Are there any plausible reasons that a culture would remain stagnant, such that since the time of your grandparents' generation your society's way of thinking remained the same?
In our comfortable modern world, we're afforded a fantastic amount of safety within which to innovate and change and experiment. Certainly in the First World at least, and comparatively in the Third World too. It's very easy to experiment with moderate change generation-to-generation if the only real downsides are disapproval of the previous generation, and maybe having a tougher time of things getting on in the world.
Roll the clock back to pre-history and the situation was very different. The risks around not doing what other people have done before you are much, much greater.
Maybe today I'll try those red berries that no-one eats. Turns out there's a very good reason no-one eats them, you poison yourself and die.
Maybe today I'll stay out a little later at night to get some stuff done. Turns out there are predators out at night and you get eaten.
Over generation, cultures living in dangerous situations develop a very sophisticated understanding of what is wise and what is unwise to do within their environment. To our modern eyes it can come across as stagnant, when in actuality it is consciously and intelligently adapted to their circumstances.
It's also important to note that the danger a culture is adapted to mitigate can be real or imaginary (I'm probably going to out myself as an atheist there). For cultures like the Inuit, deviation from established practices meant a very unpleasant death. For cultures like the Amish, deviation from their way of life carries the danger of not being granted the favour of God and access to heaven (alongside the very real danger of being shunned or excommunicated from their support network).
In a way, we all do roughly the same things as our grandparents' generation. It's just that over the generations we've established an environment where innovation and societal change isn't as dangerous as it used to be. In fact, if you look back at our current grandparents' generation they were part of significant innovation and societal change. Same thing if you look back at their grandparents.
tl;dr If you want to find out why a culture appears not to have changed for a significant amount of time, find what dangers their conservatism is mitigating.
Prevent progress and development
What @AngelPray said in their comment: that is how much of the history of humanity has looked like.
We have boom periods; we are in one now. We have had a few bust periods; like the Black Death epidemic that set us back 200 years. And in between these pretty much nothing happens for a long time.
How to prevent progress and development
- Prevent trade; impose self-sufficiency or centralised sustenance. Trade encourages people to travel and to bring news, innovations, and ideas far and wide.
- Prevent science and academia. Shut down universities and other academic institutions. Declare activities that challenge The Established Truth™ to be illegal, heretical.
- Promote tradition. "This is how we have always done it, and this is for a reason: it works".
- Abolish public discourse. One of the reasons we are so keen on declaring Free Speech / Free Expression to be a basic human right is because Free Expression boosts progress and development. Some even say it is a requirement for development and progress.
- Foster paranoia and sectarianism; a a culture of Us vs. Them. Not only will this prevent people from coming together to develop new things; it also leads to conflict.
Let's reverse the question, and see where it leads.
"Why WOULDN'T a society be the same between the grandparents and grand children?"
I posit that it comes down to three things: a critical mass of scholars, universal education, and the printing press.
In order for society to have developed as fast as it has over the last three hundred years, it took a critical mass of intelligent, well-educated people who could study from the wisdom and knowledge of those who came before. That is, an organized system of advanced learning, wherein knowledge of others was recorded in writing, mass distributed among a large body of scholars, and systematically and methodically transferred to new students.
When scientific scripts had to be reproduced one at a time by scribes who laboriously manually copied the material, there was little dissemination of knowledge. It remained exclusive to a small body of scholars, predominantly in religious institutions, constrained to a local area. Knowledge was passed down orally from generation to generation, and this demanded a high degree of standardization and repetition. Any new knowledge would quickly be prone to extinction unless it was repeated often enough through a widespread population. That is, any new knowledge would have to reach a critical mass of people before it was integrated into the common narrative.
So your 'plausible reason' becomes 'lack of formal educational institutions, no ability to reproduce and distribute a permanent record of knowledge that can be built upon by others, and a lack of critical mass of interlinked scholars'.
The contribution of universal literacy can not be understated in the advancement of knowledge.
Tradition or lack of technology
I think a plausible reason why a society doesn't change too much can be either preserving tradition or lack of/ refusing technology.
- The Amish in the US
- The Shaolin in China
- some tribes in Africa
I'm going to add another reason why society become stagnant:
Revolution occurs when there is a social discontentment
The middle-class are the pillars of the country. If given the opportunities and skill sets required, they will create active demand that industrial/productive activities rely to keep moving. If majority of the people are middle-class and content with the status quo (or manipulated), kingdom/government wouldn't be overthrown and new ideas wouldn't be born. That is why any society that wants to keep advancing must not undermine the middle-class.
