Another factor in the math of reincarnation (How can reincarnation be true when we suddenly have so many more people?) is that in times of low population, souls space out their lifetimes more due to there being fewer bodies available. Since time is fluid on the astral plane and in general seems much faster than it does on the physical plane, this is not a big deal. The decision to incarnate has a lot to do with things being interesting on the physical plane. In times of higher population, life is more compressed and there's more going on, so it "pays" to incarnate more frequently. When the world is changing slowly, souls might decide to take a thousand or even ten thousand years "off" if there are no pressing karmas or projects. There are other things for sentient souls to do beside incarnating--between lifetimes, souls often travel far and wide, for instance.