When trying to explain fertility rates, economists look at the costs and benefits of having children. In the modern, industrialized world, the incentives to have children are very different from the incentives in a rural, agricultural world. In the past, children worked, and the evidence from the studies of slavery suggested that a child of only about six could earn enough to pay his keep. In today's urbanized world, children are a financial drain until they are in their twenties and then they leave the family. In most past societies children were the way people prepared for old age. It was a disaster to be childless when one got old because then there would be no one to take care of the old person. Now people prepare for the future with financial markets. The benefits of children have been socialized with government transfer payments, and that socialization reduces the private benefits of children (though not the social benefit).
A while ago I saw a report of some study, and I cannot find a link, that reported the obvious, that young children were amusing. In primitive societies one of the major forms of entertainment was to watch the kids. Having spent a weekend with a child just learning to crawl, I was reminded of how entertaining little ones are. And it made me realize that one of the effects of television is to reduce the entertainment benefit people get from children. Unlike previous ways of entertainment, such as concerts, movies, and even radio, modern cable television provides around-the-clock and hugely varied entertainment. In the village of the past, if you wanted to be entertained, you needed young children, or else life would be pretty boring. But in today's world, you do not. You can simply flick on the television and be entertained. The effect of television has on fertility rates by influencing what people consider normal has been noted many times, but the medium itself may also change fertility rates and I have not seen this discussed anywhere (which does not mean that others have not written about it.)