Something odd about couples has puzzled scientists for more than a century

Once I saw it, I couldn't unsee it.

I noticed it among friends. Then within my family. It is rife among my colleagues.

Romantic partners who are strikingly, in some cases, uncannily, similar to one another, whether in their appearance, personality, habits, attitudes or characteristics such as age, ethnicity, religion or education.

It began with noticing just how many romantic partners have the same job. Then I started to see it across all sorts of traits, from the logical (couples with the same education who met at uni), to the quirky (the outdoorsy couple who both had big teeth, talked loudly, and always wore hiking boots and puffy jackets) to the, well, unfortunate (the backpacker couple who lived in a van and both had bad body odour).

I was intrigued. Had I noticed something real or was it just the people around me? If it was real, how common was it? As someone whose partners have tended to be quite different from me, I wanted to know: was I weird or were these couples weird? I set out to find out what the data had to say.

Turns out, I'm the weird one. The phenomenon, known as homogamy or assortative mating, is real. And evolutionary biologists, sociologists, anthropologists, psychologists, economists and geneticists have been puzzling over it for more than a century.

"It's the most common mating pattern globally," Queensland University of Technology (QUT) behavioural economist Stephen Whyte says.

"You see it in every single culture, in every single country. It's universal."

Yet when it comes to Australia, very few studies have examined the data. So after months of back and forth with the obliging consultants at the Australian Bureau of Statistics, I held in my hands a series of customised census datasets, provided exclusively to the ABC, showing the occupation, religion and level of education of each partner in every couple in Australia.

That is up to 5.2 million married and de facto couples in 2021 and up to 3.6 million couples in 2001, both opposite-sex and, for the first time, same-sex.

Delving into the data, I found that when it comes to these demographic characteristics, Australians are no exception: we tend to seek out partners who resemble ourselves. But dig a little deeper and another trend emerges: a growing tendency to embrace difference.

Falling for your mirror image

You might think there's no-one like you out there. But one of the most remarkable things I learned is just how unremarkable it is to fall in love with someone just like you.

Take Sydney-born teacher Gabe and her Welsh partner Kelly. The pair have the same height, build, complexion and hair colour. They wear each other's clothes, although Gabe swears Kelly looks better in them. They look so similar, the kids at the school where they teach struggle to tell them apart. (Yes, they are both primary school teachers and they work at the same school.)

"We have occasionally swapped ID cards to pick up each other's post or other errands," Gabe says.

"I don't think we look that similar but in a culture where all women have long hair and are very feminine, I guess we stand out."

On the other hand, the 42-year-old later admits that she once tried to have a conversation with Kelly, thinking they were both in the library at school. "When she didn’t answer, I realised it was my own reflection in the glass dividing wall."

But their similarities go beyond appearance. They both love camping, board games, rock climbing and reading. They both play goalkeeper for the Slovak Gaelic football team. They're both left-handed and have the same favourite colour: green.

"We both always dreamed of having a VW campervan as our primary life goal," Gabe says. "We have one now and it's green."

The pair met on a tram in Bratislava, Slovakia. "When I saw her short hair, clipped fingernails and Birkenstocked feet, I thought, 'Here is someone I want to get to know,'" Gabe says.

That two people who share an uncanny resemblance could come from opposite sides of the planet to meet on a train in Slovakia and fall in love is not as improbable as you might think.

Research on couple similarity dates back more than a century, to 1903, when English biostatistician Karl Pearson found that spouses tended to match on height, arm span and forearm length.

Since then, the field has expanded, documenting couple resemblance across a whole gamut of traits, from intelligence and political orientation, risk-taking and religiosity, to psychological disorders like depression and bipolar, substance dependence and antisocial behaviour, to physiological attributes like body shape, blood pressure, kidney function and finger length.

More recently, geneticists have unearthed evidence suggesting something more extraordinary: that couples also match on traits that they are unlikely to have known about when they met or logically wouldn't choose if they did.

These studies focus on a person's genetic predisposition for traits, not just the traits themselves. Across tens of thousands of couples, researchers have found couple matching applies to the genetic markers linked to height, body mass index and years in education.

It also applies to susceptibility to diseases such as diabetes and coronary artery disease, and psychiatric disorders like schizophrenia and depression.

So it's clear that "like attracts like" on multiple levels when it comes to romantic relationships. Far harder to disentangle is how this happens.

