Among the baker’s dozen of known galactic species that crawled their way to sapience, sociopsychologists were astonished to find that every one of them had the same intelligence. The bipeds from Earth, the avian dinosaurs from that one outer rim world, the furry bear-creatures that ate methane, put any together and they score within 10 points of each other on an IQ test. This wasn’t true for any other attribute. (Im)mortality? widely varying. Genders? Different systems. Biochemistry? Carbon through Arsenic. Size, shape? Hell no.
But intelligence? Why that?
For some species, this is an extension from a lifespan of decades to millennia. This is bad.
It turns out that entry-level sapience evolves as a survival trait. Hunt/find your food, develop technologies to make that easier, maybe do some farming, and so on. After basic establishment of civilization, mortality drops by factors in the hundreds or thousands. Population booms, and you start getting plagues from the species concentrating in cities.
This is where it gets interesting. See, once you have plagues, you need doctors. And once you have doctors, you start thinking about all of the other ways to cheat death. So the plagues are beaten back by vaccinations or antibiotics, and then your civ starts concentrating on welfare and quality-of-life.
Pretty soon, your species is living at the maximum, or nearly, of their theoretical longevity. For some species, this is an extension from a lifespan of decades to millennia.
This is bad.
At best, evolution stagnates. Your weak and stupid have the same chance of reproduction as anyone else—and they’re certainly not going to die before influencing their environments. Diseases that should have killed are mere annoyances, chomping futilely against a barrier of solid medical science. Predators that once ravaged tribes now are confined in zoos or hunted to extinction.
So no one gets any smarter.
The long and short of it is, after a certain point, intelligence is no longer a tremendous advantage to survival and, subsequently, traditional selection factors are abrogated completely. That is point at which medical science develops, which itself happens only when sapients begin the process of introspection and develop sympathy—that is, shortly after the development of sapience itself.