What evolutionary pressures would lead to massive differences in intelligence between the sexes?

Question

I am writing an alternate history timeline about a habitable Venus. On Venus are two species very similar to humans. They are Machos and Divas; they live on opposite sides of the planet. Both Machos and Divas are all dark-skinned due to increased sunlight.

Macho men are like human men, only with more extreme sexual dimorphism. They are larger, more muscular, hairier, and have higher testosterone levels than human men. Macho women also have extreme sexual dimorphism. They are smaller and curvier than human women. They are also hairless below the neck and have very high estrogen levels. Macho men go through puberty around age 14 while Macho women go through puberty around age 5. More importantly, Macho men have normal human intelligence while macho women are mentally-challenged (they can't understand middle-school level topics) and are very obedient. Macho women are generally domestic servants, surrogates, maids, cooks, sex workers, seamstresses, and waitresses. Most other jobs, especially careers requiring major physical or mental capability, have to be done by Macho men (or robots). Even jobs that are viewed as "women's work" in real life like teaching, daycare, and nursing, are done by men in Macho society.

Diva women look androgynous compared to human women, and Divas in general have less sexual dimorphism compared to humans. Divas have short scalp hair, but lots of body hair. Diva men look similar to human men, but they're wider due to larger muscles and body fat. Diva women possess high cognitive intelligence, but low emotional intelligence. Diva men are mentally-challenged and are very submissive, similar to Macho women. Diva men do all of the dirty, dangerous, and demeaning jobs; along with all other physical jobs, adult entertainment jobs, and unskilled labor jobs. Diva women do everything else that requires brainpower.

How would natural selection, sexual selection, or any other biological factors cause one sex to possess much greater intelligence than the other? Assume that the Divas and Machos have a similar evolutionary history to mammals. Answers that involve genetic engineering or artificial selections by aliens aren't acceptable.

Answer 1

The evolutionary pressure could be internal, with their genetics being an inescapable Y-split.

Lets assume that you have an species at the verge of intelligence, with two main genetic sequences responsible for the development of intelligence in a new organism, SMART1 (female intelligence) and SMART2 (male intelligence), both descended from a primeval SMART0. Due to the similarity between SMART1 and SMART2, and their placement in the genetic code, we get an unfortunate split:

• If biologically female + SMART1 - high intelligence.
• If biologically male + SMART1 - low intelligence/mental retardation.
• If biologically female + SMART2 - low intelligence/mental retardation.
• If biologically male + Smart2 - high intelligence
• If any sex+ Both SMART1 and SMART2 - no brain develops, miscarriage.
• If any sex but no SMART - no intelligence develops, specimen remains animistic and non-sapient

Effectively, the species had not choice but to slit into Machos and Divas eventually, to avoid miscarriages. Moreover, the only choice to preserve and ensure SMART was for the smartest males to mate with the dumbest females, and vice versa. This enhanced and amplified the effect, because the smartest Macho men were sons of the dumbest Macho Mothers, and the smartest Diva women were daughters of the dumbest Diva Men. The rest of the sexual dimorphism came after the SMART1/SMART2 split, as physical adaptations to the differences in intelligence. The evolution of both Divas and Machos would also be very rapid, since the intelligent sexes would suddenly greatly benefit, while the stupid sexes would be under significant evolutionary pressure to adapt, making the process self-accelerating.

Answer 2

It's all about sociality and lack of it

In evolution, it's theorized that humans are so smart because of a need to socialize with their group.

Macho Males

• Bulkier: Bigger bodies require more energy therefore they need more food than the women.
• Hairier: Less capable of regulating their own temperatures leading to more overheating.
• Testosterone: While I dislike gender essentialism, we'll assume Macho men are more violent due to greater testosterone for worldbuilding purposes here.

With this established, we can imagine an environment where Macho Males, because of their bodies, cannot mimic the Homo Sapien tactic of exhausting prey to death. They discover that banding together means better chances of survival and so they do.

However, they don't always secure enough food for all, resulting in violent competitions for who gets to eat, exacerbated by overabundant testosterone. Social groups defined by extreme violence and might-makes-right forms.

Macho Females

Macho Females are the opposite of Macho Males in the sense that while they are physically weaker, they trade with greater stamina, endurance, and thermoregulation. For the heightened estrogen, we'll go the opposite direction and assume this makes them more pacifistic and violence-averse.

Due to fundamental incompatibilities between Macho Females and Macho Male society, Macho Females are scared away and detach from the main social group when old enough. Macho Males allow it, viewing it as cowardice and less mouths to feed. However, since defying one's group is a scary prospect, Macho Females rarely do this and only in isolated instances.

Macho Females can mimic the Homo Sapien tactic so they can feed themselves. This constant exertion combined with their diet could lead to curvaceous bodies. Lone Macho Females can occasionally meet with a wandering Macho Male group, join temporarily to make babies, and leave when finished giving birth.

