Why Aren't People Having Kids?

Throughout the world, birth rates have fallen dramatically, and most countries are now below replacement fertility, or the number of babies needed to refresh the population. Only recently, the world was worried about a "population explosion". Now an implosion seems more likely. Why is this so? Why aren't people having children in the same numbers as they used to? The core reason is simple: People aren't having children because the costs outweigh the benefits.