Women’s education and women’s rights. I don’t personally think we have an overpopulation problem, but the above are proven to reduce birth rates in an entirely non-coercive manner, as well as being a good thing in their own right.
Somewhat surprisingly, the answer seems to be "nothing".
In a primitive society, population self-regulates through early deaths, caused by diseases, physical injury, famines, etc... Then there is a transition to modern society (demographic transition) where though improvement in medicine, agriculture, and the general idea of not letting people die, population grows exponentially, but it doesn't last as people simply stop making children.
It has already happened in the first world, most countries would have a negative growth rate if it wasn't for immigration. Asia are just at the end of their transition and Africa is in the middle of it. Assuming the trend still holds, it will be the end for Africa and therefore, the entire world in a few decades. It is actually reason for concern, as it will result in an aging population, and unlike today in first world countries, there will be no immigration to compound the problem.
But changing the question and asking "How was it done?" instead, then I would say "birth control", the voluntary kind, the pill in particular. Safe abortion too. As someone mentioned, women in the workforce also contributed, but I would go further and say that for many women, having a job is not a right, it is an obligation! Many couples simply can't live decently on a single pay, especially with a child. Also, with laws regarding child labor, long studies, and changes in mentality, children are more of a burden than they once were, as most can't support themselves before their 20s. It used that at that age and even way before that, they supported their parents instead, especially boys (that's the idea behind dories).
Besides the inability to live on a single pay, these are generally good things, but the result is less children, which is a good thing for ending a potentially catastrophic exponential growth, but it may turn out to be a problem later. Not an unsolvable one though.
Yeah, at some point we’re going to have to solve the opposite problem. Luckily that’s also pretty easy to solve as well: mandatory maternity/paternity leave, free childcare, &c
We are hardwired for sex and reproduction. It's a simple argument. Assume there's a genetic trait, "horny" (pardon the language). Some have it, some don't. Genes that cause horniness are selected for, those that do not, against. Nature selects for the horny allele every time. Reproduction is deeply intertwined with selection. Look no further than the duck penis [https://www.researchgate.net/figure/Photograph-of-the-425-cm...] as an example, one of many.
As women are rationally aware especially at this politically polarized and contested time - acting on such instincts have consequences! Pregnancy! That part is not generally seen as much fun by many women.
How I would do it - as far back as the 1880s, ovaries were removed. C-section, as the name tells us, goes back to Caesar. All very old surgeries. So women could have the prescribed number of kids they wished for, particularly if a c-section, and then be sterilized. Now they can freely have sex. Now women have 2,3 kids, not 15,20 at least among the higher classes.
Now move onto the 1960s - the pill. Now you don't even need a surgery and it's reversible - just stop taking the pill.
Now the behaviorial instinct to have sex (side effect: reproduction) has been satiated without the downside. Women are very happy. So now we are simply going through a human genetic bottleneck. There exist alleles instead of "horny" it is "really wants a baby" - now this gene is selected for.
So to summarize the argument:
- breeding has been dialed down for over 100 years. satisfying sex drive without pregnancy, in many different ways over time
- life always finds a way so in the future, perhaps a few generations, procreative genes will be favored versus horny genes.
> If you wanted to dial-down our incessant breeding without getting lynched, how would you do it?
Dialing it down is easy. Dialing it back up is impossible. The Chinese successfully dialed fertility down to 1.0 (officially, unofficially far lower), but try as they might they cannot make it move back up above that. China is doomed, even if that doom lies centuries off.
Other places didn't even have to try. Breeding has been dialed down far below the replacement rate, and many other nations are on the road to extinction. India has likely sunk below replacement fertility, or will in the next year or two. Only a few nations in central Africa have above-replacement fertility.
> There is a taboo against constraining reproduction. Legally, medically or whatever.
I've never personally seen evidence of such a taboo in the western hemisphere. Even in the United States. Instead, a few years ago, we had media talking head saying how the law should do something about the Octomom, that it "wasn't right". We live in the "shout your abortion" era. We're seeing the rise of the r/childfree culture. Constraining reproduction isn't taboo, it's celebrated. We live in a dying civilization hellbent on committing collective suicide.
Which is unreasonable of course, given that reproduction is arguably more destructive than bombs.
If you wanted to dial-down our incessant breeding without getting lynched, how would you do it?