# What would be a plausible reason why humans have lost their intelligence? [closed]

So, I’m writing a fiction story about a society of chimpanzees that are intelligent and have advanced technology. Cars, air travel, cities, cellular phones, and space travel.

My human characters travel here by spaceship crash, and land in a dense jungle. There they encounter humans, that do not act human at all. These humans look like them, but live deep in jungles, wearing strips of fur, climbing trees, and etc. Basically, these humans act like chimps in our universe. The intelligent chimps see these humans not as peers, but as zoo exhibits. But the apes learn the truth about humans. Over 3,000 years ago, in the “dark” mysterious times of history, humans were intelligent, but destroyed themselves in nuclear war, and the chimps' ancestors took the humans' role as intelligent rulers of earth.

My question is, what would be a plausible reason why humans lost they intelligence?

• This is very close to the "Planet of the Apes" scenarios. IANAL but It might be too close for copyright purposes so consider that when writing. – StephenG Jun 21 '18 at 21:15
• You need to also figure out, how do these humans survive. Chimps have quite a lot going for them physically, compared to humans (such as being able climb the way they do). And 3000 years is way way too shot time span. It took some millions of years for us to develop from level of current chimp intelligence to what we are now. And same for human intelligence devolving, it won't happen that fast. You need some kind of gene tech, like a virus which alters genome, if you can't make the time longer. – hyde Jun 21 '18 at 21:18
• Humans are no longer physically adapted to a living-in-the-jungle lifestyle. Human social development is far more sophisticated than chimps, and led to cultural adaptations (clothes, tools, domestic animals, etc) that permit us to live together and cooperate in many environments and thrive. There is no plausible natural reason for humans to cease cultural adaptation and regress the social development that seems hardwired into us. Look for an artificial cause that retards frontal lobe development. – user535733 Jun 21 '18 at 21:54
• "So, I’m writing a fiction story about a society of chimpanzees that are intelligent and have advanced technology. Cars, air travel, cities, cellular phones, and space travel. " Did I miss anything in this part of the question? – Valerio Pastore Jun 21 '18 at 22:21
• Following US political news. – pojo-guy Jun 21 '18 at 23:27

We built AIs that did everything for us.

Before the nuclear war that destroyed us, we had all of our infrastructure managed by AIs, which could do it better than us. With that, and a society that (somehow) does not reward individual intelligence with reproductive success, we remove any evolutionary pressure to remain intelligent.

(People becoming less intelligent could also be written in as a factor in how such an advanced society brought itself to nuclear war...)

• The not rewarding individual intelligence with reproductive success already seems to be a thing, it just needs to be maintained rather than explained how things ended up like that – user2813274 Jun 22 '18 at 2:03
• Pixar called. They want their plot back. – Mazura Jun 23 '18 at 2:44

I would like to post it in the comment sections, but it'd eat up too much space, so here we go:

Losing our intelligence would be easy: a full scale nuclear war would turn the last survivors of humankind into scattered, helpless small groups too busy to scavenge food to care about education. As time passes and more people die and diversity wanes, those few who remain will be inbreeding and giving brith to a new generation plagued by growing genetical defects, among them idiocy.

Well, of course at this point, chimps should need to evolve fairly quick to take our place...but whoops, there is no time at all. For us to become who we are now, it took about 50,000 years if not more, and during this long, long walk we had a living, thriving ecosystems at disposal.

After WWIII, the world would be a monstrously cold, unforgiving, dark, radiation-filled place where the ecosystem is no more. Chimps die by collapse of food chain. We die for the same reason and the rapid and traumatic loss of infrastructures.

Your scenario, thus, cannot happen. You need another background for a starting point that will see us becoming idiots and chimps the rulers of the world

2007 study on global nuclear war

A study published in the Journal of Geophysical Research in July 2007,[140] titled "Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences",[141] used current climate models to look at the consequences of a global nuclear war involving most or all of the world's current nuclear arsenals (which the authors judged to be one similar to the size of the world's arsenals twenty years earlier). The authors used a global circulation model, ModelE from the NASA Goddard Institute for Space Studies, which they noted "has been tested extensively in global warming experiments and to examine the effects of volcanic eruptions on climate." The model was used to investigate the effects of a war involving the entire current global nuclear arsenal, projected to release about 150 Tg of smoke into the atmosphere, as well as a war involving about one third of the current nuclear arsenal, projected to release about 50 Tg of smoke. In the 150 Tg case they found that:

A global average surface cooling of −7 °C to −8 °C persists for years, and after a decade the cooling is still −4 °C (Fig. 2).

