ZGene

"This disease is going to destroy everything we've built up here. We colonized the asteroids and the gas giants on nothing but grit and trust. We knew we could trust the other guy, because we knew he wanted to prosper and he didn't want to die, just like us. We trusted technology to bring us a brighter future. But that trust is gone now. People are afraid... they're afraid of what's going to happen next... I've just heard someone in this studio, he wants to review his partner's genetic records before they get hitched."

"...I realize that ZGene hit us where we hurt the most. But you people out there, baying for blood, you have no idea the kind of future you're bringing about."

- Jimbo Haddock, mid-2100s-era political commentator and radio personality

ZGene is the name for both a class of in utero gene therapies with near-universal adoption in Human space colonies prior to the Interplanetary Wars, and for a recessive gene created by the therapy. The gene therapy minimized the musculoskeletal and cardiovascular atrophy caused by microgravity. Because it allowed Humans to live normal lives in microgravity, ZGene was commonly credited with making space colonization possible for billions of people. However, ZGene caused infertility when both recessive genes were expressed in a woman.

The economic, and social fallout from this side effect ravaged Sol's outer colonies throughout the mid-2100s, ending the Golden Age of Space Colonization and giving rise to extremist political movements like the Frieden.

Summary
Prior to the hundred year anniversary of the Apollo Moon Landings, Human space colonization began to increase at an exponential rate. Part of the expansion can be explained by the perfection of reusable launch vehicles and the development of space-based industry, both of which made space affordable for the middle class. Deteriorating conditions in Earth's megacities was also a driving force, as people looked to the stars for prosperity.

This was the start of the Golden Age of Space Exploration, when millions of Humans left Earth to homestead in the Sol System. A generation of science fiction authors predicted dismal living conditions with shortages of air and water, and that Humanity would have to change itself to live in space. Instead, Humanity proved to be infinitely resourceful and resilient. Engineers, doctors, technicians and pilots spearheaded the expansion. Where there was a shortage of water or medicine, a new innovation eliminated the shortage. Where living space grew short, new habitats were fabricated from the nearly infinite resources of the solar system.

Large families, euphemistically called "Pioneer Spirit", was a symptom of this prosperity. The cause is uncertain. Some evolutionary psychologists posit that people who inhabit unexplored territory are driven to have a lot of children in order to lock down unclaimed resources. Others say that the large family sizes are the predictable result of shoving enterprising young men and women into a wilderness with few avenues of cheap entertainment. Whatever the cause, the effect was so pronounced that lunar colonists soon earned the nickname 'Moon Rabbits', and the average family size on Mars and the Jovian habitats was 4.3 children.