Since everyone in the Reach has had high-speed transportation ever since they got there, as well as easy birth control and a traditional appreciation for ecological values, population is thinly spread, each planet bearing a scattering of widely-dispersed towns and cities. Larger cities are surrounded by haloes of suburbs and smaller towns, but there are no more than a handful of these megalopoli per planet.
The population of Imperial Space is almost wholly human. AI people are very rare; neo-beasts are unknown; aliens are found only near the borders of the Imperial Space, mostly in human-incompatible environments.
The human population divides first along two important criteria: political (Imperial or not) and ethnic (Old Settler or general).
The general population is a hybrid of New Terraformer and Old Settler. Some old families and clans claim high status on the grounds that they are pure New Terraformer; these claims cannot be proved or disproved. The general population also includes relatively pure but culturally assimilated groups of Old Settlers.
New Terraformer stock and the average general populace usually resemble Eurasians – straight, dark brown hair, eyes with slight epicanthic folds, thin lips, and straight short noses. The skin is usually light to medium brown with little yellow cast. Thanks to a fashion in cosmetic mutation which passed into a tradition, much of the general population has green or hazel eyes. But deviations from these norms are quite common.
Medical science has been able to halt and reverse aging since long before the Reach was settled. As a result, the population turns over very slowly. What turnover there is, depends strongly on the wars in Reach history. Children are rare. There are people who date back to almost any period in the history of the Reach.