How couple similarity happens

Researchers debate four main explanations for the phenomenon. Most agree that there is probably some truth to all of them.

University of California, Davis psychologist Paul Eastwick reckons one explanation stands above the others. "It's really hard to overstate the importance of proximity," he says.

"We tend to live near similar people on many, many dimensions," Professor Eastwick says, and most people choose from the people around them.

After all, it's hard to fall in love with someone you've never met.

Take one study that randomly assigns students to a seating arrangement at the start of the year, then assesses the friendships they form.

"People are vastly more likely to be friends with the person that they're randomly assigned to sit next to, rather than somebody even one row over," Professor Eastwick says.

Neil Sharkey and his wife Pam are a prime example of how proximity brings like people together. They met as students in Armidale and have been married for about half a century.

"We are similar ages and ethnicities, and both come from farming families. We have both participated in a wide range of sports and have coached extensively with our respective sports," Neil, 72, says.

"We both recently retired from long careers in education, one primary, one high school. We have similar political views, enjoy similar TV programs and movies … and are both cricket tragics."

He adds: "We travelled to England for the last two Ashes series and both agree that Jonny Bairstow was out fair and square and is a complete goose.”

A second explanation, known as "market forces", focuses on competition between people of varying desirability.

To put it in layman's terms: we're all ranked according to our desirability. (Let's call this a hotness scale.) Research shows that the more desirable pair with each other first, and that most people end up with a partner who roughly matches their own hotness score.

For Professor Eastwick, the "market forces" explanation is the second-most powerful piece in the similarity puzzle. It explains why, for example, partners tend to be similarly attractive, even though attractive people aren't geographically clustered.

Throwing a spanner in the works is that there is no universally agreed upon hotness scale.

"Once people know each other, they actually stop agreeing so much about who's hot and who's not," Professor Eastwick says.

In other words, beauty is mostly in the eye of the beholder (about 75 per cent, anyway, according to Eastwick).

A third explanation, known as "convergence", suggests partners who share experiences, lifestyle and routine lead them to become more alike over time.

This explanation brings to mind that classic modern love story: two people meet, fall in love and before they know it, they've spent a decade or more on their couch together, watching TV and eating takeaway. It's not surprising that over time, their body shapes, BMIs, risk of certain lifestyle diseases and even their interests and other habits have become more similar.

The evidence for this is particularly strong for appearance. In a widely cited 1987 experiment, for example, participants were more easily able to match partners in a couple based on photos from their 25-year anniversary than their wedding.

The fourth and, arguably, most obvious explanation is that we actively choose partners who are similar to ourselves.

Professor Eastwick reckons the evidence for active choice theory is "terrible". However, University of California Santa Barbara (UCSB) evolutionary psychologist Daniel Conroy-Beam says it depends on the trait you're measuring.

"What composes your mating market is determined by a lot of higher-order, structural things," he says. So, for demographic traits like your social class, where you live, your education, age, ethnicity, religion, and so on, proximity or "social segregation forces" will matter a lot.

"But then what you do in that market also acts to shape the kinds of partners you wind up with."

As for why we follow this pattern, the short answer is that we don't know.

One theory suggests people choose partners who resemble themselves because it increases the chances that their children will inherit the same useful traits, such as intelligence, ability or certain behaviours.

Birds of a feather flock together

So what did the data reveal about Australia's 5 million or so couples?

(Keep in mind that census data doesn't tell us whether the two partners in a couple were already alike when they met or they became more similar over time. Nevertheless, some clear trends emerged.)

Our analysis of some 43,000 job pairs spanning over 470 jobs found that in 38 per cent of those jobs, the top choice of partner was someone in the same job.

That figure jumped to 66 per cent after adjusting for different workforce sizes (since there are far more, say, sales assistants than surgeons).

Digging deeper revealed different patterns for men and women. In most occupations, women were more likely than men to be married to someone with the same job, with some of the most noteworthy examples among high-status professions.

To give three examples: the top choice of partner for female chief executives, general managers and surgeons was someone in the same job. By contrast, male CEOs were most likely to marry an office manager, male general managers were most likely to marry a general clerk, and male surgeons were most likely to marry practice managers or registered nurses. (Read the full story to find out why.)

