
The heritability of fertility makes world population stabilization unlikely - barry-cotter
https://www.sciencedirect.com/science/article/pii/S1090513817302799#bb0180
======
bad_user
All of my grandparents come from big families. My grandmother on my father's
side comes from a family of 12 children.

Yet my grandmother only had 4 children and my father only 2, and now I only
have 1.

This is an anecdote, but I don't think heritabiliy has anything to do with it.

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.

Nowadays access to contraceptives is common and in Europe for example it's not
feasible to have a lot of children due to costs (property, education, women
having to eventually quit their job, etc).

Anyway, I think the evolution of poverty is a much greater predictor of
population growth than anything else.

~~~
barry-cotter
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.

~~~
maratd
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.

~~~
bin0
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.

~~~
barry-cotter
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.

~~~
bin0
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...](https://watermark.silverchair.com/151663.pdf?token=AQECAHi208BE49Ooan9kkhW_Ercy7Dm3ZL_9Cf3qfKAc485ysgAAAkMwggI_BgkqhkiG9w0BBwagggIwMIICLAIBADCCAiUGCSqGSIb3DQEHATAeBglghkgBZQMEAS4wEQQMNDbyu9VK0lVH1VRPAgEQgIIB9nVdpx5WY2VFTM3siR6zUpI0NIrJ8G9sLgHflEENvKJn66C3kpwvmovR88zIGT4Zf1EjM37ovBkwyBefyQUyxX2tV9MlM9OHvTxWrEG41Otg_iehqYE75eFXqRmaREkeDxT_ISS78RZiRXb-i6WkAIIleISQsmOwkTX_aGVOTOkFKNtHYQXN69Db-
DMkH_Qh_wtjf6TLPgDfnShgZ8_YwcfGThvT1EwSpazZAHxh82Bfn8zKsfuwaEsObTJsVAuYACQcigL0RYheX7RHBn42fW2eq6IdG6702SQWhlpEfeTuez0kPQ-
r-Jh44nomPMhRLMZMtXrtYVkXfDFCs3m1le7xk9YLjvNKZDdJ-
LvJwtp9ouuvl1cQajuaSPN6VI5sD2SjGkd5NDs1CAaODx9hwmhKL-6S_WyMlktkw9keaDWvnIRyjLjiTO1AOHs_eURKqxutChnO9xeQYOwKjgJBrT5vt9hMPao9Weab89ykbOwiApEo4NGBAxHY4PGdgm8K_1O8N18g_De-
fFCTeY7gMp0OYbWDAxEMRLrKNdTl1ON9sAU488gm7D6FHCQ71yPmc5ucl_aVoTm4sb85QLxJykbamXeBKCIr40bB_0M59bG37P1swc6dWvojGic6c5Vqr9HwB-
ItUGp8afq2_zBBOMBTV7Ih4FU)

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

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

------
colechristensen
This is ridiculous.

The replacement fertility rate to maintain population before modern medicine
could be closer to ten than two not so long ago.

War, famine, disease, accidents, and infant mortality made it so surviving
until you were old enough to start having your own children was rare and women
were often pregnant or nursing continuously as long as they were fertile.

The premise of heritable fertility driving population and reproduction rates
in humans is really far off. We're not having litters of kittens.

If it were true fertility rates could not possibly have dropped so far in so
few generations after modern medicine and the insane reduction in violent
death rates seen in the last thousand years.

~~~
barry-cotter
> The premise of heritable fertility driving population and reproduction rates
> in humans is really far off.

Yes, until reliably accessible contraception fertility was driven by the
desire to have sex.

> If it were true fertility rates could not possibly have dropped so far in so
> few generations after modern medicine and the insane reduction in violent
> death rates seen in the last thousand years.

This assumes no change in the environment, when it has manifestly changed. In
this new environment in which desire to have children leads to having more
children, on average, the desire to have more children will be selected for.

~~~
ema
I agree, however this doesn't necessarily mean that average fertility will
rise again. The environment could keep changing faster than the genes can
adapt. Also "desire to have children" might be difficult to encode in DNA so
genetic adaptation to the availability of contraceptives might be really slow.

~~~
im3w1l
This is the strongest (only?) counterargument I've seen. I don't believe it
will happen soon though. Politicians only do something when doing something is
less controversial than doing nothing and that's pretty far off.

