@KitlerIs6 @alex @dave @SpaceElf @bot This isn't an ironclad rule, though. Out-group favoritism DOES, in some situations, have benefits. It is exemplified in humans by the trait of openness (you can think of it as progressiveism). It's a way to accelerate evolution in unstable environments, or environments with newly available niches to exploit. Any individual probably has a worse chance of producing long-term surviving offspring, but one of the out-breeders has a very good chance of eventually hitting upon a good combination that exploits the new niche better than others and can be the foundation of a new species (or in the case of humans, tribe, nation, or civilization).

I can illustrate it in lower organisms with things we've observed. For example, there was a population of birds on a small isolated rocky island in the galopagos that was unable to eat tough opuntia cactus seeds because they didn't have a strong beak, but the persisted in the environment because they had an instinct to build nests on the ground. (I may have details of this story wrong but the basic idea is intact) During a storm, a bird the researchers named "big bird" blew in from another island, and he had a strong beak and could eat cactus seeds. He wasn't closely related to the native finches, but he eventually convinced a couple of females from species that were native to the island to mate with him, and his hybrid offspring that could both nest in rocks AND eat cactus seeds expanded into the previously unexploited niche on the island.

Anyway, this all loops back around to the closely related favoritism-- the newly created species and the native species on the island wouldn't mate with each other. Even in such a new (sub)species, they had an instinct to only mate with their own kind -- makes sense, because any shuffling of the genes when you're already in a stable environment runs the risk of deleterious combinations of traits. Don't mess with something that works, and all that.