There was a discussion several threads back about what is and isn’t RGRE. I’m obviously somewhat unqualified to answer unilaterally, but it made me think about the topic from the perspective of civilizational evolution.

I’ve grabbed a set of qualifiers that I could reasonably ascribe to mares and stallions some couple thousand years ago, say this was their state at the start of recorded history, or deep in the tribal phase. Some of this is based on IRL horse behavior. Some of it is conjecture. Some of it is filling in blanks or oversimplifying somewhat complex interactions. Either way, this could serve as the state ponies have been in at the start of recorded history, or at least the deep tribal phase. The traits are as follows:
IRL HORSE BEHAVIOR (ish):
Females: Guiding, Hierarchial
Males: Protecting, Competitive
MAGICAL PROFICIENCY:
Females: Active, Finesse-focused
Males: Passive, Force-focused
(FICTIONAL) PSYCHOLOGY:
Females: Proactive, Flight-oriented
Males: Reactive, Fight-oriented.
These set of baseline proclivities would determine biases in gendered behavior. Females, for example, would be more prone to thinking ahead, and utilizing magical means to solve problems, but be less able to react in situationally satisfactory ways, and less able to rely on passive magical effects. Males, on the other hand, would be poor planners and incapable of elegant magical solutions to problems, though excel at split-second decision-making and instinctive responses to events, though in the case of magical problems, would have to rely on brute force, or relatively non-magical solutions.

I highlight these distinctions and dichotomies so that we may start from an initial state where neither sex is less capable, merely specialized for different functions. The purpose of this exercise is to chart a course where one may start at this point, and arrive at the mare-dominated society we know and write for. On that note, an issue I never saw what was, to me, a satisfactory response to; what is the biological underpinning of the gender gap? With the above conditions, I can construct a hopefully interesting solution.

Our world is not static, and human development tracks with even environmental considerations, such as the Medieval Warm Period and following Little Ice Age. Reproduction, too, can be influenced by environmental factors, and some species have offspring sex selection determined by factors such as ambient temperature. Review the above conditions, then, and consider that foal sex selection could be linked to ambient magic levels.

Males would thrive in a high-magic environment, allowing them ample force to apply to their power-intensive magical implementation, bolstering their innate advantages in passive magical application (pegasus flight, earthpony plant-sense, etc), and foster a resource-abundant environment where the species can readily afford “competition booms” where the individual can be optimized to pass on the best traits, over the optimization of the species as a whole, to which such trait selection would eventually lead in a slower, yet more thorough, form, though with some waste.

Females, meanwhile, would thrive in “magically thin” conditions, where magical solutions require forethought and precision to increase efficiency and require less power overall, and where obstacles need to be planned around and avoided rather than challenged. Species survival as a whole is a greater concern than individual survival. The lesser-able need to be either fully integrated or entirely culled, and it’s key to know which. Competition is limited in favor of a stratified society where direction is more important than absolute defense, as sacrifices must be made.

Higher ambient magic environments could then bias foal sex towards males, causing more colts than fillies to be born. This would allow ponies as a species to take better advantage of conditions in which males thrive, and cause more competition between males for a lesser number of females, ensuring only the best can pass on their traits. Meanwhile, lower ambient magical levels could bias foal sex towards females, in an environment where females’ traits are better suited to thriving, and more females means maximizing conception chances in an environment where conception is much more difficult (relying as it does on a certain amount of magical influence), at the cost of (male) genetic competition. Therefore, the specific state of the species’ gender ratio could depend on the ambient magic levels of the time period.

Then, our initial states: Our ponies start separated by tribe, in an average or slightly-high ambient magic environment. A frequent, even if not exactly common, trope in the thread is that pre-Unification, males may have been more active in the defense of the band or herd, and as seen before, I follow this assumption here. Like IRL horses, bands would have been lead by a dominant mare, who keeps the band in line by an implicit hierarchy, leading the group, while the (lead) stallion brings up the rear of the group to make sure no pony falls out of line or behind, and to protect the rear from attack. As tribes are still alone, cooperation is very formalized and detached, while intra-tribal meta-hierarchies apply between bands and even herds, ensuring mares remain in positions of authority. Threats are frequent and varied, keeping stallions otherwise busy with their core essential task of band and herd defense.

With the Unification, however, a major cultural upheaval begins. Closer cooperation between once-disparate elements between tribes threatens strictly hierarchal societies; who, exactly, is in charge? Stallions step into this space, their lack of concern over stratified power structures allowing them to initially meet as equals, and while relationships develop over time, their competitive nature allows bands and herds to relate to those of other tribes by natural means of efficiency and production. Stallions, not mares, then become the ambassadors between other tribes, while mares concern themselves with the core operations and maintaining intra-tribe relationships. At the same time, however, ambient magic starts to drop. Threat stratification occurs where the lesser threats that can outmultiply the resource scarcity emerge alongside apex threats that can compete by simply dominating resource allocation from the top of the chain, with all in the middle falling to one camp or the other. Stallions have less threats to face, but more dangerous ones. Meanwhile, their preferred methods of magical combat failing, now must rely on mundane strength, which disfavors older stallions, and reduces stallion life expectancy.

