A belief that the best humans should survive, the weakest should perish, thus improving the human race.
How would Social Darwinism explain the existance of religion? Most religions tell their people to help one another, instead of fighting. Even in non-religious societies, their political systems outlaws things like murder. This should be wrong to the logic of a Social Darwinist - murder should be encouraged because it will separate the weak from the strong.
The answer to this puzzle is that religious and political systems have been in a process of memetic evolution. The belief systems that best encourage their people to cooperate (instead of compete) are the ones that survive and are passed down to future generations.
The logic of competition would imply two groups of people fight wars with one another over resources and territory. The result of war is that both groups suffer many deaths, economic production is redirected into destruction rather than creation. The beliefs that led to this war have made both the groups less fit to survive.
If instead the two groups had been encouraged by their beliefs to cooperate, they would be helping each other survive. Some growing crops, some developing medicine. In the end, both sides benefit and thus, their beliefs have more survival value.
While genetic evolution occurs in humans, I believe genetic evolution has taken a back seat to memetic evolution. The future of humanity will be defined by its knowledge and beliefs, not by its genes.
In the distant past, someone born with bad eyesight might mean he'd quickly become a tasty snack for some predator. These days? Eyeglasses. Bam - genetic selection for better eyesight takes a big hit. People who are nearly blind without their glasses live normal lives and have children who survive just like everybody else. Whatever genetic deficiencies some humans have, we overcome them much sooner with technological evolution than genetic evolution. There is no longer a need to wait around millions of years to evolve better genes when we can invent our way out much faster.
How many people are "smart" enough to start a fire if they were in the woods, without a match or other modern conveniences? Well, society has basically moved past that kind of selection - matches, lighters, stoves, etc are the norm. Soon we'll be raising a generation of kids who can't spell because they will all have auto-correct on their computers - does that mean they are "mentally and genetically disadvantaged"? No - it's just that spelling won't really matter in their world.
Communication the Key Difference?
Once a species has developed a sophisticated enough method of communication, evolution begins to lose its individual flavor and begins to happen at the level of societies. It is communication between individuals that allows for cooperation and the transfer of memes - thus the transfer of survival advantage without having to wait for genetic change.
Most societies have religious and moral systems that try to get their people to help one another survive, rather than leave "the weak" to die, as Social Darwinists may suggest. Why cooperation? The evidence seems to indicate that cooperation is a better survival strategy than competition - which is why philosophies based on human competition (such as Hitler's concept of a master race) tend to go extinct.
A Matter of Scale?
On the much smaller scale, "might makes right" causes the cancer cell to multiply like mad and spread throughout the host's body. However, since the cancer destroys the normal functioning of the system it depends on, the host dies, and the cancer cells die with the host.
When it comes to property, you could have a lot of individuals fighting over land and resources. However, fighting turns out to be an inefficient use of time and resources. It also often ends up injuring both sides, thus reducing the overall "might" of the population. In order to increase the "might" of the overall population, humans evolved systems of cooperation. Thus you are no longer looking at the might of single individuals, but the might of the civilization.
When the civilization loses its cooperative edge is when its individuals start fighting each other - whether it's over land or other things. It is a sign that the memes currently present in the civilization are no longer as mighty as they used to be, or could be in the future.
The Clash of Civilizations?
One might define "cooperation" as between just the people who, for example, speak the same language, and then this "cooperative" group engages in "competition" against another group who speaks some other language.
But again, it comes down to the scale of cooperation. If there are viruses / cancer cells in your body that "cooperate" with themselves (or at least compete) against the other cells, it isn't really "cooperation" in the big picture view. To the host that contains all these cells, they are not cooperating, but competing.
It would be more efficient for both sides to work together rather than attack each other - which is probably one of the reasons most religions and moral codes have evolved prohibitions against murder.
When it comes to other species, if there's another species (perhaps alien) that is smart enough to kill off large portions of the human population, or is smart enough to help large portions of the human population, then the logical thing to do would be to find ways to cooperate with their civilization, rather than try to compete with them.
Even if they're militarily weak, if you don't cooperate with them, then they can withhold their help (or at least sabotage you if you've enslaved them), or even help other civilizations that do pose a threat.