When it came to religion, over two-thirds of the partnered population were married to someone of the same faith (or type of non-belief, such as agnostic or atheist).

This figure soared past 90 per cent among brethrens (95 per cent), Muslims (93 per cent), Hindus (92 per cent) and Sikhs (92 per cent).

The reasons are nuanced and overlapping, and often strongly tied to migration patterns, the religion's size within the society, whether it has a history of persecution, and how its practices intersect with the cultural identity of its followers.

Our analysis of education uncovered two striking trends. First, that the millennia-old tradition of women "marrying up" the social ladder is on its way out. Across Australia, women are now more likely to outrank their male husband or partner when it comes to education than vice versa.

As the chart below shows, in 2021, 34 per cent of men were married to women with more education than themselves, compared to 30 per cent of women. (In 2001, 22 per cent of men had married up, compared to 36 per cent of women.)

The second trend in the education data is that, despite this stunning reversal in who marries up, the top choice of partner in almost every education group is someone with the same level of education.

This preference to "marry within the group" is strongest at the top and bottom of the education ladder, with university degrees degrees now a dividing line in dating and marriage.

Those with no educational attainment are over 80 times as likely to partner with each other compared to the average, while those with doctoral degrees are nearly 15 times as likely to marry each other compared to the average. (The average is the rate of marriage to that group across the partnered population.)

These trends raise tricky questions about class mobility. After all, marriage has been the golden ticket up the social ladder for millenia, particularly for women.

So while marrying someone similar, in social terms, your "equal", might sound like a good thing, it presents an uncomfortable paradox: is increased equality within marriages actually leading to greater inequality across society?

On the other hand, as with most data, the numbers don't tell a single story. If similarity is the rule rather than the exception, there are hints that this long-standing pattern is shifting.

But more and more, opposites do attract

Australia's cultural and ethnic diversity, the rise of dating apps and the ubiquity of international travel are some of the reasons millions of couples have found love across social, educational, cultural and other divides. Joanna Jrejie and her husband are among them.

"My husband is ADHD, OCD, emotionally charged, and always in a heightened state of anxiety over everything.

"This translates to him being energetic and constantly active, meticulously clean and uncluttered, and always needing to be around many people to talk and debate with," the 57-year-old says.

By contrast, Joanna is quiet and unemotional, prefers to be alone and "could easily sleep 16 hours per day". She also confesses to being prone to hoarding and clutter.

But the two complement one another and balance each other's strengths and weaknesses. It's tough at times, but Joanna reminds herself that things could be worse. Like, if the pair were more similar.

"If we were both like him, we would have killed each other years ago in a super clean and uncluttered house while hosting a get-together with hundreds of friends," Joanna says.

"If we were both like me, we would be sleeping under the piles of old boxes and broken gadgets we had collected and when we died it would be months before anyone realised we were missing."

Joanna and her partner may be old hands at the art of give and take (they've been married for 40 years), but the data suggests younger couples and same-sex couples are leading the charge when it comes to love that embraces difference.

We found gen Z and millennials were the most likely to partner outside their occupation. The percentage of jobs in which the top match was someone in the same job dropped to roughly 30 per cent among gen Z, compared to roughly 56 per cent for the silent generation.

The trend was similar for education. In all three of the youngest generations, gen Z, millennial and gen X, men are now significantly more likely than women to marry up the education ladder.

Much of this is the result of a lopsided dating equation. Under-35s, for example, count just three university-graduate men for every four university-graduate women, according to Australian Bureau of Statistics figures.

Add to this a very different economic context that in many ways undermines the traditional view of marriage premised on a division of labour between husband and wife.

"Men are now looking for wives who can also earn," Professor Hewitt says, citing a pattern that first caught the attention of sociologists in the early 2000s.

"You can no longer sustain a family on one income. Women's incomes are … essential for the financial stability and viability of a family."

When it comes to religion, the rate of same-religion matching falls with each generation born in Australia. Among the partnered population, 77 per cent of first-generation immigrants are married within their religion, compared to 65 per cent among second-generation immigrants (the Australian-born children of immigrants born overseas) and 62 per cent among subsequent generations, which cannot be further separated in census data.

Among some faiths, the fall was far steeper. Between the first and third generations, the figure dropped from 92 to 47 per cent among Hindus; from 86 to 38 per cent among Syrian Orthodox followers; and from 83 to 45 per cent among followers of Greek Orthodox.

