Everyone wants their kids to have a good life. If I have to support the King's son becoming King if it means my son will replace me as Duke, so be it. At least, that's how I've always thought of it. The nobles want to uphold the status quo and the peasants don't really get a say. Plus, propaganda.

Sometimes it's not the question of who your father is, but who back your claim to the throne. Kings and princes always need to form personal connection with nobles in the court to gain their support in matters of the state, and in return award them with title, land and power. As long as you manage to keep the close circle happy they have no reason to rebel against you, and they'll put their trust on your heir to honor this system. Not to say the status quo can't be changed; rebellion and usurpation happened all the times even in countries with long hereditary tradition. When the nobles dissatisfy with their monarch and see some other person, or other political system, as a better alternative, they're willingly to revolt (look at the English Civil Wars and Glorious Revolution as examples).

A lot of good answers have been given pertaining to this not being an absolute concept. Still, there was a traditional order that, by and large, remained accepted in many regions. If we stick with, for example, medieval Europe , the example of Henry II before Toulouse may be instructive.

The Angevin king Henry II ruled one of the most powerful kingdoms of 12th century Europe, and this created a fair amount of tension in the complicated diplomatic web that was feudal Europe. So in 1159, the king of England had set his sights on conquering Toulouse, a piece of territory disputed between him and count Raymond. Now Raymond was the brother-in-law to the king of France, Louis VII. Louis was worried since Henry - also his vassal on the continent, as he was only king in England - was already so darn powerful, acquiring Toulouse would only strengthen his position. Yet ultimately, he made a genius move. As Henry II arrived before Toulouse at the head of one of the largests contemporary armies, his heart sank when he saw the banner hoisted over the city: Louis VII had taken up quarters inside before his army arrived...

The game was up, and Henry knew it. He ordered his army to ransack the countryside and went back home, failing his ultimate objective. So, what happened? Why did Henry hold back & why is this instructive for your question?

Because Henry had sworn fealty to Louis (for those lands for which he was beholden to him). Henry himself had more than enough rebellious lords under him, keen to resist his royal authority. To attack Toulouse with Louis inside was to break all his oaths he had sworn (under his god), making him that thing medieval society as an honour-culture despised so hard: the honourless oathbreaker. So Henry knew that if here and now he pressed his claim on Toulouse and violated the person of Louis VII, he would enable each and every lord who held a grudge towards him from Scotland to the Pyrenees, to safely renounce their allegiance and rebel.

Medieval society, like many, was structured on concepts of honour, typically expressed through oaths and the like. People took these seriously, for breaking the social contract - then as much as now - renders one open to the fallout.

So many things underpinned royal authority, from the more raw aspects of money and power (soft or hard) to the those of concepts of legitimacy.

IDK, medieval history seems to be mostly about the king fighting with his vassals, and then when the king dies, vassals fighting with the princes.

In Persian and Turkish history, there always seems to be a period when one brother kills all the others and then becomes king and has to go and put down all the revolts that spring up.

So I would say mostly through superior violence is how the hereditary monarch maintains their power.

The relationship between monarchs and their nobility was often extremely fraught, so much so that most of Asian and European history involves monarchs and nobility trying to curb the power of the other.

If enough of the nobility decided that the current monarch was not to their liking for whatever reason, they could and often did choose to back someone else, who could be a relative of the current monarch or someone else entirely. Most of ancient and medieval history in much of Europe and Asia is that happening over and over again.

China did away with its warrior aristocrat class first and replaced them with scholar officials, but that didn't prevent the eventual toppling of all subsequent dynasties when economic and political troubles inevitably occurred.

> how did hereditary monarchs actually hold on to power?

Divinity might help. When the king is seen as a god or chosen by god/gods, it validates his power to everyone. If the autocracy/nobility also derives their power and privileges on the same basis they have little interest in challenging the system.

When and where?

>But the older I get, the more skeptical I am that the social class just below the monarch would blindly subject themselves to the monarch's authority for the sake of upholding tradition.

They never did. The nobility had no problem revolting or disobeying in states that were often sorely lacking in centralisation; Magna Carta was practically signed at swordpoint for instance, by King John, who was compelled to do so by a bunch of rebellious nobility.

You are not talking about a modern state where we all reluctantly agree to elect a leader, and that leader wields vast power we have no access to. You're talking about a tiny minority who already has an enormous amount of power (physical, financial, political, social, etc) being kept somewhat in line by constant application of power-sharing agreements and marriages, competition, open violence and threats, gifts of land, money, and so on.

definitely not as much of an academic answer as everyone else gave here, but your question reminds me of how when Queen Elizabeth II died, no one (or at least i didnt hear of anyone) questioned who was next in line to the throne. maybe we do just blindly accept this stuff and maybe they did the same thing back then? plus they had all the religious arguments that it was their divine right to rule- cant mess with gods choice!

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Adding to all the excellent answers here, there were also cases like that of Charles VI of France, who ascended the throne at the age of 11 in 1380. However, he was regulated to the position of a regent by his uncles who called most of the shots in how things were done. He had a very hard time, even after coming of age, asserting his position as the monarch. He eventually did, though that was also debilitated by the madness that seized him in later years.

Interesting question honestly. You got to wonder what was going through a noble's mind when he saw that the heir was young, sick, or mad. They respected traditions but they were also pragmatic enough to see that there were ways of working around an invalidated monarch.