Why was/is such a high value placed on "heredity"?

To be clear, I’m not talking about the right to inherent wealth and property from your parents.
I’m referring more to how back in the day, leadership was often determined by hereditary blood lines. The concept that someone was the “rightful ruler” simply by virtue of being born to particular parents. Granted, it didn’t always prevent someone from making a grab for the throne. But it seemed to matter enough that one had to take birthright into account.

Even today, in some Old Money circles, there is a sense that being born to generations of wealth is somehow superior to having earned most of it within your lifetime.

How did this sort of thinking come about and why did (does) it persist so long?

Because it’s easy to believe

Especially if you’re uneducated and illiterate and born into a society where this is the established order.

Given that examples of hereditary monarchy existed in societies throughout the globe, the ultimate answer is likely biological or sociological.

Well, for starters, the powerful headman/prince/king/nobleman/ whatever-is-applicable-for-historical-period-in-question would WANT his child (son, generally) to grow up to be powerful and significant after him - 'cause, you know, his kids. So it’s in his interests to promote the idea of heredity/“noble blood”, because it benefits his family. Any idea which is being promoted by the most powerful person/people in the society has an automatic leg-up.

Next, after this concept has been in a civilisation for a few generations, all the most powerful people will be themselves the beneficiaries of the concept of noble blood/rightful rulership. So, obviously, they will have a vested interest in keeping the system going.

Thirdly, in a pre-industrial civilization where there’s no universal education and frequent famines, it’s probably really the truth that the nobles and ruling families, who get properly fed all through their childhoods (or at least, have a better chance of it) and have time for education are a lot, lot smarter than the general run of peasants, on average.

And, in a violent age/society it’s quite an advantage to have a clear, well-accepted method of choosing the next ruler, because in cases where you have a number of guys contending for the job, that tends to end up in bloody wars and rolling heads. This still happened, of course, even with primogeniture, particularly when the heir was young and/or weak, but at least if there was a predisposition in society for everyone to be assumed to support ONE “most worthy” candidate, that promoted stability. It didn’t really matter for that purpose what the way was of choosing the one most worthy candidate, but “eldest son of the current king” was as good as anything.

Another factor - following from those Aspidistra describes - is self-preservation. Let’s say you’re king, and you want to stay that way. You already know you need a clear-cut heir to maintain stability after your death. But why primogeniture? Because you have better odds of being well-liked by your own children than you do your most powerful noble. If Minister Steve is your heir, it’s only a matter of time before Steve starts wondering how to speed up the succession. But if your son Jim is heir, he’ll hopefully be okay with waiting until you succumb to death by natural causes at a ripe old age.

Also, until fairly recently you didn’t understand the scientific method and the link between correlation and causation and such. You just observed things and assumed they were natural. So peasants are peasants because they were born to it. Merchants are merchants for the same reason. Millers, butchers, cobblers, etc. It’s only a short step from there to “well, kings are born to be kings.”

I think JohnT’s quote is also relevant even today. I was watching The Great Gatsby the other day (also remembered reading it in 8th grade or whenever). A key theme is Gatsby’s sense of inferiority to being a “self made man” vs having inherited his wealth like Daisy and Tom.

I suspect that there is a strong notion of entitlement from simply being born into wealth. Like being surrounded by it all your life (and thus fairly indifferent to it) gives you a false sense of superiority over those who actively coveted wealth and had to scrape and claw their way to success.

Kind of a “people should be happy with their place” attitude (which is easier when your “place” is a big mansion.)

It still exists today, in America.

http://psycnet.apa.org/journals/psp/105/2/247/

Terms like “noble blood” and “good breeding” suggest to me that people also believed that the traits that make a good ruler were inherited. And to some extent they were right. We now know that there’s a genetic component to traits that might be valued in a feudal lord, like physical size and intelligence. Environmental factors like nutrition and education also play a role, but as Aspidistra points out, the ruler’s children would have an advantage over the peasants there too.

In feudal times people didn’t know about genetics, but they did understand that offspring tend to resemble their parents. IIRC the belief that acquired characteristics could be inherited by one’s offspring goes back at least as far as Aristotle, so it may have been accepted that the son of a king would inherit the benefits of his father’s education and experience as well as his strength and intelligence.

It could be an agricultural type of worldview - people have always valued heredity when it comes to horses, grapes, etc.

Size, yes. Intelligence… Well, some VERY long SDMB threads can be found debating that…

What muddies the waters a little is that things like education are pseudo-genetic. They do follow family lines, but they’re a part of one’s social heritage, not actually in the DNA. Still, the children of educated people are much more likely to be educated themselves, and this is part of the illusion of “good breeding.”

I didn’t read anything Lamia posted as being supportive of those theories.

Stability was some of it as well, that you knew who would follow the present incumbent and not expect any surprises like persecution once they took over. Or perhaps that you knew where to run to to avoid persecution when that happened.

I think part of it can be traced to genetic survival strategies, if only subconsciously. After all, if you are in a superior position socially, you assume that you are superior in all other ways. Maintaining that status quo, for the higher classes, improves the chances of genetic survival and quality.