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I have a sapient (alien) species that behaves more or less similarly to humans, and that includes their romantic interactions. However, although they court/date each other much like we do, there is a certain mechanic that shapes their cultural understanding of romance. If a courting pair of these creatures lock gazes and share a warm fuzzy "love struck" moment, as we might describe it, their pheromone production spikes, and the ensuing surge of endorphins is so strong that it rewires their brains permanently, devoting the two individuals to each other for the rest of their lives. Pairs who have had this bonding moment have zero interest in infidelity and simply can't fathom getting a divorce, no matter how much they might argue. Non-bonded pairs exist too, but they are much rarer than bonded pairs and are generally either judged or pitied by the rest of society.

The question is, what external stimuli could cause this soulmate mechanic to be favored by evolution? Right now it seems to me like it only serves to make individuals grieve all the harder when their mates die, because they can't "move on" the way humans can. Any suggestions?

EDIT: Further details about the species:

  • The bonding process takes several seconds, and the individuals have some control over whether they want to allow it to complete. Often they're too swept up in the sensation to care, but it's possible to "snap out of it" if they recognize the sensation early enough and don't feel ready to commit.
  • The bonding moment typically happens after the couple has either been mutually crushing on each other or outright courting/dating each other for a while. There are rare cases of it happening at first sight, but these cases are usually one-sided and are considered to be foolish mistakes.
  • If one of these creatures loses their mate, either to early death or because they made the mistake of a one-sided bonding, they are capable of mating again with another person...they just typically don't want to, because the thought of betraying their mate would make them quite uncomfortable. Their biological urges to have sex and/or children may be strong enough to nudge them into taking a new mate anyway, but they'd most likely feel unsatisfied or even guilty.
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    $\begingroup$ If you want a little more on this, you might look into this wikipedia page on r-strategists and k-strategists. $\endgroup$ – Roland Heath Oct 29 '15 at 22:20
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    $\begingroup$ can't post a full answer without the above questions answered, but one thing you need is for the species to be able to, with 100% certainty, detect cuckolding. The male has to always at the moment of 'birth' know if he is the genetic father, and presumably attack the mother or children if he isn't harshly. This is the only way to make an obligatory monogamy remotely plausible compared to pair-bonding with cheating option animals of today take. Make cheating always result in losing parental care, and possible young to angry male, and 'true' monogamy becomes an evolutionary possibility... $\endgroup$ – dsollen Oct 30 '15 at 0:59
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    $\begingroup$ A long time ago I read about a theory (was it Bronowski?) that romantic love does indeed have an evolutionary advantage. The idea was that people tend to fall in love with persons whose gene sets complement their own, so as to produce optimal results when combined. So if a mating pair has good compatibilty they will improve the gene pool every time they reproduce. So staying together is good, and the same would be true for all animals. $\endgroup$ – RedSonja Oct 30 '15 at 9:51
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    $\begingroup$ I'm afraid I have to agree with Agent_L. From an evolutionary stand point making children is sort of the only reason we exist. Passing up a chance to create more children without somehow increasing the fitness of existing children (or occasionally non-children family members) is an evolutionary dead end. You could say that it takes a significant amount of effort to raise a child alone and so they don't take new mates for awhile due to the increased burden of caring for existing children without their mate, but they would take a new mate after existing children mature or they don't evolve. $\endgroup$ – dsollen Oct 30 '15 at 17:25
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    $\begingroup$ World Building Stack Exchange - casually tackling the most difficult topics facing humanity one question at a time. $\endgroup$ – corsiKa Oct 30 '15 at 21:27

12 Answers 12

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For raising children.

Let's say their world was rather hostile. Early in their evolution they could be attacked at any time. Having two parents to protect the young was therefore safer for the children. That is, parents who stay together will more successfully raise children.

Once they've raised children that bond is no longer required, but there is no significant evolutionary pressure to stop being partners. They've already passed the pair bonding trait on to their children who will pass it on to their own children.

It's especially likely to be as strong as described if the children are vulnerable for a long period. Only the very strongest pairs would stay together and would therefore be slightly more successful in raising children than those who did not.

EDIT:

There seems to be a lot of confusion here. This answer provides a possible reason that these creatures mate for life. There is no guarantee here. If you have an alternate mechanism or believe there is a way to guarantee pair bonding without possibility of cheating, then post it as an answer, there is no need to post extended comments here regarding your theories of pair bonding for animals on Earth.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Nov 1 '15 at 1:21
  • $\begingroup$ And also grandchildren $\endgroup$ – Salmoncrusher Sep 11 '16 at 19:32
  • $\begingroup$ @John What about the advantage of continuing to mate with a partner that has already proved successful? The situation you suggest implies that evolution has hindsight, but that is not the case. Also, read the final paragraph, you seem to have missed it. $\endgroup$ – Samuel Sep 15 '17 at 19:02
  • $\begingroup$ @John Ah, I see. You're only considering the case where after raising children, one partner dies. If that was a significant portion of the time, then yes, you'd be correct. However, the vast majority of the time, the partner does not die. Not only that, but even in the cases where a successful mate does die, genes were already passed on that didn't require the ability to 'move on' (hence the hindsight issue). The 'another mate' offspring would be outnumbered and be born to older parents. I don't see how that's an advantage. Perhaps you should explain it in your own answer. $\endgroup$ – Samuel Sep 15 '17 at 20:12
  • $\begingroup$ @John Also, if what you're arguing for is "a different selective advantage", then I again encourage you to read the entire answer, including the final paragraph. Because, really, you haven't seem to have done that. $\endgroup$ – Samuel Sep 15 '17 at 20:16
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To prevent sexually transmitted diseases.