Evolution

Inevitably, traditions form to solidify this culture of social Macho Males and lone Macho Females. If we assume nothing ever changes about this set up in evolutionary time scales, Macho Males eventually develop enough intelligence due to socialization while Macho Females intellectually atrophy due to lack of need. When Macho Males develop agriculture and create civilization, they'll need to forcefully incorporate the Macho Females into it. Thus, Macho society as you describe is achieved.

Divas

Similar conditions can be applied to the Divas even with consideration of the more equal physicality. The only difference is finding a reason why Male Divas would leave which could be explained by any manner of proto-culture. As for Female Divas' imbalanced cognitive and emotional intelligences, it could easily be explained away by looking at how humans historically treated those who they viewed as intellectually inferior outsiders. You don't have to be a psychopath to treat the out-group terribly. All you need is to love your in-group more.

Answer 3

There is a theory that human intelligence is attributable to a genetic flaw which caused the pre-human jaw muscles to be massively under strength compared to other primates. This resulted in both a larger cranial capacity and the need to find alternate ways to acquire food (driving innovative thinking), which increased brain size and intelligence. You could make your mutations gender based. But seriously? Machos and Divas?

Answer 4

How would natural selection, sexual selection, or any other biological factors cause one sex to possess much greater intelligence than the other?

Providing nutrition to a big brain is "expensive" as is pregnancy and breastfeeding, for a species to have the best chances of survival and propagation the females may have to trade brain power for the ability to produce more children. Once a female is beyond reproductive age she'd only need to have enough intelligence to not be a burden on the community for reproductive success, she'd need only be a not-terrible grandmother. If the elder women can perform at the mental capacity of the average 12 year old then they can cook, clean, babysit, and perform other relatively routine household tasks that benefit the family.

If this humanoid species is in a more hostile environment, where they experience more extreme weather and/or are hunted by some other species, then the math changes on where the body needs to expend limited energy. Body hair aids in thermo-regulation which means fewer calories expended in heating up or cooling down the body. Greater intelligence means better ability to evade predators, find food and shelter, and generally protect any children. The males may have to trade brain power for physical strength and endurance, meaning the females are doing most of the thinking in the family group, tribe, community, or whatever. Once the females are beyond reproductive age they can add to the survival chances by being teachers, guides, engineers, tacticians, and so forth.

Answer 5

with the apparent levels of social commentary / allegory being set up in the story, your readers are unlikely to expect much evolutionary rationalization.

I beg to differ: evolutionary rationalization adds a tart aftertaste of Clans of the Alphan Moon meet Just So Stories. First a disclaimer:
This is NOT a philosophical tale about human sexuality. This is not Earth, this is VENUS. (In petto: ) It had to be Venus. Of all planets, for X'ssake. Sigh. (Aloud: ) KAPIERT?

To answer the title question: Easy. Machos and Divas originated as criminal castaways from a single Venusian species.

The original species, the only intelligent one on Venus, consisted of Macho males and Diva females. The castaways were a mixed bunch of sex offenders and criminally insane of both sexes, deported to a penal colony. Then, the whole species was wiped out by a pandemic outbreak of some STD, including the wardens of the colony.

The inmates were spared because no promiscuous contact was possible for them, so in the end they had to ward themselves as the only surviving remnant of the species. Of course, first thing in the morning, they freed themselves and started looking for sex.

Among the criminally insane were some people of both sexes with a rare genetic condition, which made them unable to learn past the age of 10-12 and sort of froze them in a state of irresponsible childishness, without impairing their physiological development; they appeared to be blind to the consequences of their actions and had been commited to the colony for the odious outcome of the most reckless pranks.

It turned out that the psychological immaturity of children is what aroused the Venusian paedophiles of both sexes, much more so than e. g. a juvenile look. As a result, some of the paeodophile inmates developed fruitful relationships with their emotionally retarded, but physiologically adult preys and multiplied, transmitting the rare "frozen in childhood" gene from parent to same-sex offspring. Eventually, they speciated into Divas and Machos.

Long story short, the founders of the Macho species were some Venusian Big Bad Wolves lucky enough to bring their respective Little Red Riding Hoods to the altar and to procreate happily ever after.
Correspondingly, those of the Diva species were Big Bad Wolvesses lucky enough to do the same with their respective Little Red Riding Tuques.

Answer 6

Gender Roles Affect Evolution

Humans are a self-domesticated species meaning that our culture has a direct impact on our physical and mental traits. Self-domestication is seen in other highly intelligent species as well such as dolphins and elephants. This means that any species intelligent enough to form a society will selectively breed itself based on the expectations of that society, not just through their selective fitness in thier environment.

This Sort of Happened on Earth

Within the human race, the greatest level of sexual dimorphism is seen in Native American populations whereas the lowest level is seen in Native African populations. These differences can be seen in how people function at the tribal level.

In most of the tribal societies of North & South America, you see women and men filling mutually supportive roles where it is the woman's role to be a mother, and the man's role to provide, and these roles are strongly enforced by thier societies. These societies are not necessary misogynistic in that they don't all look down at women's work as lesser than men's work, but they do in most cases have very strong gender role differences in what work is expected. Work outside of these roles is very rare, especially prior to the colonial period, because widows were typically taken care of by the tribe or simply killed making thier ability to function without a husband a moot point. So being smaller and easier to take care of increased thier survival chances.

