The selection effects of this transition will be really fascinating to see after the fact. The species has spent a long time under selection pressure for "having more kids", but is being subjected for the first time to "having more kids while extreme prosperity and modern telecommunications exist" which is a very different thing.
I had an evolutionary bio professor in college say this: "you don't understand evolution until you understand how contraception could lead to overpopulation."
Anything placed in the path of reproduction is a barrier to be overcome.
If there is anything in the human genome that correlates with a positive desire to choose to have children, we are selecting hard for that right now. We may see a bottleneck this century and then a gigantic population explosion next century as a result, with a world full of people with very loud "biological clocks" who just adore and crave babies.
That is assuming this is genetically determined enough to be a target for selection. There are probably correlates that are, and I could speculate endlessly about what they are, but I also know that such speculations are likely to be wrong because these systems are complex and often counter-intuitive.
One I've speculated about recently is negativity bias. It seems to me that a lot of people choosing not to have kids right now are doing so because of negativity bias, because they see the world as a terrible place as a result of their consumption of negative media. Historically negativity bias may be something that's been selected for, but this may now have flipped. Optimists may have higher fitness now while pessimists did pre-industrialization and pre-modernity. But again, speculation.
Basically “if only the Amish have kids, in 200 years there’s going to be a large percentage of Amish”.
That this somehow is a surprise to anybody is baffling.
What do you / your prof think about the timelines though? I always heard people shoot these kinds of arguments down by saying that evolution does not significantly operate on our accelerated timelines of human technology.
How long an evolutionary change takes can vary widely depending on a ton of factors: current makeup of the gene pool, strength of selection, whether it's a single or multiple gene trait, whether and to what extent there are counter-pressures selecting in the other way, and so on. It's very hard to say.
https://en.wikipedia.org/wiki/Recent_human_evolution
Human culture is subject to evolutionary pressures, just as biological systems, but it can change in a shorter timescale.
Japan’s population dropped rapidly before contraception was allowed in due to abortion.
Abortion beats a strong selection pressure.
> Abortion beats a strong selection pressure.
Only within a lifetime.
Over several generations, it doesn't beat selection pressure, is selection pressure: people who want (or are willing to get) abortions get selected against.
Human cultures are also subject to evolutionary pressures and it is likely that we will start seeing intense selection effects at that level much earlier.
> The species has spent a long time under selection pressure for "having more kids"
This needs to be carefully qualified. Infant mortality rates for the first 99% of human history were so shockingly, stupefyingly high, that a woman might only expect to have about three of her children survive to adulthood (and maternal mortality rates were also so high that a woman wouldn't expect to survive more childbirths than 5 or 6).
What this means is that the human population was only marginally above replacement for the majority of human history. Human population didn't explode until the medicinal revolution.
This article [1] claims that it's actually still quite higher than replacement, to make people around ancient Mediterranean have to resort to various kinds of birth control (be it violent or non violent). Medicinal revolution do lowers death rate significantly but I guess the reason why we haven't normalized late/late marriage pattern as birth control until recently is because of abundance of resource resulted by industrial and agricultural revolution.
1. https://acoup.blog/2025/08/08/collections-life-work-death-an...