tools to love more than one person at a time.
This is likely because these arrangements – which are in many ways incongruous with the evolved tendencies for pair bonding that have allowed us to thrive as a species – are difficult to sustain with multiple partners, especially over the long term.
I also suspect that, over time, the intimacy most people experience in consensual non-monogamy is more sexual than it is emotional, and the same issues that plague monogamous relationships: mismatched libidos, jealousy, boredom and more, will surface in open ones eventually.
As one of my friends who tried to form one once told me: ‘It didn’t work. I just p****d off two women instead of one.’
Some people are wired to cheat
The question I’m most frequently asked is why people engage in infidelity. A close second is how many people cheat or stray.
Studies can be confusing. Are we talking about infidelity over the course of a lifetime, the last year, or in your current relationship? There’s no exact answer.
If we’re defining infidelity as a sexual betrayal, we could reasonably narrow that down to 20 to 50 per cent, which is still a pretty huge range.
But in 2010, my research programme unearthed an astonishing discovery: that some people have a genetic predisposition to be unfaithful: an inherited trait that became known as ‘the infidelity gene’.
Our research centred on two genes that influence the function of the brain’s dopamine ‘feel-good hormone’ receptor cells.
The rush of dopamine comes as a reaction to anticipated rewards: the excitement and uncertainty of what we expect to happen, so is therefore associated with risk-taking and sensation-seeking.
In our study we brought 181 people into the lab and ran them through a battery of behavioural surveys on their romantic and sexual histories, addictions and their risk-taking and novelty-seeking proclivities. We then collected a sample of their DNA.
We were looking to see whether individuals carried a ‘long’ or ‘short’ version of these particular genes. Past research has linked people who have the long version to sensation- seeking and risk-taking behaviour.
One way to look at it is to think of the long gene as a clogged garden hose; in order to get the same dopamine hit as everyone else, a lot more neurotransmitter needs to be sent down the pipe.
We found that people who have the longer version were 50 per cent more likely to report having engaged in infidelity. Furthermore, they were much more likely to have strayed multiple times.
I cannot overstate how much this finding does not mean that if a person has a certain genetic makeup, he or she is somehow destined to cheat.
Our genes influence the tendencies, predilections, and desires we all carry with us into our intimate lives. But biology is not destiny.
We’re programmed to be jealous too
Humans are wired to be jealous, to protect pair bonds once they have formed them, which is probably why we have strong opinions about infidelity.
A Gallup poll from last year found that 89 per cent of Americans consider infidelity immoral (compare that to divorce, which only 19 per cent consider immoral), and 65 per cent find infidelity to be unforgivable.
We humans are in the minority when it comes to monogamy (only three to five per cent of mammals, and 15 per cent of our fellow primates practise social monogamy). Yet jealousy is something many species share.
Very few animals who do couple up are also sexually exclusive, so most will attempt to enforce their mate’s sexual fidelity, by screaming, biting and clawing any interloper who gets too close to their pair-bonded partner.
In the natural world, this behaviour is called ‘mate guarding’ – and we humans do it too.
In 2010, Dr Garcia’s research programme unearthed an astonishing discovery: that some people have a genetic predisposition to be unfaithful
We’ve all seen that man at the party who won’t let his wife out of his sight or talk to other men, or the girlfriend who constantly bad-mouths her boyfriend’s female colleagues.
Betrayal offends all the emotions, and our responses are often rooted in an unbridled instinct to defend our pair bond.
As many of us know from our own experiences, jealousy rarely brings out the best in us.
Why do people cheat?
As with most of what goes on in our intimate lives, the reasons people are unfaithful are incredibly complex. In one study we looked at the primary infidelity motives in a largely heterosexual sample of 495 young adults and ultimately identified eight distinct reasons people cheat:
- Anger: ‘My primary partner had been unfaithful to me/we’d had an argument and I wanted to even the score/seek revenge.’
- Sexual desire: ‘My primary partner had lost interest in sex/ refused to perform certain sex acts that I enjoy.’
- Lack of love: ‘I was not sure if I really loved my primary partner or if they were the right person for me.’
- Low commitment. ‘I was not very committed to my primary partner to begin with. Even though we were seeing each other regularly we were not technically in a relationship.’
- Esteem: I wanted to boost my self-esteem/feel better about myself. I wanted to assert my independence and autonomy.’
- Situation: I was drunk and not thinking clearly. I was overwhelmed at the time due to external stress factors, such as school, work, family issues.’
