Why do we choose the partners we do, and why do we get flak about it from our parents? Professor Leif Edward Ottesen Kennair and Associate Professor Robert Biegler from the Norwegian University of Science and Technology's Department of Psychology say it comes down to simple genetics.
"We see a conflict between mother and daughter because of opposing interests," says Biegler.
The researchers knew this was the case from their research several years ago. They even know why, and named the conflict the "Juliet effect" after the conflict between Juliet and her mother Lady Capulet in Shakespeare's drama.
Juliet's mother hates Romeo
Juliet's mother would rather have Juliet marry Paris, who is from a good family. Juliet has set her sights on the heartthrob Romeo from the archenemy's family.
But what's new is that you find the same opposing interests between sisters.
Your sister would choose the steady fellow for you
It's the old story. The daughter of the house brings home the handsome hunk and proclaims that he is the love of her life.
But her mother prefers the respectable fellow with promising prospects, or maybe the rich guy from a good family.
As it turns out, your sister would probably agree with your mother, and would rather you have a steady, boring partner, too. This despite the fact that mother and sister would both rather have a hunk themselves.
Everything is ultimately about genetics and mathematics.
"For their own partners, women focus on an attractive appearance that suggests good health and an ability to pass on their genes. At the same time, they prioritize qualities in their sister's partner that can provide direct benefits for the whole family," say the researchers. "This is consistent with our previous studies where we compared mothers' and daughters' choices," they add.
Studied sisters
The context for this new insight is a survey that the researchers undertook among female students and their sisters.
Participants were asked to rank 133 different characteristics that described the perfect partner for themselves or their sister. A similar survey was conducted among mothers and daughters a few years ago.
"For the most part, women choose the same ideal partner characteristics for themselves as for their sister. The qualities of faithfulness, loyalty, honesty, trustworthiness and reliability score highest when women are asked who would make an ideal partner," says Biegler.
But some clear differences also emerged. "The women perceived characteristics like being understanding, empathic, responsible, helpful, sensible and kind as more important for their sister's partner than for their own," says Biegler.
Women found being sincere, humorous, charming, sexually satisfying and fun as more important for their own partner than their sister's.
Relative's partner must contribute directly
The reason is really simple. You are more closely related to your own kids than to your sister's kids or your grandchild. The transfer of your own genes is ultimately most important.
Shared genes
You are basically and obviously 100 percent of your genes. These genes make you just you. Your behavior may be modified by culture, but the genetic basis is not changed because of that. We say that r = 1.
You get half of your genes from your mother and half from your father. We say that r = 0.5. You transfer half of your genes to your child. Therefore you are as much related to your children as to your parents. Thus, r = 0.5.
Your child passes on only half their genes to your grandchild, i.e. half of the half that you share with your children. So you are more closely related to your child than to your grandson. The grandchild is therefore r = 0.25 and is as closely related to you as your sibling. For great-grandchildren and cousins r is half that again, i.e. r = 0.125.
You share so much genetic material with your relatives that you can't be blasé about whom they have babies with. They also carry on some of your genes and are part of what is known as your "inclusive fitness". But they can't get in the way of your own direct gene transfer.
"The ideal partner for your sister or your daughter can't drain resources from you and decrease the chance that your own genes can be passed on. Preferably he should directly increase your own chances. This can be achieved in part if your sister or daughter makes big gains by choosing a particular partner, and is able to spread your shared genes much more effectively," says Biegler.
But an advantage for your sister will rarely outweigh your decreased chances. Normally you want to have the greatest genetic advantage when a relative chooses a partner that can provide direct benefits for you, in terms of wealth or status, for example.
You don't want to spend money or other resources on raising your sister's or daughter's kids, unless it can bring you a considerable advantage in spreading your shared genetic material. And then you'd often rather spend the resources on increasing the survival and status of your own children, or have more kids yourself who can procreate.
"Women prefer for their daughter or sister to choose someone who can contribute to the upbringing of their own children and grandchildren, or who at least doesn't pose a burden," Kennair says.