------
Merrill
In rural societies, children are productive members of the workforce and the
care of the aged depends on having surviving children. This encourages large
families.

In urban societies, children are expensive to rear and educate until they can
become productive, and the care of the aged depends on social institutions.
This encourages small families.

However, if urban societies socialize the cost of rearing and educating
children, then that disincentive will be removed and families may become
larger. Ethnic subgroups with strong extended families and religious
solidarity tend to have larger families, even in urban settings.

------
marliechiller
admittedly i havent read the whole article, only the abstract, but: assuming
that children from larger families will partially inherit the trait for having
larger families themselves seems like a leap to me - arent family sizes in
less developed societies generally considered to be either due to religious
superstition or because more children = more workers to provide for the family
a la victorian times? Once these factors diminish as a society develops, I
would argue (from a position of ignorance) that the "traits" governing a
bigger family (fertility) are environmental rather than genetic.

~~~
saagarjha
I found this very strange as well, so I skipped down to the section that
discussed this:

> In countries that have undergone the demographic transition, twin, adoption
> and family studies have pointed to a substantial genetic effect on fertility
> (Fisher, 1930; Kirk et al., 2001; Kohler, Rodgers, & Christensen, 1999;
> Murphy, 1999; Murphy & Knudsen, 2002; Rodgers, Hughes, et al., 2001a;
> Rodgers, Kohler, et al., 2001b). For example, Fisher (Fisher, 1930) found
> that a woman could expect 0.21 additional children for each additional child
> that her mother had, and 0.11 additional children for each additional child
> that her grandmother had. From this, Fisher suggested that the heritability
> of fertility at that time was 0.4 (40% of the variation in fertility is
> explained by genetic factors). Summarising research conducted through to
> 1999, Murphy (Murphy, 1999) noted that the heritability of fertility
> averaged around 0.2 in post-demographic transition societies, with the
> estimates increasing in recent periods. Kohler et al. (Kohler et al., 1999)
> examined data on Danish twins born in the periods 1870 to 1910 and 1953 to
> 1964. The first period covers the demographic transition and the second the
> end of the baby boom. In the first cohort, the heritability of fertility in
> women varied from close to zero in the pre-transition period to as high as
> 0.4 to 0.5 during the demographic transition. Estimates of heritability
> remained strong for the 1953 to 1964 cohort. From an analysis of data for
> Danish twins from the 1950s, Rodgers et al. (Rodgers, Kohler, et al.,
> 2001b), attributed slightly more than one quarter of the variation in
> fertility to genetic factors. Rodgers and Doubty (Rodgers & Doughty, 2000)
> found a median heritability of 0.33 in a contemporary United States
> population, and heritabilities for underlying desires, ideals and
> expectations ranging between 0.24 and 0.76. Where measured, the variance
> attributable to shared environment in low-fertility populations was
> generally lower than the genetic effects (Kirk et al., 2001; Kohler et al.,
> 1999; Kohler, Rodgers, Miller, Skytthe, & Christensen, 2006; Zietsch, Kuja-
> Halkola, Walum, & Verweij, 2014). However, the relative balance of genetic
> and shared environmental factors can change quickly over time (Kohler et
> al., 1999; Kohler et al., 2006).

~~~
toyg
That is still looking at pre-1970s populations, where the female workforce
even in advanced countries was much smaller than today and the religious
element was still largely hegemonic.

~~~
saagarjha
Yeah, I'm not sure that this data is completely relevant, especially given
that the birth rate in the US has gone down something like 25% since 1970.

------
Al-Khwarizmi
If this is true, then how do they explain population stabilization in, e.g.,
Europe, and other developed areas?

Why shouldn't what worked for Europe work for the rest of the world?

~~~
barry-cotter
Population didn’t stabilise, anywhere, ever. Nothing “worked” for Europe. Most
of Eastern Europe is losing population to emigration. If you look at breeding
populations rather than geographies you see some populations growing and
others shrinking. You don’t see steady states.

The population behaviour of the British and that of descendants of those
resident in Britain in 1900 are not the same thing. Focusing on nations
obscures that fact.

~~~
Al-Khwarizmi
Sorry, I didn't make the best choice of words. With "stabilized" I really
meant "stopped growing". And indeed, in practice this has resulted into
shrinking, not a steady state.