In contrast, most African tribes expect women to work. They either treat motherhood as a highly communal task where women will often take turns watching over the tribe's children leaving women with more time and a higher expectation to take part in the tribe's heavy labor: especially farming. Or, they will expect women to strap thier babies to thier bodies while they do thier jobs. Either way, tribal African females (generally speaking) have to be just as strong, intelligent, and endurant as tribal African men.

European tribes were somewhere in between. They had strong societal expectation of gender roles like the Americans, but wars of total genocide and proper support systems for widows were more rare in Europe than America; so, women were often left as widows and expected to work to support themselves (at least for a while) if thier husbands died which led to a greater survival rate for strong and intelligent women.

How to make this effect even more extreme

In Africa, raising children is still the woman's job, even if there is an expectation that they also do heavy labor, but this inherently means they are not expected to maintain the same amount of heavy labor as thier male counterparts. You could take this a step further if it is the woman's role to do the labor and guard the society, but the man's job to raise the children, but this leads to some significant issues when talking about Social K-Select Mammals. Every single Social K-Selected species of mammal has bigger, stronger males than females because mammary glands and internal gestation make females the de facto parents, and the males the de facto protectors. In cases of r-select mammals like rodents, the females are sometimes the larger gender, but r-selection is mutually exclusive of an intelligent species. So, making Divas androgynous among themselves is one thing, but making them female dominant is complicated. The best way to make Divas female dominant would be if they were not mammals, but laid eggs since this would take away the complications that make mammalian females the de facto parents. Then, you could flip the gender roles with very little natural consequence.

Likewise, the American system could be made even more unequal if your take on a more aggressive approach to dealing with women and men who do not fill thier roles well. Boys could be put through rigorous rites of passage that that kill them off if they are not strong and smart enough to pass them, and women who are too big or independent might be killed off by fathers who see them as too much work to raise, or they are ritually sacrificed as a way of giving thier gods the "best" women that thier tribe has to offer.

Answer 7

Not sure if this would be something you want, but I know some species of animal have high maternal mortality rates, or just die after mating. Especially in the case of the weaker women you outlined, where they're smaller, thinner in places, etc., you could decide to have a really high probability of a large number of children being borne rather than just the one-at-a-time that's normal for real humans, and justifiably say that almost every female dies during or shortly after childbirth as a result of the physical, hormonal and nutritional strain on her body. This might be a great place to put some kind of allegory too, I don't know for certain. Anyways, if all the women die in childbirth and develop sexually at an early age, it might make sense that they're essentially discarded by evolution. Basically, investing resources in a longer-term life makes more sense - so especially at scale, you'll have evolution deciding to allocate more of the limited resources to the more long-lived men and conversely to diminish the women.

However, I'm not sure if a male version of this is nearly as likely? Men, of course, don't bear children, so maternal deaths don't make sense for them. But again, you might want to have a high, early male mortality rate for whatever reason... maybe a lot of sexual selective fighting happens, competing for mates, and the majority end up dying, but this seems slightly absurd as it decreases the fitness of a group. Maternal mortality counterbalances the loss with a new set of additions, and is therefore less prone to reducing fitness.

Anyways; these ideas aren't necessarily likely to happen, but they could provide some explanation and I don't consider it to be impossible.

Answer 8

Different evolutionary origins

In spite of both of these species being humanoid, they've arrived there via very different evolutionary routes, because of different pressures of the environments they've evolved in.

Machos

Your habitable Venus happens to have a geography where most of the landmass is concentrated in one of the hemispheres, where the other hemisphere has mostly only small islands and archipelagos. Machos evolved in the hemisphere with big continents. Venus is closer to the Sun, so the average temperature on the planet would be higher, so let's say the Machos evolved in a jungle environment. Let's also say that they've evolved somewhere in the middle of a food chain, so plenty of food available but also plenty of dangerous predators. So Machos ended up living in very densely populated colonies, maybe outside of hunting ranges of especially dangerous predators. Under these conditions they've evolved a social structure a bit like an ant hive. This encouraged high dimorphism, with males being like worker or soldier ants, tasked with getting food for the colony and defending it from many predators, while the females were more like ant queens, exclusively occupied with birthing the young and caring for them. Once the species achieved sapience, the tasks of the males required more intelligence.

Divas

In spite of their humanoid appearance, divas evolved from fish. Specifically, a deep-sea dwelling fish like Anglerfish. Evolutionary pressures of that environment led to extreme sexual dimorphism, with females being much larger and more complex creatures than the males, whose role was pretty much limited to impregnating the females. At some point, perhaps due to climate change, they evolved to be land-dwelling creatures and live on the islands and archipelagos. In the course of their evolution, living in a completely different environment, the males gradually evolved to be less dimorphic and closer in size and abilities to the females, but are still a lot less capable and intelligent than them.