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Comment by esarbe | Hacker News Reader

Even if it is; the availability of pretty much perfect contraceptives negates that to a large degree. The use of contraceptives is cultural, not biological.

So we'll hit a replacement problem long before natural selection has time to catch up. The replacement issue has already real implications, this will become much more obvious in the course of the next 30 years.

Any relevant change on the population size the propensity for fertility can have; it will take (three, four, five , ...) generations to manifest.

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> Even if it is; the availability of pretty much perfect contraceptives negates that to a large degree.

No it doesn’t. My wife an I both wanted to have large families, even when we were children. How will contraceptives affect people’s basic motivations and life goals?

> Any relevant change on the population size the propensity for fertility can have; it will take (three, four, five , ...) generations to manifest.

Five generations is only 100-150 years. And anyway, we have a precedent for what happens when there is significant population reduction: the Black Death. Life got way better for the vast majority of people who survived that event. So much so that it ended feudalism in most of Europe. The only problem is finding pensions. Oh well. It’s incredibly immoral to yoke our descendants with the responsibility of funding an extended retirement for us.

Except for the millions that died.

There is a wealth of data now that shows fertility declining with increased standard of living. (Lots of correlations obscure the exact causations, but among the correlations is female economic and social mobility.) Your example is an eminently plausible mechanism for the spread of high fertility via either genetic or social vectors - but given that it contradicts the data, I don't see how it can be any more than an anecdote. Perhaps the grandchildren of large families are sufficiently removed to revert to the social mean? I don't know.

As for the Black Death, the personal and societal restrictions that result from not wanting to die from casual contact were probably quite a relief to escape and reverse. I don't know enough about history, but I could imagine that kicking off a positive chain reaction. After a cold season, people love to play in the sun.

This part is just speculation on my part, though; you're welcome to continue siding with Thanos.

The mechanism is that the propensity to have more children is partly genetic. So as people with those genes out-breed the rest of the population, those genes come to predominate in the population. Thus overcoming the demographic transition. Here is a recent paper that models this:

https://www.sciencedirect.com/science/article/pii/S109051381...

The Black Death is pretty well accepted as a contributor to the end of feudalism. I guess you’re positing that somehow that would be the result of survivors feeling elated at having gotten past the catastrophe (which is a temporary emotion), versus their facing an easier life as a result of 50% of the population dying (an effect which lasts a lifetime). Seems implausible to me.

But also, that the reduction in labor supply lead to vastly improved outcomes for laborers is pretty well attested. Lords of the manor started competing for labor because it was scarce. As a result, laborers were able to transition into owning their own property. It was a big deal. And it has to do with permanently reducing the population to well below the Malthusian limit (the plague periodically re-occurred for centuries, culling the population each time).

> The mechanism is that the propensity to have more children is partly genetic. So as people with those genes out-breed the rest of the population, those genes come to predominate in the population. Thus overcoming the demographic transition.

Is this entirely a genetic thing or are there also cultural components? I’ve long entertained a crackpot theory that North America will be almost entirely Mormon or Roman Catholic in 200 years, partially for this reason. The only reservation being that secular-humanist culture seems to convert people from more traditional cultures even faster than they reproduce....

> I’ve long entertained a crackpot theory that North America will be almost entirely Mormon or Roman Catholic in 200 years, partially for this reason.

Or Muslim, although that's more likely in Europe at this point.

> The only reservation being that secular-humanist culture seems to convert people from more traditional cultures even faster than they reproduce....

Perhaps in a few hundred years secular humanism will be seen a memetic Black Death.

The heritability of fertility is about 50%, I believe. So about half of the variance in number of offspring is genetic. The remainder is other stuff: culture, chance, etc. I would imagine that the genetic component is multi faceted. Some of it might be a desire to have children. Some of it might be a resistance to certain ideas or lifestyles that result in fewer offspring (if that seems far fetched, consider first that political orientation is also about 50% heritable).

But also consider that it’s hard to experimentally separate cultural and genetic influences because most children are raised and hence inculturated by at least one of their genetic parents.

There are natural occurring experiments which overcome this called twins. Studies of twins are what’s usually involved in figuring out the genetic component of a characteristic or behavior.

Specifically twins separated at birth. Do you know of any twin studies on fertility in particular?

Some people will want to have large families, but on average it seems that most people don't.

If the tendency to have large families is heritable in some measurable way, maybe because of beliefs or personality traits, wouldn't the carriers of whatever makes them have more babies eventually overtake the rest of the population?

It may not be the case that this is how it will work, but it's not a foregone conclusion that "on average" most people will continue to not want to have kids even in the "first world."

(Don't Mormons for instance have a fertility rate well above replacement despite also having higher than average incomes?)

That seems plausible. I think it's reasonable to suppose that people who grew up in large families might themselves prefer to have large families, but I also suspect that personal preferences like that aren't all that sticky across more than a few generations. Maybe it balances out. For now, we can just say that most people in well-off societies aren't opting to have large families and that doesn't appear likely to change in the near future.

> I also suspect that personal preferences like that aren't all that sticky across more than a few generations

There is pretty strong evidence that the variance in the number of offspring is about 50% genetic.

The Black Death killed people all at once, though. Especially the old and infirm. A demographic population decline would mean large numbers of people growing old and infirm while the necessary workload of producing the goods and services they consume gets concentrated among fewer and fewer people. One particular example of this might be shortages of doctors and other medical professionals, hospital overcrowding, and similar issues.

> And anyway, we have a precedent for what happens when there is significant population reduction: the Black Death. Life got way better for the vast majority of people who survived that event.

The difference between the Black Death and a population decline driven by lower fertility rates is that the Black Death disproportionately claimed the lives of the old and the frail. The result of that (at terrible cost) was an increase in average productivity by removing the people who would otherwise have required assistance, along with a reduction in competition for scarce resources (like food).

A population decline driven by a reduction in the fertility rate effectively does the opposite -- you end up with more elderly people compared to the number of young working people. And we haven't had a meaningful scarcity of food in centuries, so the things that are scarce now are certain types of skilled labor (e.g. doctors), which you can only really source from young people because there aren't a lot of medical school applicants from the population of people 55 and older.

>No it doesn’t. My wife an I both wanted to have large families, even when we were children. How will contraceptives affect people’s basic motivations and life goals?

they will in combination with economic incentives. In a world where skilled labour is rewarded and the cost of raising a child is higher and higher, whose going to pay for your large family? When people in developed economies today date, do you think they favour high social status and intelligence or sperm count?

Fertility and some other biological traits have always been positively selected for. Is the world overrun by virile, promiscuous musclemen and women with wide hips?

> Fertility and some other biological traits have always been positively selected for. Is the world overrun by virile, promiscuous musclemen and women with wide hips?

Fertility is a multifaceted trait; the degree to which a woman is fertile is dependent on a multitude of other traits and characteristics. It seems pretty clear that a desire to have children was not really necessary in the past in order to end up with lots of children. Having children was just something you did, so most couples had children and, consequently, fertility was driven by other traits. Perhaps desire to have sex, ability to earn income, etc. But things have changed. Now it seems pretty clear that the most important factors in fertility are A) whether or not the woman pursues college and a white collar career path, and B) whether or not both parents have an innate drive to have children (those two traits are probably related).

So yes, fertility has always been positively selected for. But the traits that drive fertility have changed as a result of our changing environment. So evolution is selecting for different traits at the moment.

My friend's sister has a lot of children despite being relatively poor because being pregnant makes her feel good. It's a high. Such a trait will probably spread.

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