What are you measuring to determine over or underpopulation? Food production? We grow more food that is required to feed current population, much of world's farming is not efficient, done without modern equipment, etx., and then there is the subject of meat - if we stopped growing feed for animals, and instead seitch to vegetarian diet, we could feed a lot more people.

Then there is the argument that glasshouses are 10x as productive as open fields, so if we were to use them predominantly, we could return land to forests.