Among humans, sexually transmitted diseases can cause:

  • Infertility. (E.g., chlamydia)
  • Brain damage. (E.g., syphilis)
  • Birth defects.
  • Lots of other diseases. (E.g., H.I.V.)
  • Annoying itching and scratching. (E.g., lice, herpes, cold sores)

Syphilis-induced brain damage is suspected as a factor in one of the World Wars.

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    $\begingroup$ can you provide a source for that last claim? I hadn't heard anything like that, I'd be interested to read more but can't bring myself to google it! $\endgroup$ – jammypeach Oct 30 '15 at 16:13
  • $\begingroup$ A summary of the arguments about whether Hitler had syphilis is available at examiner.com/article/adolf-hitler-had-syphilis If you google the topic, you will find articles published by respected research institutions. $\endgroup$ – Jasper Oct 30 '15 at 17:12
  • $\begingroup$ I've wondered whether human nature today would have been greatly changed had AIDS emerged a few centuries or millennia ago. $\endgroup$ – nigel222 Oct 30 '15 at 19:12
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    $\begingroup$ Whether Hitler had syphilis or not, he didn't create the WWII by himself. Germany was in a tough spot, many Germans wanted revenge, there was tension between colonial powers, etc. Besides Hitler's Germany there was not just Italy (who both invented and entered fascism before the Nazis rise to power) but also Japan and half of Europe (who eagerly joined the Axis). Like WWI and then the Cold War (and two centuries of inter-European wars that preceded them), WWII was going to happen one way or another. Wars stopped between Europe's powers when the stakes got low -- as the US had took over. $\endgroup$ – Hejazzman Oct 31 '15 at 1:41
  • $\begingroup$ The enormous time lag between development of a social reaction to a biological problem is so huge that I think this is an impossible conjecture to support -- especially since many of the most common STDs are New World diseases, and yet a huge variety of living arrangements and sexual mores existed back to back throughout African, European and Asian history. Africa has not reacted to HIV, for example by becoming broadly monogamous. $\endgroup$ – zxq9 Nov 1 '15 at 9:13
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I promised an answer, but I already made half my points in comments. I suppose I will make one more one. Most people have given justification for 'monogamy' but not life time obligatory monogamy like you want, so presume the answers above all apply to encourage serial monogamy, what will force a life time pair bonding?

Paragraphs in parenthesis go into more detail explaining likely confusions about evolution in general, they better explain why an idea makes sense evolutionary if confused, but can be skipped for those familiar with evolution.

Honestly, as I already implied it's pretty hard to justify. I do have an answer, but unfortunately I'm sticking to evolutionary principles, and the best way I can justify mandatory monogamy starts from a rather unfortunate start so umm...My answer is rape...lots of rape...cough Hear me out first there's a reason here...

to be more accurate I'm referring to rape in the distant evolutionary past, particularly due to the resulting evolutionary arms race. Males try to rape females, females want mate choice so they develop means to prevent undesired pregnancy even if raped. This sort of arms race happens quite often, though less often in mammals.

A side effect of this sort of arms race can lead to females being less fertile, they are so good at preventing unwanted pregnancy they start preventing wanted pregnancy as well because their defensive mechanisms are too strong. Again, this is not unheard of and has occurred in many species.

( why this happens. In an 'ideal' evolution a female would evolve a way to prevent pregnancy from rape without preventing pregnancy from desired matings. However, evolution is not guided, or quite so 'smart'. It is very hard to evolve such nuanced defenses. Instead it's likely that if rape is harmful enough to female they will ramp up defenses against rape, as an undesired, but better-then-the-alternative side effect successful matings from desired mates may also drop by a smaller amount. After this happens an independent evolutionary phase may work to help increase odds of successful matings with desired partners. Many species have these mechanisms, most obvious being numerous species that will abort pregnancy if exposed to a foreign male due to the odds of a foreign male killing their child after born. This can cause undesirable abortions at times when the foreign male was not a threat of killing the resulting child, but it usually saves resources to abort and so it was evolutionary favored even if it sometimes harmed mating success. After such defensive mechanisms exist later mechanisms can evolve to help better distinguish which mating's are desired if defenses are hindering successful matings too much. Which brings us to the next point...)

So now the evolutionary problem to solve is how to allow the mates you want to mate with to have success when you have such strong defensive mechanisms. The answer is to have means to help accept a mate that your body recognizes. Pair bonding can be part of a process of creating not just a psychological but also physical connection to a mate which will actually help insure female anti-rape defensive mechanisms will recognize her mate's sperm and not attack it.

