What is the lowest median age of any human population group?
February 12, 2011 8:13 PM   Subscribe

What is the lowest median age of any human population group? How low can it go? Is there a theoretical lower limit?

The latest US census survey showed that the median age of the Satmar Hasidic village of Kiryas Joel, NY, (which is not far from where I live) is 11.9 (median age for the US as a whole is 36.8). More than half the population of the town is under 12 years old. Yikes.

This seems to be without precedent in human history. Although this population has benefitted from the low mortality rate of modern life, they have very consciously chosen to avoid the usual path of demographic transition, by which fertility rates also fall, and population growth slows. I'm trying to imagine what happens in the next few decades, as their median age continues (presumably) to fall.

We hear that the "youth bulge" in the Middle East is reason for concern; the lowest median age among Middle Eastern countries is Yemen, at 16.4. The lowest median age among countries is Uganda, at 15.0 .

This town is already having problems supplying adequate water, sewage capacity, housing, schools, cemetery space, and income--it has the highest poverty rate in the US (but reportedly no homelessness).

I realize that one problem with this question is how to define "population group." This particular group that has my curiosity piqued is a village of ~23,000. I'd be interested in any measurable group larger than a family unit, anywhere in history or geography.

Another problem with this question is that it's very hard to research or discuss it without bumping into judgments about religious beliefs and family choices, xenophobia, anti-Semitism, women's rights, the Holocaust, population control, and other sticky-wicket issues. As compelling as those issues are, for now I'm not interested in the right or wrong of it, the goodness or badness of it, just: What Happens Next? Is there a lower limit to median age in human population, and what happens as a group approaches it? Is this as unique and unprecedented as it looks?

Previously on the blue; and also.
posted by Corvid to Society & Culture (19 answers total) 6 users marked this as a favorite
Presumably, some of the sub-Saharan African countries with high rates of HIV infection have low median ages because they also have very low life expectancy.
posted by easy, lucky, free at 8:37 PM on February 12, 2011

Well, just as a starting point, the absolute limiting case seems like it would be (0.5)*P, where P is the age of reproductive maturity. That's the hypothetical scenario where you have just a single generation (no parents), they are all female, and they all get pregnant and give birth at the same time.

So I think that puts an absolute lower bound on it of 6.5 or 7, if you assume P=14 (which is cringe-inducing in the first place). It's an obviously trivial case but it provides a place to work from; although you have to decide on a lot of assumptions if you want to move forward from that. If you assume a completely closed system, with each female becoming pregnant as soon as she is able and staying pregnant throughout her reproductive lifetime (say, one child every 18 months?), all live births, and then make some reasonable figures for average lifespan, it ought to be tractable to figure out the median after an arbitrary number of generations.

I'll have to mull that over for a little while, though, if nobody ends up doing it first.
posted by Kadin2048 at 9:02 PM on February 12, 2011 [2 favorites]

This documentary, which is available streaming on Netflix, might explain a little bit about what's behind the Hasidic impulse to have so many children and especially how Hasidism in the US has evolved over the last 50 years or so. Knowing the Hasidic community mostly through exposure as a neighbor (I live in Brooklyn), I was under the impression that This Is Their Way, and I saw the whole thing through an ongoing, unchanging, ahistorical lense. After seeing that documentary, I'm not so sure. Certainly the evolution of the community beyond the first and second generation since the Holocaust is probably going to be a significant event.

Considering that the community is one of belief, and to an extent one of affinity, there is a strong possibility that attrition will drastically adjust the real impact of the numbers you cite. Though I'm more familiar anecdotally with the Hasidic community in Brooklyn, where there's probably a lot more contact with other ways of living.

Another good documentary to watch might be Trembling Before G-D.
posted by Sara C. at 9:05 PM on February 12, 2011 [2 favorites]

in theory, even with a self-sustaining population, the number could be well below 0.5*P if the infant mortality rate is high enough and the life expectancy short enough. given a life expectancy of 16y, reproductive maturity at 14y and one pregnancy per year, if only 1/16 girls survive to age 2, and an insignificant fraction of boys survive (just enough to keep the population viable), then without doing the numbers I guess the median age would be 3 or 4... of course though it's mathematically and biologically possible I can't imagine any actual human population functioning like this...
posted by russm at 9:14 PM on February 12, 2011

As far as I know, and this is from a few sociology classes, population transitions don't usually happen ONLY because fertility rates drop. It normally happens for a number of reasons, namely a "third-world" country/population transitioning to one with better medical resources. Therefore, people live longer and people have more access to birth control. Maybe, that country will progress further and further to a point where birth rates decrease so much and people are living much longer, and it's more of an "elder buldge".