As has been pointed out already, this is how the vast majority of societies have functioned right up until a couple of hundred years ago.
So, why was the pace of societal change so slow?
The short answer (the one that's common across the whole world, although not the only factor) is that the pace of technological change was also slow. If the world hasn't changed that much since your grandparents' time, society won't have done either - and that's really the key to keeping a society in stasis for a long time (oppression is great, but it's hard to keep going for decades, let alone centuries).
The long answer is really a question for History.SE, but as a pointer have a look at the Agricultural Revolution.
Societal change is driven by change in the world
(AKA society only changes when forced to)
This gives us a decent framework for constructing a world in which society doesn't change:
- The climate and physical factors are pretty constant. No climate change, no great changes to the land etc
- No interference from other cultures
- No real technological change
- A stable political system
Interestingly, one real culture that fits this bill quite well is Imperial China - their agricultural and political systems worked so well that there was no need for them to adapt, and China had enough size/power/commodities that it wasn't reliant on other societies for anything. ]
In a relatively modern setting this can be portrayed by the regime that the culture is under...
Leadership / Oppression
A lack of reform or change is often spun as "stability", and a society run under a despotic regime may employ this strategy.
Unless an enlightened, motivated individual is at the head of such a state, innovation and reform are not initiated, and therefore do not happen. Due to the absolute control of the head of state, the motivators for change must come from that singular individual, somewhat limiting the bandwidth for change even where it is desired.
Depending how far the egotistical side of a despot's character goes, they may even perceive making a change as admitting a mistake, something that dictators are liable to go to great lengths to avoid. (as per the old story of King Canute)
This can also explain how neighbouring cultures in the world progress, whilst the despotic society becomes isolated from its neighbours - to allow ideals from outside in could easily threaten the absolute control of the despot.
Various aspect of life are tightly managed and regulate in the interests of maintaining control. Education and media, the great informers of public opinion, are strictly controlled, Technology is restricted, innovation discouraged and swift and absolute enforcement of law is employed. Life for the everyday individual settles into the same routine, as dictated by the government.
This status quo can be upset upon the death of the leader, and the assumption of their position by a nominated successor, however if the indoctrination of that successor has been successful, they are just as likely to continue to rule in the same manner as has worked for their predecessor.
Similarly revolution or outside pressure can take a toll on such a regime, but this is not necessarily inevitable, particularly, as with the other answers, the further you go back in history for an equivalent setting.
I would say the ultimate example of this is the Australian Aborigines. Their culture is built from the hardship of surviving in the Australian outback. It changes slowly enough that we sometimes have trouble dating their work because the usual markers aren't where we expect them to be.
I choose the Aborigines, over other cultures which have such hardship and unchanging cultures, because you specifically mention having the current generation match the grandparent's generation. The Aborigines are especially suited to this particular metric. In many of their cultures (there are many Aboriginal groups, so there will certainly be counterexamples), stories are incredibly important. Stories are maps to places. Places that have water, for instance. If you have the story which tells you how to go to that watering hole, you have the legal right to go there. These stories are guarded extremely carefully. To spread your story too widely is to give more groups access to that watering hole.
Aborigines pass these stories on to their children, but not always. In fact, a reasonable chunk of them are passed not from parent to child, but from grandparent to grandchild. There are stories that are simply not known to an entire generation. This is their way.
Why? The explanation I was given when I visited was that it forces the group to stick together. No one generation has access to all the information, so no one generation has access to all the power. They can't go off on their own. They need the nearby generations to survive. This ensures the group stays close knit, whether they like it or not.
Now whether this counts as "stagnant" or merely not changing, is another question entirely. Stagnant is a very negative word, and it I would be hesitant to use it to describe a culture without first walking a mile in their shoes.
You might consider biological basis for this (https://en.m.wikipedia.org/wiki/Biological_basis_of_personality): humans seems to be hardwired to seek and value novelty, what if this part is affected? For example, say certain type of pollution caused genetic changes leading to significant reduction in novelty seeking (correlates with dopamine activity), increased harm avoidance (correlates with serotonin activity) and maybe increased forms of reward dependence that cause people to highly value peer approval and social cohesion. The pollutant was not actively controlled initially because tests for biological effect of it might not found any negative effects (because the experiments would look for birth defects, cancer, etc., )
Idle hands are the
devil's plaything mother of invention
I just have one thing to add that I didn't see the other answers mention explicitly.