(To be more accurate this isn't a simple chronological progression from A to B. Instead this would be a cyclic process, where increases in A or B encouraged further increases in the other. As The first anti-rape defenses likely only prevented a small number of matings, say 10% of undesired matings and 4% of desired matings. The original difficulty with desired matings put pressure on evolving the first limited mate-selection methods. As mate selection methods grew better it lowered the 'cost' of anti-rape defenses, as they didn't hinder desired matings as thoroughly, and so these rape defenses ramped up further, encouraging mate-selection to further evolve etc etc. As those defenses further increased the need to better identify desired matings increased and so on, each time one trait grew more refined it allowed, or encouraged, the other trait until eventually both were fully formed.)

Mate selection mechanisms are not unheard of. Kissing in humans is not just cultural, it's evolutionary practice shared with many primates. There are now many evolutionary advantages to it (once something exists in a species they usually find multiple ways to exploit it), but the original cause of kissing was likely to help women be able to carry a mates child to term. In this case this was due to a very common STD that was dangerous if caught while pregnant or when conceiving, but not before. The best approach for primates to deal with it was was to intentionally catch it if you mate has it, via kissing and swapping saliva, when it was harmless, so that by the time your ready to mate you already have CMV and thus don't risk catching it when mating which could result in severe birth defects to the resulting child. The point being kissing, a romantic gesture now, started as nothing more then a physical act to help ensure a success when mating with a chosen mate.

I would do the same with pair bonding, it is a way of helping the body recognize the mate so that mating success will exist later. I would change one thing, remove pheromones (which radiate out from someone so you couldn't have pheromones work on one person and not anyone else near by anyways) and make the bonding a more physical act, something that allows exchange of DNA so the female's body & immune system can start to recognize the mate. The obvious answer would be to use the above example, kissing. Kissing is the start of getting DNA from saliva to start recognizing your mate. It's not love at first site, it's love at first kiss.

(note, it would have to be more then just kiss or a male can force a female to kiss him. However, if it requires a long continues length of time to build up the bond a rapist isn't going to hold a female captive for months to develop sufficient compatibility to mate)

The more mates are close together, and exchange err 'body fluids' through kisses or otherwise, the more the female immune system recognizes the male and the better the odds of mating success. Thus life-time monogamy may be preferable because the odds of successful mating increases the longer the pair is together.

From an evolutionary perspective a female may benefit from a system that causes a mate to spend extensive time with her before mating is guaranteed, not just to prevent rape but also to have more time to 'judge' the fitness of the male, to gain benefits from courtship (which likely involved male offering 'free' resources to the female to earn favor, and to encourage the mate to stick around to raise the child since they now have a better chance of successful mating with the female in the future. This is important because it would give evolutionary advantages for women to maintain traits that lead to pair-bonding even after the threat of rape was mostly mitigated.

I would couple this with other mechanisms to prevent cheating, particularly to prevent females from consensual matings with males. as I mentioned cheating is quite common in all monogamous species and would discourage such strong pair bonding in most species. Have males have a method of reliably recognizing their children with a very high degree of accuracy. If a female births a child that is not biologically the males he will usually kill the child immediately in a rage. To make this work you would need to imply certain specific markings are carried exclusively on the male chromosome so that the male alone controls the marking and can recognize his marking with others (there is more to this, ask another question if you really want a full description for how a male can be confident he was not cuckold). This removes the advantage for the female of cheating on the male, and male would have little advantage of cheating since the female will rarely conceive from a mating with someone they haven't bonded with first.

The strong ability to detect paternity could have evolved back when rape was common due to the increased odds of cuckolding due to rape for males, and would have further driven females to excessive biological mechanisms to defend against conception during rape due to the gaurentee that the child conceived will be killed at birth by her partner making conception from rape an entirely expensive waste with no evolutionary success.

Eventually the rape-tendency will go away as it proves to not result in children, due to female defensive strategies, but the pair bonding will stick around for awhile longer.

This would also slightly encourage what you wanted earlier, about a mate not trying to mate again after their mate passes. I stick with the statement that this would not normally happen. However, if the mates are older when one passes away the remaining mate may find it not worth trying to find a new mate because it would take too long to build the bond to the point where a mating was likely to be successful, and no opposite sex would spend that long trying to build up a relationship with an older mate that will die sooner leaving them in the situation of old maid unable to find a mate. Thus if your mate passes after a certain age the remaining mate may find it adventitious to instead focus on supporting his current children and family to help them achieve better reproductive success because no mate will choose to start bonding with them so it's not worth wasting energy trying to court a mate. Younger mates will likely still court new mates if their first bond dies young. Then again, if the bond builds over time they may not have the same emotional bond yet...

Keep in mind the above only works if a mate that gives up on finding another mate is also driven to help it's family members find reproductive success, not just giving up and sitting around doing nothing with their life. That likely means providing grown children and siblings with food and resource etc so they can focus on raising their young.

