If a trait is not strongly selected for but variation exists it will be maintained in the population. See height, hair colour, propensity to gain weight, etc.

Then you change the environment so that the trait is under strong selection. The people living in Scandinavia 3,000 years ago couldn’t digest lactose. Now ethnic Scandinavians who are lactose intolerant are basically nonexistent. It’s at above 99% homozygotes.

Imagine taking a population and killing everyone over 18 under 1.8m and doing this for four generations. Assuming anyone was left after the first generation the fourth generation will be substantially taller than the first.

Desire for children was never under strong selection pressure before because desire to have sex led to children pretty reliably up until the birth of the demographic transition in France in the ~1700s. Now it is under selection pressure, everywhere with reliable access to contraception so the people who like children are going to have more children, and their share of the population will grow.

> Large families simply happen due to poverty. When poor, you want more children because there's strength in numbers. Plus the women are staying home and can afford to have children.

This will be news to the Mormons, Amish, Hutterites and the Hasidim among a number of Orthodox sects of Jews. Large families do not just happen due to poverty. Some people want more children and arrange their lives around that.

> Large families do not just happen due to poverty

You're right, they happen due to culture. However, the cultures you listed are notably insular and traditional, meaning they do not have large families for the same reason much of the world does (high infant mortality). I doubt these will ever change, but it is very possible the rest of the world will.

Though I’m sure the Mormons would cop to traditionalism calling a religion where the median man spent two years of his life in a foreign country insular is slightly odd, considering most Americans never leave their own country. Even aside from that cosmopolitanism Mormons have been state governors and been contenders for the presidency. Insular they are not.

Insular was probably the wrong term to use, culturally rigid or conservative might be better. The point is they maintain a high fidelity to the cultural and behavioural patterns of their group. One of which is to eschew birth control and purposefully create large families.

Mormon's do have and encourage large families. But there is no modern prohibition or even suggestion regarding birth control.

They're commanded to "multiply and replenish the Earth."

> It’s at above 99% homozygotes.

Meanin 99% of people who have this allele (trait) have two set of the corresponding gene/genes.

Okay, but is lactose tolerance a dominant allele?

Yes, lactose tolerance means you have functioning enzyme. That makes it a dominant allele.

That's not how it works.

https://en.wikipedia.org/wiki/Dominance_(genetics)

If it was advantageous for us to have lots of children, then women would have a dozen at a time, rather than one. There are plenty of mammals that do that. There’s clearly a tradeoff there and the selective pressures for humans tend to lean toward fewer. Not sure where we’ll end up, but time will tell.

I think there was such a selection pressure when we were nomadic hunter-gatherers. Agriculture is a very recent development anthropologically, so we were still in the mode of one-kid-per-pregnancy, though multiple might have been advantageous. It's possible we developed higher fertility due to that, but we didn't have enough time for that new pressure to kick in.

It’s been 5,000 years since the dawn of agriculture. If multiple births were fertility increasing they’d be, if not normal, at least common in some groups. You don’t even get 1 in 3. Single births are clearly not being found wanting by the Blind Idiot God.

Not necessarily. Mortality of mothers is higher in multiple births; maybe that's a counter-pressure (fewer children). Children do worse as well ([0], [1], [2]). Ladies are not built to carry more than one child, especially considering the size of our children, and agriculture has been such a blip on the evolutionary/anthropological timescale that we have not had time to evolve so significantly as to change our basic reproduction.

[0]: https://watermark.silverchair.com/151663.pdf?token=AQECAHi20...

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5541191/

[2] https://bmjopen.bmj.com/content/4/4/e004514

The size of the human brain makes this physically impossible.

Ok, explain triplets. https://en.wikipedia.org/wiki/Multiple_birth

Three and twelve are very different from each other. Even three is very rare and often comes with complications.

Minor correction: Advantageous to the children (conferring evolutionary advantage). Not to us.

I'm not sure I understand your distinction. From a gene-based perspective, the gene, whether in the child or in the parent, is the same. The selection pressure is on a gene level, not on a species or individual level.

> Large families simply happen due to poverty. When poor, you want more children because there's strength in numbers. Plus the women are staying home and can afford to have children.