~~~
toasterlovin
Fertility rates declining for several generations is not stabilization. It’s a
bump in the road while humanity evolves resistance to whatever it is about
modernity that causes us to have fewer children.

~~~
ForHackernews
I don't think humans are every going to evolve past being a K-selected
species.

~~~
username90
Human females can easily have over 10 children each, so we are R-selected
compared to a lot of species like Bonobos. Just a minor behavioral adjustment
and we'll growing by a factor of 5 per generation, that will never happen to
Bonobos who can only get pregnant every 5 years.

------
andybak
Could someone with more time than me summarize how they reached the conclusion
that fertility was heritable? It seems very counter-intuitive unless I'm
misunderstanding the precise meaning of "heritable" that enables them to draw
this conclusion.

~~~
mxfh
So far the consesus was that migrants from traditonally child rich societies,
acclimated already a generation later at local lower levels if prosperitiy and
education level was reached as well. So it's the economy and social stability
through the state that replaces children as the only "pension and care"
provider for retirement.

FRANCE: "We show a convergence towards French standards that differs across
groups of origin. Women of Southeast Asian descent deviate from the fertility
pattern of their parents, while those of Turkish descent preserve their
parents’ cultural heritage. These different paths of adaptation between groups
partly reflect cultural distance between parents’ country and host country.
They also depend on family social capital, family structure, and family
values. Access to a higher level of education is a crucial factor in erasing
differences between groups" [https://hal.archives-
ouvertes.fr/hal-02081758/document](https://hal.archives-
ouvertes.fr/hal-02081758/document)

~~~
acqq
I suspect that “Southeast Asian” women observed had different religion than
those where “fertility pattern of their parents” is preserved.

Not all religions have the same effect on the outcomes, ignoring that in the
studies is misleading.

~~~
car12
What it is pussyfooting around is that there is not that drastic of a drop in
fertility rates of muslims.

I guess it shouldn't be hard to conceive where Islamic parties start having a
major voice in elections because of demographics.

~~~
mxfh
By numbers, global fertility/population trends and religion are not a "muslim"
one, that's a made up western "identity" problem. If anything it's catholic to
evangelical sectionism, if you look at Western Africa.

Overall religion is secondary at best, but economically weak tribal/familiy
social nucleus cultures vs established nation states that provide reliable
social safety not just for relatives.

"It's the Economy, stupid", look at fertility rates of Saudi Arabi, Iran, UAE
Poland vs Nigeria.

~~~
car12
The post that I was replying to was talking exactly about fertility rates of
second, third and beyond generation immigrants in Netherlands.

And what was found is that females from south-east Asia had a big drop in
fertility whereas Turks continued to have large families, comparatively.

So, no, the above research specifically disproves your argument.

------
LeanderK
I have always wondered whether something like this could be observed.

I wonder, shouldn't this be already observable? I would look not at the
median/mean fertility, but on the outliers on the upper end. If the share
increases/becomes more expressed in social groups that are not associated with
a large number of children otherwise, one should see indicators that the
theory is has some thruth.

Also, we are talking about heritability and entire generations. If such trend
can be identified, I would expect it to be very small and take time (measured
in generations) until it will have a real effect.

~~~
DissidentSci
This study
([https://www.pnas.org/content/114/5/E727](https://www.pnas.org/content/114/5/E727))
measured the speed at which selection against cognitive ability is occuring in
the Icelandic population (they measured this genotypically rather than
phenotypically). The rate at which it occurs is meaningful on the scale of our
lifespans. Same is probably true for fertility rates, but as fertility is
being selected for directly (rather than the indirect effect on cognitive
ability) it's probably even more rapid.

Then this study more directly measured fertility's rate of evolution:
[https://www.pnas.org/content/115/1/151](https://www.pnas.org/content/115/1/151).
They used UK Biobank data (genetic data from 157,807 female and 115,902 male
unrelated samples). They found the 'Age at First Birth' is the most strongly
selected trait in the British population. So in the future we can look forward
to a world filled with teenage mums.