*note, this implies a larger sexual dimorphism between males and females, with females being smaller, at least back in the more past, which is why females had to defend themselves via biological methods instead of claws and teeth. If you really want the two sexes to be close to our level of dimorphism there are some tricks you can use to justify dimorphism decreasing during the time it took for pair bonding to evolve, due to unrelated evolutionary pressures...

Now, why this is still a lie... There are a few places where I'm bending evolutionary principles a bit to 'fit' the desired result, since as I said I don't honestly think this would evolve. I think this is a best-fit option and you likely can get away with handwaving the cheats, but I want to at least mention them so you are aware. Here is a quick list of my cheating:

Rape-mitigation strategies usually involve physical changes, not immune system

usually counter-rape evolution involve more complex vaginal tract that make the act of mating harder for rapists, or some way to stop the sperm of the rapist from entering the main reproductive tract. This is easier to evolve then some sort of immune system response to stop rape as in my suggestion.

To counter this I would combine rape-migration strategy with disease migration. Some strong disease forced heavily ramped up immune responses in the species about the same time that rape-prevention mechanisms were evolving. The 'accidental' result of the ramped up immune system attacking sperm had the side effect of working as a rape-migration strategy which it got selected for.

To make this work better I'd suggest an STD (or probably a collection of rapidly evolving/diverging STDs) being the cause of the ramped up immune system, to better explain why the ramped up immune system would attack sperm in particular. To make this work better you would need to explain why rape was still worthwhile to males if there was a non-trivial risk of STDs. Best explanation I can think of is a particular STD could be transferred from male to female via ejaculation but either did not migrate from infected female to male or, more likely, did not harm the male as much. For instance perhaps the virus only harmed a female's later reproductive success by making future implantation harder (but not ones resulting form the rape since the virus needs more time to spread before it has adverse effects). This would put the virus as a negative to the female but not the male, causing males to feel free to rape but females to develop strong STD immune system, that then also got specialized into an anti-rape defense.

Evolutionary pressures aren't enough to push such a strong extreme

This is my biggest cheat. All of this process could start as described, but probably not end like this. The problem is at some point women would become 'good enough' at preventing rapes that rape wasn't cost efficient, and at that point they would stop happening. This point would likely be before pair-bonding became 100% mandatory for successful matings. There are costs to this system as I defined it, and at some point rapes would be uncommon enough that they didn't warrant further opportunity costs, or simply didn't provide sufficient evolutionary pressure to get out of an evolutionary 'valley'.

There are some things you can do to mitigate this risk, though I like one most of all. I suggest adding a competing evolutionary cousin living close to your species, one that is still very rape happy. The cousin species would attempt matings with females of your chosen species. These matings could lead to pregnancy, but rarely fertile children.

This helps to ensure that females always need anti-rape defenses, because even after males of their own species have evolutionary pressure to stop raping the cousin species, who's females do not have sufficient anti-rape protections, may still maintain rape as a successful mating strategy and thus the males of the cousin species may continue to rape females of your target species.

The males of the cousin species would not be as tempted to mate with females of your species, Kinophillia in general would make the females less attractive and they would not be gaining any major evolutionary advantage from it. However, they would not be suffering any significant cost for the attempted rape either, meaning there is limited evolutionary pressure towards the cousin species males evolving an instinct to avoid raping the females. Put another way raping of females (the ones of their species) works well enough for there to be strong incentive to evolve a 'attempt rape of any available female' instinct, and there may not be enough of a cost to failed rape attempts for males to evolve a more nuanced understanding of which females are worth mating and which are not.

The males may also gain some secondary evolutionary advantage from attempted rapes of females from your target species. For instance perhaps it serves as practice for attempted rapes of females of their own species, with the learning experience being enough to incentivize attempted matings continuing.

This isn't a perfect solution, since it's likely that females of your target species would evolve simply evolve a means to prevent pregnancy from ever occuring from matings with the cousin species (Since there is a difference in DNA between the two species any number of mutations may prevent hybrid children from being viable but not affect success of matings with males of the same species). Still, this could place an extra layer of pressure on the females at the same time attempted rapes from males of the same species, even if decreasing in frequency, was adding it's own pressure to help further push towards stronger immune response.

For this idea to work the cousin species would have to have once been reproductively isolated, long enough for them to split off as a separate species, but that they now are less isolated. They likely were isolated for a relatively short time, thus the reason they still are similar enough to keep kinophillia low. This suggests either one species migrated away from the other, evolved into a new species, and then migration brought the two back into contact. Alternatively something 'temporarily' isolated the two species but limiting their ability to travel. For example after an ice age a river fed by glacier ice water grew too fast to be able to cross, separating the species, then after all the glaciers melted the river flow dropped down enough to make migration across the river easy again.

A separate solution from the cousin-species one is to use my CMV example above. Make mate-selection partially about getting immune systems in sync to counter virus or disease. If you do this the original mate-selection process may not be just encouraged by rape by by immune system merging. I'd use both options honestly, you need all the excuses you can to encourage such a strong evolution.