As far as that community is concerned, this part:

According to 2008 census figures, the village has the highest poverty rate in the nation. More than two-thirds of residents live below the federal poverty line and 40% receive food stamps.

makes me think that not only are their fertility rates way up, their elderly are not getting the kind of care that those in surrounding communities are getting. Historically, the next step in this story will be that the population continues to grow exponentially until someone, whether it's and outsider group or not, makes it their business to change things. A good example of this is China's population control measures. Whether or not you believe it is the correct thing to do, this is one option governments take. Another example, mentioned above, are various populations in Africa with little to no access to proper medical care and an outright deadly epidemic on their hands. In that case, charitable organizations and countries are stepping in to help.

Again, this is all very general, and I really have no idea how bad it can get, but I'm imagining that once the average age of a population gets below a certain point, there would literally not be enough people/experiences/resources to support the birth rate and the infant fatality rate will rise drastically. That's just my not-completely-well-informed theory.
posted by two lights above the sea at 9:24 PM on February 12, 2011

Er, infant mortality rate. Sorry.
posted by two lights above the sea at 9:28 PM on February 12, 2011

[few comments removed - OP is not looking for ridiculous examples or random speculation on US population control policies.]
posted by jessamyn (staff) at 9:49 PM on February 12, 2011

Remember, this is median, not mean. The median age in a family consisting of a couple who are both 25 years old and who just had triplets is about 15 minutes old. Their average age is 10 years old.

In a less contrived situation you can still get some pretty extreme numbers - consider a new housing development largely consisting of starter homes. Even if a few older couples move in, if the younger couples really average 2.5 kids each, pretty soon the median age is going to be of the oldest kid in the neighborhood.
posted by Kid Charlemagne at 12:04 AM on February 13, 2011 [21 favorites]

This is so off topic, please delete if inappropriate, but: Kid Charlemagne, that was the most excellent explanation of the different between median and mean ever. I'm good with numbers and I understand it better now than I ever have.
posted by i_am_a_fiesta at 12:42 AM on February 13, 2011 [1 favorite]

Kadin2048: That's very helpful, Thanks. The practice in this group is that pretty much everyone marries between 18 - 22, starts having children right away and keeps having as many kids as everyone's health permits. So that would make the real-world equivalent of "P" around maybe 22, so it looks like this population might be approaching the lowest median age it's going to hit - ? The next question would then be, what would the continued growth rate of the population be?

Sara C. - yes, my fascination has already led me to those 2 films. I'm betting that lots more similar films will be showing up, cause this is a great and wild story.

There seems to be only very minimal attrition (so far) from this community, as they are far more insular and self-contained than the Hasidic communities in Brooklyn, or even Rockland County.

Yikes again. No one has come up with an example of a population with a lower median age. Is anyone in the world of Demography noticing this?
posted by Corvid at 11:09 AM on February 13, 2011

Consider something like a very large orphanage - if you have a disease/disaster/war that kills adults and leaves kids, you could get the surviving kids being concentrated in large group situations with only a few adult caretakers. (Also if you had a situation where lots of kids were sent to eg a monastery or institution or quarantine.) This could bring the median age below the hypothetical reproduction-based lower bound described above.

I don't have in mind any specific situations like this, but that would be something to look at if you only want a group of a few thousand and you don't require the low median age to be sustained over time (ie you only want that median age at a single time). Maybe that wouldn't count as a "population group"?
posted by LobsterMitten at 2:15 PM on February 13, 2011

Thinking about this some more, I have an idea for you: Not too far from my house is a neighborhood where pretty much every house was built between 1946 and 1950. Now it's pretty diverse because every house has changed hands a couple times. But if you can find information for a place like that, I'm betting you'll find some pretty extreme numbers from 1950 to 1955, relative to what you'd expect. I'm not sure how granulated the Census information is from that era (or if you're just going to get big lumps, like whole counties) but that would be a place to start.

The difference is, this was happening at the beginning of an economic boom and not during a major recession.
posted by Kid Charlemagne at 3:29 PM on February 14, 2011

The difference there is that, back then, most families weren't having 5+ children. Hasidic families typically have a lot of kids. This is the reason for the demographic strangeness.

Then again, I dunno - maybe you could find a census district from the immediate postwar period for a similarly fecund religious community?
posted by Sara C. at 5:02 PM on February 14, 2011

the absolute limiting case seems like it would be (0.5)*P, where P is the age of reproductive maturity. That's the hypothetical scenario where you have just a single generation (no parents), they are all female, and they all get pregnant and give birth at the same time.