That this is not obvious to everyone is just a testiment to how little people
understand about evolution.

~~~
j_wtf_all_taken
> That this is not obvious to everyone is just a testiment to how little
> people understand about evolution.

Exactly because things are not obvious, they need to be explained. As long as
you do not explain the things you call obvious, you cannot judge if people do
not understand a topic.

------
maire
This is the opposite of what Hans Rosling says in his book Factfulness.

According to Hans Rosling's findings family size is linked to child mortality
which is linked to poverty. When mothers see their children die then they want
to have more children. When they see their children grow to adulthood then
they have fewer children. You can also play with the data yourself on his site
[https://www.gapminder.org/](https://www.gapminder.org/).

He also looked at family size relation to culture and religion and found even
in religions that promote large families, family size is still declining.

Family size has dropped world wide everywhere except for Africa. There is an
unfortunate link between poverty and family size in Africa. The Bill and
Melinda Gates foundation is tracking this exact issue.

I am not a data scientist, but the heritability of large families seems a
difficult thing to pull out of the data when environment plays such a large
role. In my personal family history child mortality and religion did play a
large role in family size. We all now have small families.

~~~
theotherview
This is a 20+ year recollection...but while working on a Masters in Public
Health, we learned that the only causal factor that could be linked to both
number of children birthed and age at which a woman gives birth to her first
child is the education level of the mother. That is, as women in a society
have greater access to education, they postpones when they have their first
child and in the end have fewer children in their lifetime.

~~~
maire
Yes - Hans also said the same thing although indirectly.

His life work was reducing child mortality (under the age of 5.) He discovered
that the biggest factor in reducing child mortality was the education level of
the mother. It turns out that most child deaths are preventable if the mother
understands how to prevent the death. For instance, dehydration due to
diarrhea and vomiting can be prevented by sanitation. Also access to vaccines
is linked to the education of the mother.

It goes without saying that access to birth control also plays a big factor.
Melinda Gates tells a story about how she was talking to a group of African
women about vaccines and what they really wanted was birth control so they
could plan their family. They couldn't use condoms because culturally it
implied they had AIDs. This also seems linked to education.

------
ForHackernews
I'm sceptical of this result. Most previous research suggests that in
countries where immigrants are able to assimilate, second-generation fertility
levels converge with local social norms:
[https://paa2008.princeton.edu/papers/80042](https://paa2008.princeton.edu/papers/80042)

~~~
toasterlovin
Those immigrants haven’t started the evolutionary process in question, since
they have, by definition, only been exposed to a modern developed economy for
one or two generations.

~~~
j_wtf_all_taken
Which "evolutionary process in question" do you mean? Isn't the point that it
is NOT about an evolutionary process?

~~~
toasterlovin
The premise of the article is that fertility is heritable, which means an
evolutionary process is taking place that will overcome the demographic
transition.

~~~
j_wtf_all_taken
> The premise of the article is that fertility is heritable (...)

Actually, that was the conclusion, and is exactly the thing that is being
disputed.

~~~
toasterlovin
The person I’m responding to is disputing that people in developed economies
are evolving to become more fertile by pointing to the fertility rates of
recent immigrants, which fall within a couple of generations. I am pointing
out that these recent immigrants have not gone through multiple generations of
selection for fertility in a developed economy, so the fact that their
fertility rates drop upon arrival does not invalidate the hypothesis.

~~~
j_wtf_all_taken
And I was responding to your claim about the premise of the article? How is
the original post relevant in that respect?

Anyway, the point is still: if someone makes a claim, they should be able to
back that up. That is not done in the paper, and also I cannot find a post by
you explaining the reasoning for your hypothesis.

------
neffy
The Beak of the Finch by Jonathan Weiner describes (extremely well, it´s a
great book to read btw.) the evolution of Finch beaks on the Galapagos
Islands. There was one particular Finch/beak size that had relatively small
families and population growth, and was clearly being outcompeted by other,
better adapted Finches.

Until there was a severe drought, when that type of beak was able to take
advantage of a neglected source of foods, which wasn´t accessible to the other
Finches - who were more sucessful, but only when there was abundant easy food.
After the resultant die-off this particular Finch subtype dominated of course.

A lot of the discussion of evolutionary traits implicitly assumes an adaption
to an unchanging environment, but that is very unlikely to be the actual
situation, especially as in adapting, the adaption itself can change the
larger environment.

------
temac
Well, the finite nature of the world makes its population stabilization
likely.

~~~
malvosenior
There only needs to be enough resources to advance technology enough to enable
space colonization.