Considering that the global average cooling at the depth of the last ice age 18,000 yr ago was about −5 °C, this would be a climate change unprecedented in speed and amplitude in the history of the human race. The temperature changes are largest over land … Cooling of more than −20 °C occurs over large areas of North America and of more than −30 °C over much of Eurasia, including all agricultural regions.

In addition, they found that this cooling caused a weakening of the global hydrological cycle, reducing global precipitation by about 45%. As for the 50 Tg case involving one third of current nuclear arsenals, they said that the simulation "produced climate responses very similar to those for the 150 Tg case, but with about half the amplitude," but that "the time scale of response is about the same." They did not discuss the implications for agriculture in depth, but noted that a 1986 study which assumed no food production for a year projected that "most of the people on the planet would run out of food and starve to death by then" and commented that their own results show that, "This period of no food production needs to be extended by many years, making the impacts of nuclear winter even worse than previously thought." 2014

In 2014, Michael J. Mills (at the US National Center for Atmospheric Research, NCAR) et al. published "Multi-decadal global cooling and unprecedented ozone loss following a regional nuclear conflict" in the journal Earth's Future.[142] The authors used computational models developed by NCAR to simulate the climatic effects of a regional nuclear war in which 100 "small" (15 Kt) weapons are detonated over cities. They concluded that:

global ozone losses of 20–50% over populated areas, levels unprecedented in human history, would accompany the coldest average surface temperatures in the last 1000 years. We calculate summer enhancements in UV indices of 30–80% over Mid-Latitudes, suggesting widespread damage to human health, agriculture, and terrestrial and aquatic ecosystems. Killing frosts would reduce growing seasons by 10–40 days per year for 5 years. Surface temperatures would be reduced for more than 25 years, due to thermal inertia and albedo effects in the ocean and expanded sea ice. The combined cooling and enhanced UV would put significant pressures on global food supplies and could trigger a global nuclear famine.

You are severely underestimating the effects of a nuclear war here...

• Their is no other way, besides nuclear war in my opinion. And, nuclear winter would only last a few years, and most of the effects would happen in the northern hemisphere not the south so no – Menlo Jun 21 '18 at 21:21
• @Menlo please read the entire article that Valerio Pastore linked. The effects of a nuclear winter would be truly global, not restricted to the northern hemisphere. Look at the the mass extinctions of the Chicxulub impact and consider the timescale for the planet's recovery. – KerrAvon2055 Jun 21 '18 at 22:51
• The Ancestors lived and evolved though several ice ages, so you may be overstating the case a bit. – Thucydides Jun 22 '18 at 1:05
• The ancestors lived in several ice ages where, by all means, the planet had life to offer for predation, water and ground weren't polluted by radioactivity. Also, those ice ages had happened on a geological scale and life forms had adapted to stand the new climatic condition. A nuclear war would plunge Earth so suddenly in a nuclear winter, life could not adapt to it soon enough. – Valerio Pastore Jun 22 '18 at 6:03

### Feral

Tarzan and other stories that have a human being raised by animals have a significant flaw. In reality what happens is that the child never develops socialization skills or what we perceive as intelligence. These are called feral children. Note that unless the child learned some speech before the time with the animals, they never learn speech and do not act human.

The question then is why the children are feral. A child of two ferals would be feral as well. But how did the first generation become feral? We need to get rid of the socialized humans, the adults and older children.

### Nuclear war

I don't see it. Nuclear war kills lots of people, but it doesn't attack adults and leave children. And if it did in some odd circumstance, it would be difficult for the children to prosper afterwards.

### Biological contagion

There is a disease. It affects everyone. No one is naturally immune. Doctors discover a cure. The catch is that it is a genetic cure and can only be applied in vitro.

They gather as many women in the appropriate stage of pregnancy as possible. They create a garden of Eden. Food is plentiful and easily available. As the mothers die, they pair the babies with lactating animals.

All the adults are dead before the babies see their first birthday. They never learn to speak. They only make animal sounds.