- Neglect: ‘I felt neglected by my primary partner/they were emotionally distant.’
- Variety: ‘I wanted a greater variety of sexual partners. I am the kind of person who cheats; it is a part of my personality.’
In a separate follow-up study, we examined how these motives impacted infidelity experiences and outcomes.
Perhaps not surprisingly, we found that people who strayed for reasons like lack of love and neglect enjoyed more intimacy in their affairs – going on dates, engaging in public displays of affection and being told ‘I love you’. These affairs also tended to last longer, whereas affairs arising out of situational motives were more likely to be one-off sexual encounters.
The positives of cheating
For all its negative connotations, there are some evolutionary advantages to straying: it may be essential to have more offspring, or more diverse offspring with different genetic mixes.
Think of it this way: if the birds’ nest in your back garden gets hit by a predator, they might lose all their young that breeding season. But if one of them happened to also sire offspring in their neighbour’s nest, their genes might still make it into the next generation.
Or, in another real scenario from the natural world: if your offspring are genetically susceptible to a new pathogen, the offspring you have with a different partner would have different genes and might have a greater chance of surviving a plague.
Keeping the flame alive
Even in strongly pair-bonded couples, sexual passion wanes over time.
Researchers have found that somewhere around 18 to 36 months into a relationship, what some might call a ‘cooling down’ tends to happen.
On average, we move from intense and obsessive feelings into a more attached and comfortable stage. Think of this as the ‘couch and cuddle’ phase.
One stage is not necessarily better or worse than the other; while some couples find themselves growing more distant once the flames of sexual passion subside, others enjoy moving on from those turbulent times.
Nor is it necessarily a linear journey. It’s simply the warp and weft in the fabric of human intimacy. Once the companionable love phase sets in, the emotional bonds in the relationship often grow stronger.
Yet for many long-term couples this phase marks a decline in sexual frequency, as the intensity calms and partners find other, deeper ways of connecting.
So, the question before us becomes: Is it possible to sustain sexual passion throughout the life of a relationship?
My colleagues and I decided to explore this very question in a landmark study of almost 40,000 heterosexual couples who had been in a relationship for at least three years, asking them to rate their feelings of passion and sexual satisfaction.
When we focused on those men and women in our study who were more likely to report their most recent sexual encounter with their partner as ‘passionate’, ‘loving and tender’, or ‘playful’ we found that they engaged in a range of behaviours largely associated with intimacy and closeness. These included giving or receiving a massage, going on a romantic getaway, or planning a date night to have sex.
They were also doing things like trying a new sexual position, talking about or acting out fantasies, or using a sex toy together.
On the surface, this might read as though the trick to sustained passion is simply lots of sexual variety, but I believe these findings actually tell a story of intimacy.
Experimenting outside of your usual comfort zone demonstrates interest in being intimate with a partner and in so doing communicating trust, vulnerability and connection.
It becomes more than just any one particular act of sex. The power is in the commitment of effort in advancing passion, together.
What about gay couples?
We launched two more studies, asking the same questions of gay couples, comparing how
satisfied they were with their sex lives compared to heterosexuals.
In the partnered women study, we found lesbian and straight women were equally likely to be sexually satisfied, but lesbian women in long-term relationships were more than twice as likely to report having sex never or infrequently – colloquially popularised as ‘lesbian bed death’.
But when we dug deeper into this large dataset, we found that while these couples were having less frequent sex, the sex they were having was more intimate.
Lesbian women were more likely to orgasm regularly, receive oral sex and use sex toys.
Among gay men, we found similar patterns of intimacy, but without the sharp decline in the frequency of sex.
Gay men were also less likely to report a sexual desire discrepancy – that is, a scenario in which one partner wanted sex more (or less) often than the other – than straight men, particularly in relationships longer than five years.
Though the variety and frequency of sexual activities may have varied across the three groups that we studied, all had one important thing in common: communication, both in the bedroom and in the relationship in general, was a strong predictor of sexual satisfaction.
That pattern held, regardless of gender or sexual orientation.
In other words, those couples who were keeping the flame of intimacy alive, by texting, talking, leaving notes, and body language, thereby making sure their partner understood what they were thinking or feeling, were keeping the flame alive sexually as well.
- Adapted from The Intimate Animal by Dr Justin Garcia (Penguin Life, £20), to be published February 12. © Justin Garcia 2026. To order a copy for £18 (offer valid until February 7; UK P&P free on orders over £25) go to www.mailshop.co.uk/books or call 020 3176 2937.
