Costs in terms of time and money.

In the past, kids could work on the farm or in a factory and generate money, now you have to send them to daycare and buy them toys and they cost money.

Previously, more people would jump straight out of high school into blue collar jobs and raising families. Now, men and women are delaying kids to focus on college and building a career.

And mothers would more often stay home to raise kids. Now both parents are working more, and there's less time to spend raising children.