The adults tried to leave electronic devices to teach the children. Unfortunately they fail. Perhaps they just didn't work. Perhaps the power failed. With no adults to fix it, even the best of automatic systems could go wrong.

That generation and every generation after it grows up feral.

### Biological war

If you want the Frankenstein plot of we did it to ourselves, a natural contagion may not fit. But a war where someone unleashed a biological weapon should. It gives you the same critique of human behavior in a slightly different vector as nuclear war.

The development of intelligence in humans is (as I understand it) related closely to the development of communication in a person. It has been documented that deaf people who are not taught and do not develop themselves any way to communicate do not develop much intelligence (until they do develop a way to communicate).

As this "Today I found out" video explains deaf people need to be taught something like sign language early on so that they can develop a mechanism that allows them to experience an "inner voice", which is apparently important to help develop mental skills. Before the development of sign language many (most ?) deaf people were left with significant learning disability.

So any mechanism that significantly damages your "pseudo human's" ability to communicate could lead to a seriously mentally handicapped society. I'm not expert enough to suggest an explicit mechanism for this, but I would assume you could research possible medical causes. My very limited knowledge suggests that damage to the right hemisphere of the brain can do this, so perhaps a difference in brain development between your outwardly human-like people would work.

Intelligence results from succeeding generations becoming better and better able to solve problems.

I don't think that any kind of apocalyptic scenario would decrease human intelligence. It might remove our technology and our learning but we would need intelligence to figure out how to survive.

@Qami give a specific answer. In general, any situation where people no longer have to think to be comfortable would, over time decrease average intelligence.

So, a Garden of Eden type planet would do it. If the local weather and climate are very comfortable without clothes and you can eat just about anything you pick up, people no longer need to solve problems.

A nanny state government or AI would do it too. Especially if asking questions or working the system had some physical or social penalty.

The Computer is your friend. The Computer is always right. The Computer says that everyone is happy. The Computer says that anyone who isn't happy will be used for reactor shielding. Are you happy?

Arguably, our current society is decreasing intelligence. It is fashionable to be stupid. The smart kids don't get dates or mates. There are a significant portion of the population who think the Earth is flat.

• Arguably, our current society is decreasing intelligence. It is fashionable to be stupid. The smart kids don't get dates or mates That's only really a First world problem. People who are comfortable in the status quo aren't motivated to change – nzaman Jun 22 '18 at 7:16
• @nzaman, that's why I qualified it by saying "our society" under the assumption that almost everyone who has access to this site is a member of a First World society. More specifically I was speaking of the US because that is the society I know best but I have seen the same phenomenon in Europe and Japan. – ShadoCat Jun 22 '18 at 17:32

Congenital illness.

There are many infections which cause infants to be born with intellectual disabilities - rubella, toxoplasmosis, cytomegalovirus, syphilis are all examples. There are also viruses which are transmitted mother to baby, with the virus not disabling the baby so badly that it cannot itself grow to adulthood, become a mother, and pass the virus to its own progeny. Hepatitis B is a good example of this, and hundreds of millions of people worldwide are infected with hepatitis B for this reason.

In your world there is a virus. It spread after the collapse. Maybe it is a variant of one of the above. Your mother has it, and you are born with it. It causes intellectual disability, reducing humans to the intellectual level you need for your story.

It would be nice if the virus also benefitted its host in some way - for example people with this virus have enhanced immunity to other infections. Maybe there was or is some bioweapon which does not work on people with this virus. The infected survived because they are infected.

As regards the chimps, they are chimp human crossbreeds. For whatever reason, a population of these came to exist before the crash. The chimp genes make them immune to the above infection and they have much of the chimp phenotype. The human genes give them intelligence. When the world fell and the idiot virus spread among humans, these crossbreed chimps spread through the empty world. How there came to be chimp human crossbreeds is a matter for your story. Some people think they may already exist.

Deliberate Genetic Engineering by Governments

Why would a powerful government want everyone to be unintelligent? Perhaps they want to suppress the population. This wouldn't make a ton of sense for their own people, as it would ruin their economy, but maybe large-scale genetic modification to vastly reduce intelligence is seen as a "humane" way to pacify a conquered population.

Then, there's a brief nuclear war that kills off the intelligent people who did the modification in the first place, leaving only their victims behind (another bonus here is it's easy for a few intelligent people to survive in a bunker somewhere, which could thicken the plot at some point).