This also means that the man should be trustworthy, take care of his children, preferably be strong financially and have a social status that does not diminish your or your descendants' chances of spreading their genes.
Your own partner may contribute indirectly
So why would you rather have a good-looker yourself?
"The underlying truth remains: passing on your own genes is the priority. The primary consideration is to find a partner who can give you attractive children who survive. They need to be attractive enough to pass on their genes to the next generation to the greatest extent possible," said Kennair.
That's why the muscular heartthrob is a more interesting choice than the boring geek for one's own partner.
"A healthy hunk is presumably in good health, attractive to others as a partner and can transfer those genes to your children," says Kennair.
Then your children might also be more attractive than if you choose the geeky nerd. It's nice to have a stable guy, but in the end you'll be drawn to the handsome man instead.
Trying to exert influence
But it's no sure thing that you'll end up choosing the heartthrob. Your mother or sister might try to influence you to choose a different partner than the one you like best. Yes, this happens even in our society where we like to think that we choose our own partner.
Whether you opt to listen to them is another matter entirely. That can depend on your own living situation, or if your family refuses to provide financial assistance or other help if you go for the heartthrob against their wishes.
Not a moral issue
Kennair and Biegler are moving into an area that often evokes strong feelings. But, they say, none of this is a matter of morality, only of passing on genes.
"People who haven't behaved according to this pattern have been deselected through generations. A larger proportion of them simply didn't get to pass on their genes to a new generation. So their contribution to the gene pool dwindled," says Kennair.
But for those who still want to look at it all through a moral lens, it just gets worse.
Latent in us
The best possible outcome, of course, is if the heartthrob you've set eyes on is also a kind and steady-as-they-come kind of guy with good prospects.
But there's no guarantee you'll just be able to pick one that has absolutely everything, you know. This perfect guy may prefer your sister. Or your mother. It may be part of the reason they won't allow you this heartthrob.
It could be that your sister would like you to choose another partner so that the heartthrob will be available to her instead. She may not even be thinking about it, and it's far from certain that she's actually trying to steal your guy.
The same underlying mechanism may even still exist in your mother, even though she is past her baby-making days. It lies dormant in both of them, just as it does in you.
This mechanism is a result of competition and has yielded the best results over generations, regardless of morality.
No one is saying that any of this is necessarily conscious. It is a result of genetic transfer through all the generations before you. Your mother and your sister are also out after the best possible partner.
Equal, but similar
Perhaps most interesting is that this also applies in a relatively egalitarian society like Norway, where women are largely financially independent and choose their own partners.
Today, Norwegian women can usually even provide independently for themselves and their children. But they seem to be attracted to partners with exactly the same qualities as the partners of women in countries where the family chooses their partner. In very few cultures do women have much choice.
"It's the exception for women to choose their own partners. In most cultures, it isn't this way," says Kennair.
In most cultures, the mother will usually get her way. But the researchers' hypothesis is that the stronger the parents' control is over their children in a culture, the stronger the conflict between the sisters is also.
"If you can't win over mom, you still have a chance to win against your sister. The less chance you have to win one conflict, the harder you have to fight to win the other," suggests Biegler.
That's why it is more important for you that a grandchild passes on their genes than that a cousin does.
This has nothing to do with morality. It is more or less pure mathematics. We assume monogamous relationships.
But even for independent Norwegian women, it can be an advantage if the partner doesn't take off and leave you with almost all the responsibility for the kids. This can also reduce your chances of effectively passing on your genes.
Maybe you would have liked to have more kids if you had been able to afford it. Or maybe your sons become paupers who don't get support from others' mothers.
"In the end, though, Norwegian women are more attracted to the good-lookers than the boring, kind and steady types-the same attributes that have been playing out for generations before us for the greatest genetic success," say the two researchers.
"Sisterly love: Within-generation differences in ideal partner for sister and self." Robert Biegler and Leif Edward Ottesen Kennair. Evolutionary Behavioral Sciences. DOI: 10.1037/ebs0000060. Jan 2016