Today, 1:32 AM · 11 replies and 279 views.
You could possibly do this just by state alone.
Unchecked outbreak could lower the population, as you would expect. But the refugees may not settle in their destination system. But what these refugee missions will do is have an effect on the state of their target faction in the destination system. So this state, if triggered, could raise the population or reduce it and generate missions as well that will also affect their destination systems.