The biggest driver of the shift from large families to small families is communication technology and education. As these techniques come into place the switch to lower birth rates is faster than what demographers have expected. And they are more permament.
...the earth's population will reach its peak sooner than official forecasts predict and because there is no visible counterforce compelling the majority of couples to have more than 3 kids each, world population will rapidly fall after reaching its peak.
Kelly does not know about Bryan's next book, which is going to convince everyone in the world to have more kids. Bryan should bet soon, before his book comes out.
The silliest bet is that within ten years the large Hadron collider will destroy the earth. In that case, winning the bet won't mean anything.
Many predictions do not have opposing bettors (including several predictions that already are false). One of the interesting not-yet-false predictions comes from econlog reader Mark Bahner.