Poverty and risk, fertility changes lag behind medical advances / availability e.g. in europe the large families were mostly around the first half of the 20th century (to soon after WWII), where culturally you wanted to have a fair number of children so you'd have some living to adult age (and also manpower on the farm) but medicine had also progressed and spread enough that almost all of them would survive to adult age.

Culture quickly caught up and the default assumption that all children will live to adult age means even farming families aren't really large anymore (not to mention machines & automation makes the manpower of many children less relevant).

Developed countries are mostly at the point where medical advances are getting available, leading to huge families because the cultural aspect hasn't caught up yet.

I can think of, at least, another possible explanation.

Before birth control was available, there is not need for evolutionary effects in the willingness to have children directly, only for the willingness to have sex. You probably agree with me that sexual desire is pretty common, obviously we have evolved for it.

After birth control exist, sexual desire is not an appropriate proxy for evolutionary pressures, so, we could see an increase in the instinct to have children and, maybe, a decrease in the instinct of having a lot of sex.

Same trait may have different effects in different environments. In stone age, bad eyesight might have been a severe evolutionary disadvantage. After we invented eyeglasses, bad eyesight has no effect on your survival or number of children.

In a culture where large families were normal, perhaps genetic traits that push you towards having a large family were hidden by cultural aspects that made you have a large family. Only in an environment where having a large family is not the norm, will they surface and have an effect.

Just speculating here.

Actually, wearing eyeglasses is sometimes perceived as unattractive, causing lower chances of offspring.

Or perceived as attractive, as ‘studied’ if you think of peacock feathers. Probably only to be studied in hindsight depending on evolutionary success...

But your anecdote is exactly what is explained in the article: the new environment (such as contraceptives) which lead to your ancestors having less kids, will cause increased selection on fertility traits in the future. This will reverse the trend of decreased fertility.

Traits that will be selected: a desire to have more kids, physiological changes that cause resistance against contraceptives, tendency to have twins, etc.

That makes sense from an evolutionary/ biological perspective- it’s like contraceptives appear to nature like a filter - a global pandemic, an extinction event of sorts. So those who nevertheless have more children - ipso facto- pass down their trait of having larger families despite the availability of a baby pill. Fascinating hypothesis. My next question would be if there is a biological link to religion and fertility.

Yes, religiosity is inheritable. It's been proven with twin studies. Therefore, if religious people have more kids, having more kids is inheritable. (and also the world becomes more religious)

https://www.newscientist.com/article/dn7147-genes-contribute...

http://blogs.discovermagazine.com/gnxp/2006/11/heritability-...

While not “necessarily” poverty related, I read somewhere once that India wanted to bring TV to as many rural households as possible. The reasoning being is that People are not having kids because cheap labor, they’re having kids because there is nothing else to do at night for entertainment, and contraceptive use is non existent.

Somewhat related, https://www.conservationmagazine.org/2013/09/tv-as-birth-con...

I'd dispute a lot of what is in that source. Their heat-maps of various characteristics are a correlation only, and a correlation among things which tend to go together (fertility rate, tv ownership (wealth), gdp per capita, and female literacy usually increase in tandem as part of a cultural shift). This does not necessarily mean they cause each other.

Also, ought we really to be brainwashing people to live how we wish them to?

All good points. While I think TV is perhaps too specific an example, my personal hypothesis is that availability of other forms of entertainment is a cause of declining birth rate. I’d say close to half of my married friend couples do not plan on having kids, and their main reasoning is usually that they like their social life, freedom to travel, ability get out in the weekends and generally do what they want. Also often they don’t seem too keen on loading themselves with additional responsibilities.

Imo, it is more that having children has often effect of isolating you quite a lot. It can get very lonely as a result as you miss adults around. You miss friends with common interests as your social group become exclusively those with same age children but not much in common otherwise.

It is not necessary that entertainment is so great.it is more that routine and isolation can be quite big.

TV is compatible with kids. But most social activities are nowdays are not.

in the old days manual labour was an incentive to have as many hands as possible. also infant mortality was high

Funny that people are "too rich" to have kids. Or that poverty means you have more of them. Are we sure it is not due to access to contraception?

Yes this seems to miss actual examples of developed countries where fertility is approaching and dropping below replacement rate.

There's also climate change https://qz.com/1590642/these-millennials-are-going-on-birth-...