Falling birth rates are due to many factors, including:
Decrease in financial incentives:
- A change from kids as income producers (working on the farm) to income losses (costs of raising: food, shelter, and in modern times, daycare, college) decreases financial incentive/necessity to have kids
- More kids survive their infancy/childhood, so there's less need of having "spares" to do the above work
More options, for women in particular:
- Contraception, which allows people some level of control over reproduction
- Higher cost of living today. People want to launch their kids well, so many families that in the past would have have more than 2 kids, stop sooner because they feel they can't afford more kids without compromising what they want to give their children (see above food, shelter, but particularly daycare and college). [Note: I wrote this before
@RetiredAt63's post which explains this issue well, I just didn't get around to finishing my post until now.]
- People want to hit certain milestones before they have kids (e.g. get a house) which are more difficult to achieve nowadays than it was for their parents given sky rocketing costs of mortgages/student loans v. income increases
- Rise in women's rights, so they are allowed to own property, get a bank account and credit card, get a job. With this freedom, they are not solely dependent on marrying a man - young - to take care of them financially, but can get a job.
- Rise in women working also means parents are exhausted from working and taking care of families, so might choose to have fewer kids.
- Rise in women attending college and postponing kids/marriage for school/career. Delays in becoming pregnant may may mean some people never have kids or have fewer than desired as their fertility declines.
- It's becoming more socially acceptable to choose not to have kids. People no longer date in their small area of birth, so it's also easier to find like minded partners.
- Fewer people are "in the closet". Kids don't really happen accidentally in same sex relationships.
Other:
- General decline in religious belief. Some major religions discourage or prohibit contraception use (that incidentally would decrease the number of followers...).
- Belief the world is overpopulated with detrimental effects (e.g. climate change) and desire to avoid adding to the overpopulation
- Past government policies impacting desire to have kids (e.g. some women negatively impacted by China's prior one birth policy aren't willing to have kids now)