So I think that puts an absolute lower bound on it of 6.5 or 7, if you assume P=14

First of all, that's the median. Second, if you go out another year, and all the mothers have given birth a second time, it would be (0.333)*(P+1). Another year out and (0.25)*(P+2+1). Something like fertility drugs or intentional causing of multiple births would lower the median still further.

Third, the OP asked about the mean... if there was a disease or other event that caused more deaths in older members of the population, the mean could fall further than than. In the extreme case the only ones left living would be too young to care for themselves, grim as that is, which would make the mean quite low for a very short time period.

Grimmer still, there probably have been multiple times and places in human history where that has happened.
posted by yohko at 2:12 PM on February 15, 2011

OP asked about median. To wit: "Is there a lower limit to median age in human population, and what happens as a group approaches it?"

However, you are correct about the median decreasing further with future generations; playing around in Excel, with an exaggerated scenario (10 women, no men, a child each year) I can get it down to 1. It strikes me as an edge or at least fairly strained case, though. I think the issue is that with a small population the median stops being that valuable a metric, and you are better off looking at the distribution in a more direct way.
posted by Kadin2048 at 8:29 AM on February 18, 2011

To refine my question a little more: I'm most interested in what is likely to happen over the next few decades, rather than really weird hypothetical scenarios. That's not a terribly useful distinction, I know, given that "weird" scenarios, involving wars, diseases, roving armies of orphans, totalitarian governments and such, so often become Reality. But this is Orange County, NY, an otherwise affluent and boringly ordinary American suburb of NYC. And in the middle of it is this little bubble of humanity expanding like nobody's business, thriving like mad (if growth is your metric), straining resources but still growing.

I don't see increasing infant or maternal mortality putting a limit on things anytime soon. This community invests a lot of resources into care of mothers and babies. I've never seen any suggestion that old people are neglected, either; I think the life expectancy is probably not far from the US average.
posted by Corvid at 11:31 AM on February 18, 2011

What is likely to happen is either that

A) as time passes and the Holocaust becomes a distant memory rather than something that happened to your parents or grandparents, the imperative to have so many children so early in life will fade across the community and median ages will start to return to something approaching typical for that part of the US;


B) the Satmar community will expand to the point that they spill over into another town somewhere else (probably further upstate or towards Western New York where there are a lot of rural towns which have the infrastructure and need more inhabitants);

or perhaps

C) this is all moot because attrition will bring down future birth rates. If you're one of seven kids, and three of your siblings end up not sticking with the frum lifestyle, for demographic purposes you are really only one of four siblings within the community. Which isn't all that unusual a number.

I'm a little confused at what you think is going to happen, or why you're so concerned about it. Either they will continue to expand out from the New York area, or they will stop having so many children.
posted by Sara C. at 12:14 PM on February 18, 2011

Sara C.: I think that your option B will happen quite soon, with C becoming more important as time goes on. I'd wager that it will probably take a few thousand years for the Holocaust to become a distant memory in this community, though the imperative to grow will likely become less urgent.

Fair enough to wonder why I'm concerned--I wonder that myself. It's really none of my business. It's just a tremendously interesting story, admirable yet nervous-making to an agnostic, feminist environmentalist. I have many conflicting thoughts and feelings as I observe these events play out. It's a new and fascinating thing, having a population that has the opportunity to play out the consequences of combining high fertility with low mortality, and I want to understand it better.
posted by Corvid at 5:32 PM on February 18, 2011

For clarity, when I say "when the Holocaust becomes a distant memory", I don't mean the point at which these people are not aware that it ever happened.

If you watch the documentaries I recommended above, you'll notice that several Hasidic people interviewed (especially in A Life Apart) mention that they live the frum lifestyle out of respect for their parents, who are holocaust survivors, and that they believe it's important to have so many children because so many of their relatives were killed in the genocide.

Hasidic people of the current childrearing generation, whose grandparents immigrated to the US in the aftermath of WWII, still have a living connection to that mentality. But their children, the children who are part of the intense demographic shift you describe, will likely not have personally known anyone for whom the Holocaust was a concrete reality. They won't feel that immediate loss of aunts, uncles, cousins, grandparents, etc. They won't have people guilting them to stay, to be closeted, to drop out of school, to get married at 18, turn down a career, etc. because "this was very important to your grandfather, who WORKED HIS FINGERS TO THE BONE so the Nazis wouldn't be able to send you to the gas chamber".

So it's really hard to say how this generation will respond in that absence of that very concrete and immediate form of pressure. Looking at Hasidism as something that exists within history, and which responds to historical forces, it's hard to predict exactly what will happen. But whatever happens, it'll probably be a lot more boring than you think.
posted by Sara C. at 5:59 PM on February 18, 2011

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