We all know at least one couple who just make no sense, a perplexing pairing that irks and boggles us. One of them might be vivacious, gregarious, classically attractive and yet the other is a little freeze ray of misery and seems to despise socialising. So why, oh why, are they together?
Neuroscience, according to Dr Hannah Critchlow, may have the answer to this eternal question, and plenty more - why we get more opinionated and closed minded as we age, how our political leanings are formed, why some couples don't even seem to like each other much.
Critchlow, a British researcher, writer and broadcaster who has been described as "the female Brian Cox", lays out in her brain-bending book - The Science of Fate - just some of the recent research into determinism and the theory that we don't so much make our own decisions as inherit them. The fast-moving field of modern neuroscience will, she believes, "one day be considered as profound as Darwin's Theory of Evolution".
So, what can the brain tell us about the laws of opposite attraction? Well, there's a lot more to love than what meets our eyes, and it may well involve senses we didn't know we were even using.
"Scientists used to believe we only had five senses, but we're finding more and more we didn't know we had, through experiments, all the time," explains Critchlow, who found herself "happily stuck" in Noosa by the coronavirus lockdown while on an Australian book tour.
One fascinating trial, carried out at the Zoological Institute at Bern University and later replicated in the US, showed that women may actually be turning the smell of potential male partners into complex information.
Researchers asked men to wear the same T-shirt for a few days without washing, deodorising or eating smelly foods. A group of women were then given the appetising task of sniffing the shirts and rating them for attractiveness.
The results clearly showed that women would choose the odour of men whose immune systems were highly differentiated from their own. Finding a mate with different gene variations from your own produces the strongest possible offspring; a child with the greatest resistance to a wide range of infections, and thus the best chance of survival.
Just how women were able to detect their biological ideal man using optimum genetics via the smell of a stinky armpit is "quite mysterious", as Critchlow understates it.
"But we are, at some level, just animals, driven by the single desire to interact in a way that will pass on our genetic material," she says.
We are, at some level, just animals, driven by the single desire to interact in a way that will pass on our genetic material.Dr Hannah Critchlow
"Love, it seems, is largely a by-product of the brain circuitry that prioritises reproduction and the survival of the species."
Interestingly, the sniff test does not work with men, but boys are not without their own mysteries. A study of thousands of lap dances in the US found that strippers would make almost twice as much in tips on the few days when they were at the most fertile point of their menstrual cycle. Somehow, the men just found them more attractive on those days, without having any idea why.
"When it comes to sex, it seems that a choice that may feel highly personal and deeply intimate is, to a large extent, the behavioural result of our brains' coding to seek maximum opportunities for our genes to be passed on," Critchlow says.
Like many of her colleagues, she has come to accept that many of the choices we make are hugely influenced by the genes given to us by our parents, and our grandparents' parents. Even the foods we like are choices driven by what our ancestors were eating, and enjoying.
"Basically, we are designed to eat food when we can get it, because there might not be any around tomorrow, but now we live in a world where many of us can have whatever we want, whenever we want it, which obviously leads to obesity," she says.
"Genetic mutations to encourage eating less weren't passed on because food was scarce and there was no advantage in that. Mutations that made us eat as much as possible in case there was no more are a problem now that we live in abundance.
"Evolution has not caught up with Uber Eats."
The reassuring sense we have that we are making our own choices is "just our brains messing with us", in much the same way that we like to perceive the sun as "rising" and "setting", when we know, scientifically, that it is just the world turning.
"There is always scope for changing your mind, this is the basis for consciousness, but it's not as big as we perceive it - that scope to change is limited based on the genetics we've been given," Critchlow says.
"Remember that our brains use 20 per cent of our daily energy quota to fuel this enormous circuit board, and to save energy your brain filters a lot of information, and makes assumptions, based on past experience.
"Judging people in the first few minutes that we meet them is all about saving energy.
"With friendship groups, or clans, people look for individuals with a similar outlook and who have similar genetics as well (unlike the way they look for sexual partners).
"You are drawn to people, friends, who are genetically similar to you, so you are more likely to see the world in the same way and have the same biases.
"You're saving energy because you don't have to explain things."
Speaking of biases, just think how reassuring it would be to discover that people who hold political views that strike you as unjustifiable were just born that way.
As Critchlow puts it, understanding that people believe in certain things, like religion or politics, because their brains were built that way, "might have massive consequences for reducing conflict at every level - as we discover more about the neurobiology of belief formation and prejudice, we might be able to boost our openness to new ideas".
She quotes the work of Jonas Kaplan, Professor of Psychology at USC's Brain and Creativity Institute, who has found that activity in the amygdala, and the size of people's anterior cingulate cortex, can be used to predict whether they are liberal or conservative.
His researchers were able to use brain scans to predict the political leanings of American test subjects - whether they voted Republican or Democrat - "with high sensitivity and accuracy".
"It's quite incredible and it does help me to understand people a little bit more, because those who are more liberal have a less-sensitive amygdala are more able to think about collaborations and partnerships for the future, rather than being scared in the moment," Critchlow says.
"Conservative types have a more reactive amygdala, and that gives them a heightened reactivity to fear. They assess risks and react conservatively.
"But the fact is, both types of people are really important for our survival as a species. If we were all one type it would be a disaster, we wouldn't have moved forward as a species."
This, of course, raises the interesting quote most often wrongly attributed to Winston Churchill: "if you're not a liberal when you're 25, you have no heart , if you're not a conservative by the time you're 35, you have no brain." Why would people's leanings change as they age?
"There's been some research at Oxford into that, following people from the 1960s to see whether they got more conservative as they got older, and it showed a 20-point increase in conservatism by the time they were 80 years old," Critchlow says.
"As you get older, you rely on more tried and tested routes within your mind, there is slightly less potential for plasticity, so you might become more risk averse.
"You also start to weigh how you process information differently; you place less weight on signals from the outside world, and more weight on the internal capacity of your mind - the information you have stored there.
"In a way, older people are not really listening to new ideas, because they take too much energy. They're relying on their own, refined information. Or what we think of as wisdom."
Kaplan, from USC, provides the quote in The Science of Fate that most neatly sums up the way most neuroscientists now see the world, which sounds radical to most people but is, Critchlow says, very much the accepted wisdom in her academic milieu.
"I don't believe in free will. The universe is deterministic,'' Kaplan says.
"We aren't the authors of our own actions, because everything is caused by something prior."
He is aware, however, that unlike scientists, many people would find this idea hard to live with, and adds: "Decisions are partially controlled by our emotional state, and most people find it depressing to believe that they have little or no free will, so there is a lot of value in believing in it."
Critchlow says abandoning the idea of free will can actually be quite relaxing. She says she frets less about the way she parents her young son, because she's not sure there's much point worrying about it.
"I tend to forget that most people don't think this way and I was chatting with my agent recently and she said 'So hang on, you really think we're really just like machines?' And I was like, 'oh yeah, that's what all of the people in my little bubble of neuroscientists think'," she says.
"But I think it's an idea that will become more accepted, and it's starting to happen.
"Don't forget that Darwin's theories were pretty radical there for a while."
- Stephen Corby is a freelance journalist who has previously edited magazines, been a feature writer for national newspapers and manned a desk on Fleet Street in London. He began his career at The Canberra Times.
- The Science of Fate, by Hannah Critchlow, is published by Hachette.