There is no reason for both males and females to have anti-rape techniques

If males could successful identify cuckolding and kill the child then there would be no reason for rape as the child would not survive. It doesn't make sense for both female and males to have such well evolved anti-rape techniques since either one alone can make rape evolutionary disadvantageous.

This is partially fixed by saying males are not always able to identify cuckoldry, or sometimes are less confident of when they identify it. while they sometimes kill children they are certain are not theirs, they also often abandon the female with the child, when they strongly suspect, but can not confirm, cuckoldry occured. The combination of possible infanticide with abandonment combine to put a heavy cost on females for having a child conceived from rape while giving enough of a chance for the child to survive to adulthood that rape may still be evolutionary advantageous to males.

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    $\begingroup$ I really rather like this! I must thank you for such a thorough, detailed answer even after all the comments already. One question, what evolutionary advantage is there for the female body to shut down a pregnancy? Not poking holes in your theory, I'm just curious. $\endgroup$ – Fuzzlewuzzlekins Oct 30 '15 at 19:05
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    $\begingroup$ How would "preventing wanted pregnancy" be a successful evolutionary mechanism? The trait which would be more successful and passed on would be the inability to prevent pregnancy, not the other way around. $\endgroup$ – Samuel Oct 30 '15 at 19:15
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    $\begingroup$ Basically, it's better to take an extra 4 months of mating with your partner to have a success then have a 60% chance of conceiving via rape and having your partner kill the offspring at birth and you lose 9 months and a huge caloric investment. Thus it's better to make all matings harder. Ideally you would just prevent successful mating from rape without the other, but it's pretty hard to be that...exact in non-guided evolution. So creating a mate-selection method is the solution. Tell you body what mate to select, exclude all others... $\endgroup$ – dsollen Oct 30 '15 at 19:21
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    $\begingroup$ Do you have a source for the Kissing = STD Prevention? If you do, I'd like to see that piece of interesting research... $\endgroup$ – Malandy Oct 30 '15 at 19:58
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    $\begingroup$ Perfect! Start with world with super-strong parasites, and necessary strong immune system. So strong that female immune system could kill sperm if raped, and needs her immune system to be "primed' to accept chosen male's sperm. Of course priming will be more involved that just passionate look into eyes. It would be more like vaccination. $\endgroup$ – Peter M. Oct 30 '15 at 21:56
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Make the connection get stronger over time

At first, the connection is just a deep bonding with no other clear advantages, but with time, the creatures become gradually accustomed to each other's pheromones, subtle body language, ect. These things are unique for each member of the species and are able to quickly convey complex ideas. The bonding attunes the partners to learning to communicate with each other non-verbally, but the learning takes long enough that it's prohibitively expensive to start over with another partner. Their minds are only complex enough to extensively catalog one other person's idiosyncrasies, so polygamy is out of the question.

Partners who have been together long enough might be indistinguishable from mind-readers. This amounts to huge productivity gains. Partners often have the same job and go to work together. Losing your teammate confers a feeling of nakedness/emptiness and makes you much less productive. Societies evolve to have disdain for people not in an effective partnership.

External pressure for this evolutionary path might be that the species engages in a lot of activities where verbal communication isn't possible. If the species spends a lot of effort hunting highly-intelligent prey, having a mind-reader to flank with would be very valuable. Diving would be another example (are they amphibious?).

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  • $\begingroup$ Ooooh, now this is a creative answer! I'm not immediately sure how applicable it is to this particular species (to answer your question, they're mammals that evolved in the desert and are best suited to digging/burrowing), but I'll definitely mull this one over some more! $\endgroup$ – Fuzzlewuzzlekins Oct 30 '15 at 17:43
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I'll add that if the children stay with the parents (pack animals,eg) the "selfish gene" theory comes into play. Since a sibling shares much of your genetic variability, it is genetically advantageous to support and nurture your siblings. Half-siblings do not share as many genes and the advantage of supporting them is reduced.

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Shared responsibility for child rearing or greater female selection

In the human species, women bear most of the cost of child-bearing. For a woman, creating a child is difficult, dangerous and expensive. It involves 9 months, millions of calories, pain, blood loss and significant risk of death.

For a man, on the other hand, generating a child takes a few minutes of effort and a teaspoon of gamete cells. A man can potentially father many thousands of children in a lifetime. A woman is limited to 20 or so at the absolute maximum.

Women are therefore much more likely to be choosy about a mate. The risk of taking on poor DNA or abandonment is much higher.

Evolutionary pressures

This disparity creates several evolutionary pressures:

  1. The woman wishes to choose a mate who is stable and committed. Her children will be more likely to be looked after and will enjoy a survival advantage. She wants a good man.
  2. The woman wishes to bear children by a man who is promiscuous. Her children will be more likely to sire many children by multiple partners, spreading her DNA. She wants a bad boy.
  3. The man wishes to appear stable and successful to attract a mate who will be committed to him. The successful male.
  4. The man wishes to appear wild and promiscuous to attract multiple partners to spread his DNA widely. The rock star.
  5. The man wishes to bed other women who are already in a relationship, so another man will expend effort raising his children. The woman looks for the bad boy. Marital infidelity.

