Modern science has an aversion to the so-called anecdotal evidence. What is it? Well, all those stories about husband ordering black coffee while the wife drinks latte. She eats his salads, picks vegetables from his plate and fruits from cocktails, and instead he eats her rice or fries. He’s an introvert, she’s an extrovert. The husband votes for the left, and the wife votes for the right - et voila! – "you can see that opposites attract."

But it's not that simple. As well as the fact that although these days, thanks to the patient's genome sequence, we can afford to individualise the therapy, it is still a "case study" and this therapy cannot be generalised without serious research in many fields. However, the incredible development of IT tools has made it possible to compare the results of many partial research works with low statistical significance. And from them, large meta-analyses containing thousands of items were created. And so, especially in psychology and social sciences, many previous judgments turned out to be myths and began to collapse.

In this way, we were recently confronted with the myth about the attraction of opposites in love, as described by researchers from the University of Colorado in Boulder in "Nature Human Behaviour". The meta-analysis of genomic sequences and phenotypic features deposited in the UK Biobank was performed by PhD student Tanya Horwitz, from the team of the Institute of Behavioural Genetics lead by prof. Matthew Keller. The research covered over 130 characteristics and millions of heterosexual couples over a hundred years. In detail: analysts looked at 22 characteristics from 199 independent publications (the oldest from 1903) covering millions of couples, including co-parents, fiancés, spouses or household-sharing couples. In addition, they used data (including fully sequenced genomes) from the UK Biobank to examine 133 traits in almost 80,000 people - heterosexual couples of different ethnic or social origins from Great Britain. It's surely impressive.

Beauty doesn't want a beast, she wants a handsome man

This study shows that, after all, birds of a feather flock together - as the proverb puts it simply. “It turned out that in the case of 82 to 89% analysed features - ranging from political inclinations, through age of first intercourse, to habits of using psychoactive substances - partners were more often alike than not. Only in about 3% characteristics, the opposite phenomenon occurred", the university's press office summarised the study.

Particularly high correlations between heterosexual couples were found for political views (high correlation ratio: with 1 as the maximum it was almost 0.6) and religious views, education level and IQ ratio (according to some types of tests, because some are more sensitive to the gender of the test subject). Addictions (or total abstinence) also most often came in pairs. Interestingly, external appearance (height and body weight), chronic diseases and basic personality traits (the so-called Big Five, i.e.: neuroticism, extraversion, openness to experience, amicability and scrupulousness) showed much lower, but still positive correlations. Similarly to the rather rarely studied number of sexual partners of a given person or the fact of being breastfed in the neonatal period.

Researchers from Colorado, among the 133 analysed features, managed to find only three that had a negative correlation - that is, they may be opposites. These are: the so-called chronotype (i.e. one is a morning person and the other is an owl), tendencies to worry (which is probably linked to gender, occurs more often in women) and... hearing difficulties (on the other hand, men are more likely to be "deaf to requests to take out the garbage", as the local rumor has it - but we were not supposed to listen to rumours).

Among the reasons for similarities between partners, we cannot, of course, rule out coming from from the same area geographically, which marks us for life in terms of culture. It was also not possible to test the effect that their durability has on the similarities of people in heterosexual relationships. Because it can happen in two ways: the relationship is lasting because we were alike at the beginning, or it has changed us and we have become more similar to each other. There is even a witty proverb for the latter: “after seven years, marriage starts to resemble incest”. I must point out that the study only included the genomic characteristics of people registered in UKBiobank, and did not include the composition of their microbiomes. It is known that the microbiome influences our well-being, the development of neural connections, and during a relationship, which includes sharing a table and a bed, it becomes shared to some extent.

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  Of course, psychology has already questioned this common belief about opposites attracting. About beauties only waiting for their beasts. However, not completely - many, but weak, opposite features that attracted each other were found. Regardless of how diverse we seem to each other (and the shorter the period of time in a relationship, the less we know our true faces) and how attractive we find the features of a potential partner (on the basis of interesting exoticism or satisfying our own desires to be different), the attraction of opposites most often works at the beginning. Common interests and "going to the same kindergarten" (i.e. basics learned at home and generational experience) provide a firmer foundation for mutual attraction and a lasting relationship - said psychologists in the last decade, such as Ramani Durvasula, bestselling author of "Should I Stay or Should I Go?"

