So...societies regulate some behaviors of its individual members in a way that is beneficial to the society as a whole. But, since those who create and enforce these values are individuals, and seek to maximize their fitness, moral values beneficial to society as a whole (fidelity, truthfullness, will to sacrifice oneself for the group) are sometimes not obeyed by those powerful enough to ignore them.
Not just those powerful enough to ignore them. Everyone has the very real potential to ignore this particular rule. Actually infidelity, or at least infidelity leading to offspring is more common among the least powerful. False paternity in poor areas is 40% for example, averaged to 10% over the population.
Also, it is not so much individuals that create the rules. You can think of groups (societies, cultures, tribes.. whatever size scale you want to talk about) as being units upon which selection acts. They compete with other groups in the same size scale for resources, living space, sometimes people. Now these groups are made of individuals (or sub-groups at larger size scales but lets keep things simple).
The group-level competitiveness depends on how individuals act in relation to eachother. To use a thought experiment. Say you have an island with a bunch of groups on it. The groups that cooperate better will steam-roll the ones that dont. However, A person within a cooperative group that does not cooperate (thus gaining the benefit of the cooperation without expending his own energy to maintain it) will outcompete the cooperators, destroying the cooperative group from within over evolutionary time.
So, cooperative groups by definition succeed at developing systems to either force non-cooperators to conform, or make them go away. This is what ethics are, and what sanctions do. They force conformity or make the offenders leave (one way or the other. A third option is of course harnessing the uncooperative individual to serve a group benefit. See: the difference between communism, and a regulated capitalistic mixed economy. In the one the group tries to crush individual fitness and suborn it to group-fitness. In the other, the system itself uses the individual quest for fitness for group-benefit. ) Groups that do better at this will out compete groups that do poorly.
What most morality is, is this group-level stuff it regulates behavior that has group-level consequences. Like murder.
But, because morality is transmitted culturally, it can also regulate behavior that does not have group-level fitness consequences. WHich leads to cases where an action is looked down upon by society, even though everyone does it. I will use masturbation as a more neutral example.
When you masturbate, the frequency at which you do so optimizes the ratios of the different types of sperm you have for different kinds of sperm competition with other males. It is in the best interests of males to keep other males from masturbating, while continuing to masturbate themselves. So, people who masturbate are teased, etc. To shame them into not jerking off. It doesnt work... but it is an attempt to reduce your fitness while still maximizing your own.
Infidelity is similar. It by itself does not have strong group-level consequences(they are there, but probably not too bad). Some of the evolved countermeasures however do negative impact the society very badly. Jealousy, posessiveness, abuse, all of these are counter measures to infidelty that you see in humans as well as birds. And they have the same root cause. But I digress.
Infidelity in this analysis is like jerking off. It is viewed as undesirable, sanctioned agains but the risk does not outweigh the reward often enough to keep it from occurring at high frequency. And this is because of the trade off that is made in the attempt to suppress the reproductive fitness of others, while maintaining the ability to use the same methods to enhance their own. When this happens across a population, you end up with a rule that might as well not be there.
THis is not to say that everyone has or will cheat at some point. The decision to cheat is, at the level of ultimate causation and evolutionary cost-benefit analysis going on in your subconscious (what you are thinking is "Oh she is really hot... but I love my wife..." your brain is putting that into a Pro vs Con system) If someone has a really good mate they cant risk losing, they wont cheat. For example. If they find that individual early they may not ever cheat because prior to that they didnt have the opportunity, and now have no reason to do so. This is part of the area where I didnt think things through completely in prior posts.
Just because individuals will try to fuck other members of society over doesn't mean they should
Whenever you ask the question "what should someone do" you have to ask the question "where do we as a society get that ideal and what purpose does it serve?" In the end, I am not trying to say what is or is not desirable. I want to answer that fundamental question. Evolutionary theory cannot prescribe morality. But, I think it can explain it, and answer some questions about why some of our moral rules have more teeth than others.
Furthermore, it's entire possible reproductive success depends much more on the mother/father staying with his/her children and making sure they get a decent upbringing and education, rather than spawning as many brats as possible.
Basic biology time
Females do indeed stay with and invest heavily in a few offspring. When they cheat, they cheat to produce better offspring, not more. To either provide them with better genes, or better provisions. They will play on the strengths of one male and make up for his deficiencies with another. For example, if she has one very very good social partner who is sub-optimal genetically, she might go shopping around for a better pack O sperm. And her reproductive biology is actually built around making sure she doesnt get caught, and to bias the sperm competition in favor of the genetic mate she prefers...
Males on the other hand cheat to have more, resource investment free offspring. They stick with one (or a few) mates and raise the kids (that might not be theirs hehe) and cheat in order to produce offspring they dont necessarily have to provision as much. In other words, they are trying to receive the benefits of focusing their attention on one mate's kids, while not paying the opportunity cost of not being able to father children with other females.
Also bear in mind that no one but biologists sit around and think about trying to maximize their fitness. There are proximate mechanisms that the evolutionary ones run beneath.
