Mankind has prospered generally after sharp population reduction events e.g. the Black Plague epidemics in Europe were followed by the Renaissence. After the extensive blood letting of WWII the world has prospered for 60 years etc etc. So what is going to work this time? Maybe a dangerous variant of haemorragic fever (Ebola)? bird flu? Global Warming droughts? depletion of natural resources (oil,coal, minerals)?; the expansion of the great Islam-vs-the-West conflists into a WW? Various combinations of the above?
We can leave it all to the above mechanisms or we can decide to manage it. I don't much like Mao Tse Tung's philosophies but his one child/family policy was a step in the right direction and as far as I know was the only government that targetted population control through specific limits on family size.
This is the only humane way of bringing the overpopulation problem under control. The recent Florida mother of octuplets who already had six other children is a prime example of all the population excesses that people indulge in. My personal view is that the sole goal of this lady was to get higher child welfare and support payments. If all of mankind started having litters like this we would run out of standing room in short order and have to become amphibians.