Broad vs. Narrow

So there are several valid strategies. Do we spread our DNA widely, do we focus on a few children, or does a male attempt to pass off his progeny as another male's progeny?

Is it better to spread seed widely, or to focus on a few offspring? What are the costs and benefits of each strategy?

The woman will evolve to choose the partner or partners who will be most effective at spreading her DNA, as will the man. The difference between sexes is caused by the disparity in the cost of child rearing.

On earth currently both strategies are in balance so both are extant in modern humans.

The solutions

  1. Increase the cost of infidelity. Make infidelity more obvious. Perhaps pheromones, or a distinctive inherited callsign. Perhaps infidelity is punished severely. Do this for a few hundred generations and the gene will disappear.
  2. Increase the cost to the male of child rearing. This might be a calorific cost or a cultural cost. The male can no longer produce lots of free children, so will choose more carefully and commit. The pressure on the woman to choose a bad boy will be reduced as the bad boy strategy is less successful.
  3. Decrease the survival chances of unguarded children. The male selective pressure to procreate widely is reduced if almost all children, not guarded by a male, do not survive. The female selective pressure to choose bad boy genes is also reduced. The dangerous environment need not be current, it only needs to be in the recent evolutionary past.
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As I understand you want a society that's very much like humans except for being truly monogamous - so here's an idea on how to not solve, but circumvent the issue:

Behavior-altering parasite

Not a total takeover by hostile intelligence, like the one SF novels. Something very real and well known, Toxoplasma Gondii, that subtly nudges it's host in the direction it wants.

For those who're not yet familiar with our unseen overlords, Toxoplasma infect rats and then wants to move on to a cat. How to do it? Just turn a rat into careless risk-taker - and a cat will come along. Of course, it can't tell apart human from a rat, so infected humans get more courageous and underestimate risk just as well.

Now this is just taking an issue from one place (your aliens) and putting it into another one (their parasites), however it has some advantages:

  • No longer your protagonists chose to give up breeding after losing mate. This choice is forced on them by another species, so it's more likely it can be made plausible and beneficial - for the parasite, of course.
  • The parasite might have evolved to control another species. Maybe one that lives shorter and in harsher, more monogamy-requiring environment. Then it dead-ends in your aliens just as Toxoplasma does in us and the lifelong monogamy is unintended side-effect.
  • Your readers are more likely to spot inconsistencies in design of human-like species (hey, if they are so much like us, why they do X so differently?) than to question your design of some "evil" protozoan.
  • If you chose to explain toxoplasmosis (eg. by a human scientist explaining the parasite to other humans) at least some of your readers will be so shocked by discovery THEY (eg 84% of France) are being mind-controlled, they'll stop caring about plot holes : )
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It could be a survival trait, if there was an evolutionary reason to need a partner you can implicitly trust.

For example, let's say there is a species of tree on the alien planet. The tree has a fruit that is both highly nutritious and highly delicious to your aliens. The fruit grows fairly high up, and by the time it falls is mostly rotten. And the trees do not have terribly large yield. Enough to feed two or three people.

But the tree also has a tentacle, with a poison barb on the end. The tentacle is quick, but only goes after anyone climbing the tree. That said, it is also not very strong, and can be restrained. So getting the fruit is most effective in a two person team.

The female aliens are lighter and more dexterous, making them better climbers to get the fruit. And the males, being stronger and heavier, have an easier time restraining the tentacle while someone gets to fruit. So said two man teams tend to be male/female.

Of course, the problem is that it is fairly easy to betray your partner. Then male lets go of the tentacle after the female has some fruit, letting her get stabbed and taking the fruit for himself. Or the female could throw the fruit to a third person, or just run from the slower male (who is still holding the tentacle) when she gets to the ground.

That said, not betraying your partner, and trusting them not to betray you, leads to a higher survival rate as both are happy and well fed. Over time, this bond of trust begins to become part of their DNA. Looking at someone, knowing them and thus knowing that you can trust them, it is a huge advantage in the harvesting. Such pairs tend naturally to mate, and thus over time the harvesting bond becomes a mating bond.

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    $\begingroup$ Game theory. This is the iterated prisoner's dilemma. Defect and take all the fruit, or cooperate, take half the fruit this time, but with the potential for more fruit next time. $\endgroup$ – superluminary Nov 4 '15 at 20:34
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Monogamy of any kind as a mating strategy in Earth biology is the simplest and easiest form of teamwork in a competitive society with two-gender sexual reproduction. The basic idea is that both the male and the female parent have the most vested interest in getting their offspring to maturity, and two providers are better than one, especially in harsher climates. On Earth, prolonged monogamy is actually fairly uncommon; virtually all species traditionally thought to exclusively mate for life, on closer inspection, "cheat" on a semi-regular basis and so are closer to serial monogamists. The majority of social mammals have developed a patriarchal harem structure; one or two sexually mature males, a larger number of females and their immature children. Those that pair-bond for a mating season tend to be serial monogamists.