The reason technological change accelerated so dramatically in recent history is that it has been a self-reinforcing loop. People cannot invent radical new technologies if they don't have time to do it. In older times, a huge fraction of society's total work output was tied up in the production of enough food to feed everybody. It simply was not possible to devote any significant fraction of resources to scientists. In fact, many of the first scientists were nobility who essentially found science to be a hobby, like Lord Kelvin.
Similarly, people that are kept constantly reacting to things they have no control over have to exert everything they have just to keep where they are. They don't have time to waste trying to invent new things.
Our modern technologies have led to a massive productivity gain. That has given modern society the breathing space that it needs to allocate a significant fraction of its resources to developing better technology.
In short: One way to keep things more or less in stasis is to have some not-quite-apocalyptic struggle force the society to forego things like 'having a class of people that do nothing but invent new things' as being too costly in the short term to be sustainable. Perhaps some BBEG-sealed-in-a-can is threatening to break out and absolutely everything must go towards maintaining the seal. Perhaps everyone has to go all-out producing weapons and food to fight the endless zombie horde created when the neighboring country did something really really stupid.
Forces that help traditional patterns to continue:
- "Proofs" of the correctness of traditional ways.
- Limits on concentrations of power. (For example, Amish businesses have a maximum of 10 men.)
- Selection bias. (For example, Amish and Mormon young men spend a year or two in the outside world. 90 percent of the Amish young men return to Amish society. Any young men who prefer the outside world can join it. Similarly, a small fraction of hobbits "went off to sea.")
- High prestige for people who are experts on traditional ways.
- Strict controls against money-lending-at-interest (usury) within the community.
- Limits to what money can buy. If purchase of visible status symbols is discouraged, envy will not push people to make changes to the society. And it is likely that the best available investment will be raising more children in one's footsteps.
- Restrictions on propaganda by the outside world. (For example, a lack of televisions, or a distinct language.)
In many ways, this is the case in the developed world today, although we may not notice it because of all the hype around relatively minor changes. In Mark Steyn's book America Alone, he considers a hypothetical young man catapulted from 1890 to 1950 (about two generations' time, wouldn't you say?) compared with a man transported from 1950 to 2010.
The first half of the 20th century overhauled the pattern of our lives: The light bulb abolished night; the internal combustion engine tamed distance. They fundamentally reconceived the rhythms of life. That's why our young man propelled from 1890 to 1950 would be flummoxed at every turn. A young fellow catapulted from 1950 to today would, on the surface, feel instantly at home – and then notice a few cool electronic toys. And, after that, he might wonder about the defining down of "accomplishment": Wow, you've invented a more compact and portable delivery system for Justin Bieber!
In this blog post ("Holding Pattern") he reviews a 2014 WSJ article by Tim Aeppel (behind a paywall, unfortunately) which reaches a similar conclusion. Both observe that major, life-changing innovations ended around 1970. The only areas we've really innovated in since then are electronics, telecommunication, and medicine. Not that those are insignificant, but they really don't change "the rhythms of life". In 2018 you might spend your idle hours surfing the 'Net whereas in 1970 you might have spent them watching TV or doing crossword puzzles, but basically you have the same kind of leisure time budget.
By contrast, the first half of the 20th century changed where we live (more suburban, less of both city-center and rural), how we get our food (buying it at the supermarket and putting it in the refrigerator), what kinds of jobs we do, how we commute to work (more cars, fewer trains and horses), how we schedule our days, etc. It was an extraordinary period of change.
We like to imagine we're still living in that period, so we hype the differences that seem big but actually don't change our lives. For example, the iPhone seemed like magic compared to the feature phones that came before it (no buttons! touch screen! apps!) but if you take a step back you realize that it still does two things: make calls, and entertain the user. Yes it has a calendar and a maps application, but before 2007 you could have solved the same problems by looking at an actual calendar or an actual map, so it didn't really change our lives from a practical point of view.
I will grant that this is a pessimistic view of the past 50 years, but I would not say it is out of the mainstream. Possibly, this view is providing an impetus to younger would-be innovators to work on projects that have more practical real-world impact (e.g. Jeff Hammerbacher, when he quit Facebook, famously complained "The best minds of my generation are thinking about ways to make people click on ads. That sucks.") Maybe a decade from now, because of companies like SpaceX, Steyn's criticism won't be valid any more.