Cyril Kornbluth (IIRC) wrote “The Marching Morons” in the early 1960’s. The premise of the story is that the smarter people voluntarily limit reproduction. (And the dumb ones breed like rabbits - cue the rant about welfare babies). Over a long span of time this has to have an evolutionary result.
OTOH, in Freakonomics, in a discussion over reproduction, abortion, and crime levels Levy suggests that modern statistics point to the fact that a woman has as many children as she chooses to have. Nowadays, that seems to be less than 2 as an average. Women who get pregnant early in life (by accident) simply choose to have fewer children later in life; age at which one starts does not affect the birthrate.
I won’t say “selfish”. The choice not to have children is logical.
As I mentioned earlier, the Quebec government, concerned at the low rate of reproduction, has offered some fairly good incentives for having children, with minimal effect. This suggests that the (perceived) cost of having children, in money, lifestyle changes, effort, whatever - is very high. Depneding on who is trying to scare you and how much, the cost of each child is in the hundreds of thousands of dollars over their lifetime.
Modern lifestyles are best enjoyed with a certain level of income. The way society is structured nowadays, the only way to achieve this for most people is as a dual-income couple. Having one or more children will severly impact that lifestyle.
The cost of a house is fairly high - I would assume it is high even adjusted for inflation, even after the recent bubble collapse. I recall when a 2-car family was something special; a 2-income family was not rare, but less common. Nowadays, 2 people work they NEED 2 cars.
The toys we “need” have also changed. Consider the big-screen TV, surround sound, DVD/BluRay player, computer(s) and internet, video-games, cell phones, cable TV, satellite radio, car stereo… and we haven’t even gotten into sports or exercise equipment. All toys that cost significantly more than 40 years ago or had no equivalent then.
A modern lifestyle takes too much money, and kids subtract from that; not to mention maternity leave’s lost wages and child care expenses. No surprise that people choose to be childless.
OTOH, most of the parents I know who have talked about having kids say they would do it all over again… unless their kids were teenagers at the time.