Why our romantic choices matter

Beyond the personal repercussions, your choice of life partner has profound social, economic and genetic consequences.

"Understanding how we choose our partners is core to the human condition," Dr Conroy-Beam says.

"It affects every single part of your everyday life but also, on a grand scale, when you're thinking about human behaviour from an evolutionary perspective … there's very few things that affect that more than mate choice."

Genetically, couple matching on specific attributes increases the proportion of people with extreme levels of the attribute.

In the US and UK, for example, studies suggest couple matching between overweight people may have contributed to the obesity epidemic. One model found that switching from random matching to complete matching on obesity had the potential to more than double the percentage of obese people in the population within just two generations.

Overlapping social, economic and genetic impacts can also lead to the certain traits or diseases becoming concentrated in particular social classes or groups, in turn leading to greater inequality. Alzheimer's, for example, is more common among lower socio-economic groups, says Macquarie University geneticist Loic Yengo.

"So we can imagine that assortative mating on [socio-economic status] will contribute to the increase in prevalence of those diseases," he says.

Socially, couple matching has a powerful influence on class stratification and social inequality, which are linked to social conflict. In the same way that couple matching increases the share of people with extreme levels of a genetic trait, so, too, does it increase the share of people at the top and bottom of the social ladder.

Numerous advanced economies, Australia included, are witnessing the rise of education and income "power couples" formed by two high-status spouses, whether measured by income, occupation or education. The broader societal ramifications are still playing out but in the US, researchers have estimated that up to half of the increase in income inequality is linked with increased couple matching on earnings.

The effects carry across generations, since parents tend to pass advantages to their children, entrenching inequality and further reducing the chances of intermarriage.

Lastly, the question of who we share our lives with is a key measure of how deeply different parts of any society have truly accepted and integrated with one another.

"[Marriage is] really the ultimate connection between people within a society," Professor Sherkat says.

"It's the real test of whether or not groups of people within a multicultural society are connecting with one another."

Georgia Brazier, 29, says it's definitely an "opposites attract" situation with her partner Pat. She describes his heritage as "exotic Macedonian/Czech" and her own as "standard white Australian".

That's just the start of their differences.

"Pat is an adrenaline junkie and I am not. He codes and I teach … he plays lead guitar in a deathcore metal band. I run the art club at lunch at the school I work at. He races cars on the weekend, I happily work in the garden. He loves horror and gore, I love Disney and Paddington 2," the 29-year-old says.

But the pair share core values around "the essential things: money, lifestyle, love, health," she says.

"We have just enough in common to bind us and just enough that sets us apart to make things interesting. I love the growth and change in me that he has inspired naturally, and he says the same about me for him."

Georgia and Pat's story is among more than 500 messages I received in response to a call-out published at the start of this project, asking readers to share their love stories.

In these tens of thousands of words about love, two recurring themes stood out.

The first theme was shared values. The youngest person who wrote to me, Trung Duc Nguyen from Thomastown in Melbourne, had this to say:

"We met in high school, at a sport festival. We got different hobbies. I'm more an outdoor, active person while they like to stay inside and watch Netflix," the 16-year-old wrote.

"[But] we share the same beliefs and certain values about relationships and love, which makes me feel connected to them."

The second theme was accepting the good with the bad. Or, in the words of Liza Newby, one of the oldest people who wrote to me, having the wisdom "not to sweat the small stuff!"

The 80-year-old from Mallacoota, Gippsland, shared what she described as her "MALS — mature age love story".

"My husband and I got together when I was nearly 60 … we are now 80 and 82 respectively — been together 21 years. Our next 'challenge' will be the inevitable decline and illnesses of old age, and one of us losing the other to death," she wrote.

"If your project is looking at love, don’t forget us oldies. I still thrill when my man walks into the room and our bedroom is not just for sleep!"

Now that sounds like a love worth waiting for.

This is the fifth and final article in a series of data stories looking at why and how we choose our partners. Our first story analysed who you'll probably marry, based on your job. Our second story examined the most and least likely religions to marry. Our third story revealed how men "marrying up" became the norm. Our fourth uncovered how university degrees became a dating deal breaker.

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Posted Sat 18 Jul 2026 at 5:04am

Sat 18 Jul 2026 at 5:04am