There are two main reasons this is so:

  • Females have all the biological equipment necessary to carry, bear and care for the baby in its early life. That means the male's primary job is done in about ten to thirty seconds; if he sticks around after that it's as a guardian.
  • The best overall strategy for diversification in the genetic pool (the primary advantage to sexual reproduction versus everyone just popping out exact clones of themselves) is to have as many matings between different males and females as possible. That produces a large number of genetic combinations that are tested for fitness by nature to produce beneficially-evolved specimens.

That generally leads to four dominant strategies for males of Earth species:

  1. mate and die/be eaten (most insects/arachnids),
  2. mate with any willing female and then move on to the next as many times as you can until you die (most reptiles, solitary mammals and birds),
  3. build/take over a harem of females and mate with as many of them as possible for as long as possible, until another male dethrones you and you either die in the fight or live out your old age alone (most social mammals), and
  4. pair-bond with a female to guard and provide for your mate and the kids until they're big enough to fend for themselves, and then move on (a few mammalian and bird species).

Anthropologically, the rise of human civilization has given our species a fairly even mix of genders as of the age of sexual maturity. We look down on incest because it produces misshapen children, so we don't keep our daughters around to mate with, while kicking our sons out to fight another man to the death for their women. That makes monogamy the path of least resistance, with a few societies tolerating polygamy among the very successful.

The average length of human monogamy is notable primarily because the gestation and maturation periods of our species is so damn long. Most other mammals are able to fend for themselves in one to two years and sexually mature a couple more after that. Human children don't even start their sexual maturation until 12 to 14 years after birth, though they can become productive members of the society somewhat earlier, and really puberty doesn't wind down until the mid-20s. By the time a human child is really ready to strike out on their own, the parents are usually long past their sexual prime anyway. So, the human strategy is to stick together, have a bunch of kids, and raise them together as a couple, after which time you might as well continue to stay together for mutual support.

So, that would be the first evolutionary advantage to exclusive pair-bonding in some non-human species; if the species takes the majority of the adult lives of the parents to mature, that child has the best possible chance if the parents stay together the whole time to provide for and defend it. Any evolutionary trait that makes that more likely, such as forming a neurobiological bond, is a trait that will advantage the species as a whole.

Additional traits not seen on Earth that would require both parents might include some basic biological need for increased male participation in childbearing/childrearing. Picture a species that is otherwise humanoid, but the male has the mammary glands and/or some other necessary biological feature, like a marsupial pouch. The female carries and bears the child, but the father's required to be around to feed or care for it or else it will die. That makes the fourth male strategy the only truly viable one; all the others depend on the female to be able to handle everything else after mating. A biosphere with animal species that required significant participation by both genders would benefit greatly by hard-wiring an attraction to exactly one of the opposite sex.

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  • $\begingroup$ It could be something as simple as raising the child being taxing enough on the female that the female depends on the male to provide for them, at least intermittently. In that situation, if the male leaves the female and offspring, then the offspring (and possibly the female) dies, and the male has failed in his biological mission. (superluminary discussed a variation on this theme.) $\endgroup$ – a CVn Feb 22 '16 at 20:46
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It benefits the alphas at the top of the collective structure, not anyone down below. Actual behavior in historically "monogamous" societies tend to reflect this. Consider that at the height of the Catholicization of Europe ordinary people had monogamy imposed on them based on a rather creative interpretation of 1 Corinthians, chapter 7, the ultimate outcome of which was that there were three tiers of society, and only two were permitted political power:

  1. The clergy were to remain unmarried "as Paul was" which implies celibacy (otoh, with great power comes great privilege). The church wielded profound political authority, but this was tempered by the fact that establishment of dynasties was problematic, as unmarried clergy were unable to have legitimate offspring under typical circumstances (consider, though, the interesting tales, political and genetic, of Popes who have been named Alexander)
  2. The ordinary peasantry, ungraced by God and limited in privilege in almost every way, but absolutely necessary to the maintenance of every nation, were permitted monogamy.
  3. The royalty, extremely wealthy, various forms of aristocracy and other elites were ostensibly to live under the same rules of monogamy as everyone else -- unless they didn't. And very few did. Fascinating processes for absolving marriages, overlooking multiple marriage, flexible rules regarding incestual unions (by varying definitions of "incest" depending on the case), legitimizing bastards, and simply overlooking extramarital unions (or even celebrating them publicly) is the norm throughout history.

This is just Europe, but we find parallels all over the place, in every period. Consider that today a large patch of the "conservative Right" (an hilariously misleading label, but whatever) supports Donald Trump, of all people, but he certainly is a man who has found it perfectly reasonable to selectively breed with many of the top lines within his scope of preference. And evidently people think that's just great. The most staunchly monogamy-interested section of the political landscape is this guy's voting base (for some this particular issue is a hangup, but the majority seem to just overlook this entirely).

