Arcology: While SecLand was being terraformed the only living spaces available were in pressurized arcologies. This shaped the early SecLander culture profoundly. The living conditions required a centrally planned economy and population controls. Housing was assigned by government housing authorities and subject to change at a moment’s notice. A large minority of the population lived off of the nutrient dole, with their only hopes of seeing actual money being a work crew or the “grey market” in home-made consumer products, often intoxicating. Families were kept together as much as possible so the basic social structure existed, but since housing was so mutable they tried not to form attachments to their dwellings and none of the customs humans attached to land or home ownership developed.
After terraforming was completed and the air became breathable the vast majority of the population stayed behind in the arcologies out of convenience if nothing else. Even when the government opened up free settlement only a few daring individuals and enterprises set out. That changed with the plagues.
When the centuries-long cold war between the two states on SecLand went hot with bioengineered weaponry the arcologies became death traps, the population fled in droves to the countryside. The dispersed population organized along family lines, with the majority of lands being claimed in quick succession by the more organized genera.
Of course, after microbots cured all known diseases the arcologies were swiftly reclaimed, and remain the domicile of the majority of SecLand’s population. However, the rural population is significantly more eager to claim new worlds and emigration accounts for a notable degree of that disparity.
Arcologies appear on unterraformed planets every so often, but they tend to emerge as a conglomeration of interlocking family homesteads rather than from central planning. And since terraforming technologies have significantly improved they tend to open up within the first century of colonization.