How to have the best relationship in the bedroom

Successful love relationships of more or less (rather less) total opposites do occur, but they need to be cared for a bit more rationally. At least to really realise that we are dealing with complements, not oppositions, and treat it as an opportunity, which is probably easier for people who are internally development-oriented or curious about the world and other people. Minimise confrontations, especially over trivial matters. The motto becomes “2xC” - communication and compromise. Finally, the ability of both partners to show deep respect for each other and the best possible relationship in the bedroom becomes a sine qua non here. The truth is that taking care of a relationship ensures success, regardless of how good we are. It's just that most of us would prefer it to play just like that, without any effort. And therein lies the problem.

As we discuss the American analyses, there are interesting consequences from the point of research on human population and evolutionary genetics. Well, if it’s not completely indifferent who we choose as a sexual partner and potentially as the parent of our children, then it is no longer accidental what these children are genetically like. And so far, all evolutionary models assume such randomness. Among humans, this is not as simple as in animals such as apes or wild horses, where there’s usually a male surrounded by his harem. This male became the main sperm donor for the group of females as a result of fights with other males aspiring to this role - and winning these fights. In a situation where the choice of a partner is free and faithful for at least one breeding season, it matters whether we match at random or similar with similar (which will strengthen certain features in the offspring). This determines which genetic variants will begin to accumulate in the genomes of subsequent generations.

So how can we account for this non-random accumulation in the genealogy of the human population? Eugenics and selective mating of people, even breeding pairs, have been consigned to the history books, hopefully forever. How can we ensure biodiversity, which is necessary for the population to survive all obstacles, from hunger to pandemics? In this last point, numbers are helpful. The more of us, the less problems. However, in closed communities, even for religious reasons, this can be a serious issue.

Finally, it should be noted that in today's world, education and the so-called cultural capital acquired from home is the basis for a chance of economic success. This means that the relationships of similar people, not princes with orphans, will again strongly and NATURALLY stratify the supposedly socio-economically egalitarian, post-revolutionary and post-industrial societies in the First World countries. Or even widen the existing gaps and... the percentage of educated and single, childless women who, although they would like to, cannot find partners at their level.

So what? All of us, proverb lovers and evolutionary biologists, will have to understand this phenomenon, contrary to popular belief, which has just been precisely, numerically analysed and described. One clings to its own, because, as Tevye the Milkman wisely observed in "Fiddler on the Roof": a bird and a fish may fall in love, but where will they build their nest together?

Of course, as it happens in psychology, every school has a theory of building relationships. What appeals to me is the theory of attachment, formulated by John Bowlby (who observed post-war infant orphans) and Mary Ainsworth. To summarise it briefly: attachment is a developmental process lasting the first six months of life, during which the child learns to accept separation from the mother (caregiver) and is able to separate, differentiate and integrate from her, thanks to which he is a separate individual. If during this time the caregiver ensures the child's safety but allows for autonomy (appears when called, does not restrict movements beyond safety requirements, etc.), the child develops social and emotional skills. If it is different, e.g. the child avoids caregivers because they were unavailable, so he or she becomes convinced that he or she can only count on himself; whether the child remains uncertain about the availability of caregivers and, consequently, always doubts his or her abilities; or the infant is exposed to conflicting, confused, and unusual behaviours from caregivers - harmful, lasting consequences for the offspring's social-emotional development are inevitable.

This also results in our attraction to specific people with a type of attachment similar to ours. At least that's what Marco Pistorio claims, as well as recent work in the field of neurobiology showing that our brain has been literally programmed during the first two years of development by the type of attachment we build with our caregiver. To put it bluntly: if we developed an insecure type of attachment as an infant, we will be attracted to that same type because it will be from our comfort zone. This is also the source of dysfunctional patterns of insecure couples repeated over generations. I would like to point out that here too, similar and similar attract each other. To escape from the vicious circle, you need to know your type of attachment and be aware of its consequences in relationships, trying to counteract them.

– Magdalena Kawalec-Segond

TVP WEEKLY. Editorial team and jornalists

-translated by Maciej Sienkiewicz