So its not about "evolutionary benefit" in human genetics as much as "social control" in human society. At the level of structural evolution of societies there may be some significant benefit, but that is a tier of evolution entirely above chemistry, and one we refuse to allow ourselves to understand well yet. I don't think we've ever seriously studied that without projecting our own politics into it (and corrupting the results).

I imagine in terms of letting genetics experiments occur and die -- pure organic progression of the species -- monogamy is probably a bad thing, especially in the case of low-birth rates (each new person then becomes a one-shot recombination experiment). In absolute terms, prohibition against incest is a similar problem -- but its not like we're going to treat people like animals and inbreed them, culling the bad ones until we actually purify a desired set of traits.

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    $\begingroup$ I don't really understand what you're trying to convey. Is that alphas at the top benefit from monogamy? It's exactly otherwise, monogamy hinders the alphas - weak males benefit from monogamy the most. Unless you're trying to say that those at the top enforce rules they don't obey - but that's a truism valid in all areas, from marriages through taxes to land ownership. $\endgroup$ – Agent_L Nov 2 '15 at 9:27
  • $\begingroup$ @Agent_L Yes, elites don't follow the rule, which is exactly what I stated. It is, as you indicate, a case of rules being made to be broken: constrain the plebes, enable the elites. The OP's question is whether this has a genetic benefit, and I say "no", I believe it to be a distinct evolutionary hindrance to the species as a whole -- but it may have some positive (or negative) effect on the evolution of societies, but I have yet to see a study (or even speculative paper) on this idea that wasn't motivated by some prior moral position (and accordingly slanted to support that position). $\endgroup$ – zxq9 Nov 2 '15 at 9:55
  • $\begingroup$ Monogamy is one of pre-programmed (evolved) human breeding strategies, so it's much more than just a social construct. Social construct it to force this particular choice upon rest of the population (but that's one of the reasons society exist in the first place). And this forcing is not even very successful, as it's estimated that 20-30% children are sired by different father than officially stated. $\endgroup$ – Agent_L Nov 2 '15 at 10:26
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    $\begingroup$ But your argument in this particular case doesn't make any sense. Polygamy directly benefits the successful individuals (elites), and takes women away from the least successful (peasants). Elites would have much better if they could practice polygamy in the open. The problem is that peasants would die out. $\endgroup$ – Agent_L Nov 2 '15 at 10:28
  • $\begingroup$ @Agent_L That little bit about 20~30% of children being fathered by another man conflicts rather strongly with your statement that "Monogamy is one of pre-programmed (evolved) human breeding strategies". As for your second comment, elite do practice polygamy, it is just not called that because it tends to not be official marriages that are involved. Pharoes, Caesars, Greek patriarchs, Semetic patriarchs, European royalty (Henry the VIII being a peculiar example, considering the regulatory system tie-in), Donald Trump, the Khans, etc. $\endgroup$ – zxq9 Nov 2 '15 at 10:30
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Make the cost of breeding equally high for both parties.

As several people have pointed out, being choosy about your mates is advantageous for females, while being blindly promiscuous is advantageous for males, the upshot of which is that it is evolutionarily most advantageous to appear to be monogamous but cheat when you can get away with it. I don't feel the need to repeat the full explanation; others have covered that quite well.

However, you are talking about an ALIEN species. If males (for lack of a better term) can ALSO only successfully mate a certain number of times in their life, like earth mammal females, then they are going to evolve to be equally picky about their mates. Maybe the females carry the infant to term but then the male nurses it. Maybe the males don't have re-usable penises, but have something like a bumblebee's stinger which is used to deliver sperm, and after mating they need to heal and then re-grow the organ over a period of months. If the males have a high enough investment in conception, the successful strategy will be to stick around and ensure the survival of the offspring, rather than just sow widely and hope for the best.

Allow both partners to quickly know when conception occurs

There is one type of cheating that the above doesn't address, which is cuckolding. If a male can impregnate a female who will then get some OTHER male to help raise your offspring, that's still an evolutionary win, so, as others have mentioned, having males be able to recognize their own offspring beyond reasonable doubt is a good way to prevent cheating by partnered females. However, as was also pointed out, that's really hard to justify. However, I think it would be a lot easier to justify being able to recognize that conception had occurred. Then you just apply logic: if your partner is pregnant, and conception did not occur while (or shortly after) you were shagging her, then the kid is not yours.

Combine with other answers and shake well

The above only really gets you as far as serial monogamy, rather than life-long monogamy. However, if you use some of the other ideas for why they would keep the same partner for life, raising the cost of conception for the male enough, and reducing the chance of cuckolding going unnoticed, should strongly reduce or eliminate the advantage of cheating.

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  • $\begingroup$ Although promiscuity is advantageous to an individual's reproductive success, monogamy in the other parent is more advantageous in species where the father is able to support the offspring. $\endgroup$ – EvilSnack Oct 15 '16 at 23:03
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The advantage lies in the fact that a single partner will result in many reliably strong children throughout their lives. You don't have to worry about possible unexpected consequences of a new partner, because you already know that one partner well.

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