------
nobrains
Some anecdotal data.

My father's parents had 8 surviving children.

My mother's parents had 8 surviving children.

My parents had 4 surviving children.

I have 4 children.

My siblings have, on average, 3 children, and will probably have 3.5 over
time.

My cousin's have on average 3.5 children.

Grandparents: Asian.

Parents: Asian and lived in Middle East half their life.

Me and siblings: Living primarily in Middle East.

~~~
lotsofpulp
"Asian" is too broad to be a useful descriptor of ethnicity or culture.

------
adrianN
Catastrophic climate change makes it very unlikely that people in the future
will breed with similar success as in the past. Especially poor areas that
today show high fertility will be hit hardest by changes over the next twenty
to fifty years.

~~~
0xd171
Isn't your reasoning backwards?

We know that improvements in living conditions lead to reduced fertility. And
climate change will worsen living conditions, not improve them. So if areas
with poor living conditions have high fertility now, why would even worse
conditions (due to climate change) reduce their fertility? Sure, the outcomes
and quality of life of the children will be even worse than it is but I can't
see how that will impact rates of reproduction.

~~~
adrianN
Sufficiently bad conditions lead to children starving and women not surviving
labor.

~~~
sadris
In bad conditions humans breed more. There was a ten year famine in Ethiopia
yet there wasn't a single year in which the population actually dropped.
People stop breeding when they know their offspring will survive

------
bhouston
The main factors currently in determining family size is not genetically
heritable, but cultural. Culture appears to be heritable but that is just
because culture tends to be passed down.

Culture can change fast in either way, towards more children or less.

This does not mean we will be stable but rather culture matters.

For example soon as a member leaves a conservative religion stream and shifts
to a more liberal streams their fertility instantly drops in one generation.
Israel's government even models this in their demographics projects as the
flow between religious streams of Judaism is the biggest fertility modifier in
that country.

This does means in part conservatives will inherit the earth because liberal
culture right now cause smaller families but this is culture. I think liberals
have to fix this.

Also if a women gets an education her fertility drops and the more education
the lower the fertility. This means uneducated women (for whatever reason,
religious or other) will inherit the earth. If educated women tend to be those
with more genetic talents, and in aggregate this may be the case, then this is
quite a problematic trend long term.

I think the trends to who has fertility and who doesn't isn't great and should
be a real topic of conversation. As a society currently we do not deal with
this head on.

~~~
j_wtf_all_taken
>> Also if a women gets an education her fertility drops and the more
education the lower the fertility. This means uneducated women (for whatever
reason, religious or other) will inherit the earth. If educated women tend to
be those with more genetic talents, and in aggregate this may be the case,
then this is quite a problematic trend long term.

So is there any reason to believe that last "if" is true? Why would that be a
problem anyway, as we can simply make sure schools are in good shape to
educate those children, no matter the education level of their parents?

------
PaulHoule
How about a counter effect?

Many couples today reproduce with the assistance of reproductive technology.
This releases selection pressure, so low-fertility genes proliferate.

------
j_wtf_all_taken
The authors seem to have a background in economics, does that really fit the
research topic of that paper?

~~~
j_wtf_all_taken
One of the sources:
[https://journals.plos.org/plosone/article?id=10.1371/journal...](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0126821)

>> This finding implies that individuals with genetic predispositions for an
earlier AFB had a reproductive advantage and that natural selection operated
not only in historical, but also in contemporary populations. The observed
postponement in the AFB across the past century in Europe contrasts with these
findings, suggesting an evolutionary override by environmental effects and
underscoring that evolutionary predictions in modern human societies are not
straight forward. <<

Yeah, so basically we got a simple model by two economists vs. decades of
empirical data?

It'd be funny if it wouldn't sound like a scientists version of the
"immigrants will have a lot of children and replace us" lie.

~~~
DissidentSci
Because that never happened before in human history:

[https://www.amazon.com/s?k=Who+We+Are+and+How+We+Got+Here&re...](https://www.amazon.com/s?k=Who+We+Are+and+How+We+Got+Here&ref=nb_sb_noss)

~~~
j_wtf_all_taken
What exactly is your point?

~~~
j_wtf_all_taken
Sorry, my mistake, forget it. I should have read your other comments first
before replying.