As the Unification period progresses, the social upheavals are smoothed out, but ambient magic continues to decline, now enough to start influencing gender ratios (which may now be approaching 3:2, or even 2:1). Stallion lowered life expectancy now compounds with this additional factor, requiring a proportionally higher number of stallions to be allocated to defense. This, combined with the competition-verified inter-tribal hierarchies now established, allow mares to step back into positions of authority across the board, freeing up stallions for protection duty. At the same time, mares are quickly overtaking stallions in positions that require magical exertion, whereby they also start to slip into defensive roles, albeit rear-line ones at first, like support casters or barrier-maintainers. Stallions more and more are pigeonholed into certain roles, as their scarcity is compounded by multiple factors; a species-wide reallocation of resources to where they’re needed most. Even still, their social tendencies from the last era remain, ensuring their reputation as social butterflies.

By this time, we have mares in dominant positions of authority, on top of being in a magical environment that favors a mare’s talents. Celestia has ruled – alone, now – for several centuries. The Canterlot nobility is entrenched, the Pegasi Legions are dispersing into local defense groups rather than centralized commands, and the Earth Pony herd-analogue of ‘towns’ has become the civilizational standard, supplanting the other tribal types (see my ‘history of herding’ paste), with bands now themselves known as herds. Society is now, by and large, resembling a form we’re somewhat familiar with, even if a more primitive reflection thereof.

As we approach modern society, we see a few more changes that cement things in place. The gender ratio may reach as bad as 3:1 or 4:1 on average, though each tribe may have different ratios as ambient magic reaches an all-time low, exacerbating regional differences. Per my prior worldbuilding dumps, atmospheric magic may remain at decent levels, allowing Pegasi ratios as high as 3:2, mountain-dwelling Thestrals at 2:1, unicorns migrating to lowlands suffer 5:2, and earth ponies as low as 3:1. Threats have, however, become very rare, with mares occupying defensive "maintenance roles", leaving the few remaining stallions free to stay home and, er, make the most of themselves.

Harmony is already a core cultural institution, further disfavoring competition in an environment such clashes are already ill-afforded (and likely due to that reason). Mares as a conscious guiding force is all but required to ensure harmony is maintained, even if it means remnants of hierarchal structure linger throughout society. Between this authority and the fact that, in an increasingly complex society, their magical traits are more highly prized than ever, they almost universally fill the niche as breadwinners.

At the same time, that societal complexity highlights a stallion’s strengths in the home. Their reactivity and flexibility allows for better raising of foals, easily handling the wide variety of situations a mare may have trouble wrapping their heads around. An ideal stallion is one that can navigate the incredible array of problems facing a modern herd, from sudden reversal of a herdmare’s fortunes, to difficulties with colts and fillies among their peers, to neighborhood issues where their social skills shine, even if it means a subtle war with the neighbors whose foals are better.

We’ve arrived at a point where, from their pre-historical or early-historical distinctions, civilized ponies have settled into a new pattern of gendered traits:
In societal obligation, mares are oriented towards work, and balancing the needs of their herds with those of others in their locality and beyond, while filling any job required of them by society.
Stallions, meanwhile, are adept at handling the home life, first and foremost concerned with the well-being of themselves, their mares, and their offspring, before any other consideration.
In magical performance, mares have long since settled into any high-’dexterity’ or intricate work, their greater efficiency and precision allowing them to easily handle the widespread need for magical finesse and acuity, even if their spell talents extend not much further than the telekinetic. Many often specialize heavily in their cutie-mark-derived special talents.
Stallions, meanwhile, do not have the resources to perform the feats of magic they once were able to, preferring simple methods. Their passive strengths have become their only strengths, augmenting their physical abilities and innate tribal passive forms, but within these limitations they’re quite adaptable and magically agile, innately understanding direct and thorough solutions.
In societal function as a whole, mares are keenly in tune with social synergy, able to work well with any other mare they need in order to accomplish a goal. Ad-hoc working groups are common, and often develop into further professional or social partnerships. They excel in navigating tricky situations, whether at work or play, with friends or coworkers, and first and foremost ensure everything runs smoothly.
Stallions, on the other hand, are deeply concerned with matters of personal and familial security, making sure all needs are met and continuity is maintained. They’re natural nurturers, fostering the development of their mares and their foals in all fields by any means, making sure the next generation is brought up right and proud.

This evolution of traits into a modern psychological ‘overlay’ is what gives ponies’ gender roles such distinction; unique yet inevitable applications of their traits, their histories, and even genesis as a civilization. It gives context to the often-used trope of stallions being somehow less able without dipping into biological absurdity, justifies the gender ratio, and differentiates in positive rather than negative ways. It can also be used as a foundation for further development, example, what would outlier or outsider archetypes look like? What does being a janefilly really mean? Etc.