Taken from page 96 of Against Leviathan:

If the total incarcerated population were to continue to grow by 7.3 % annually, it would double approximately every ten years, whereas the total population, growing at 1 percent annually, would need some seventy years to double. Hence, in the decade of the 2080s, within the lifetime of many people already born, the prison population would overtake the total population, and the immigration barriers would have to be removed in order to let in enough foreigners to fill the cells. I leave it as an exercise for the reader to determine who will serve as guards.