reminds me of this comment!

copy text from source: u/kmbI654

https://www.reddit.com/r/AskHistorians/s/BwJ2u9yzI9

This is an insanely complex question that no modern study I've ever encountered has actually tackled head on (despite some scholars raising it up). Any attempt at fully explaining it is ultimately going to be somewhat speculative/not entirely from scholarly studies, since they don't yet exist. I obviously will not be able to definitively answer this in a reddit comment, but I want to at least clarify some of the facts surrounding this issue. I'll also preface that my expertise is in late antiquity and Byzantine history, so any experts on earlier periods are more than welcome to correct me when my discussion of pre-tetrarchy events will be noticeably less in-depth. I obviously can't say that the answer to this question lies in a specific study, but would recommend studies from O. Hekster concerning dynasticism like Emperors and Ancestors: Roman Rulers and the Constraints of Tradition or the more general Caesar Rules: The Emperor in the Changing Roman World.

1. Emperors did not have a lot of sons

This is very much true but needs some more exploration. First of all, I don't think that infant mortality is a reasonable explanation for this phenomenon. Sure, children (and people of all ages, for that matter) were dying all time in the empire. However, plenty of Roman families were capable of having plenty of children. In fact, pre-industrial societies typically have larger families due to lack of birth control and economic incentives to have more members in the household.

Of course, maybe Roman emperors just happened to have a string of infant deaths throughout their existence. But one reason why I doubt this reason is that so few Roman emperors actually practiced dynastic succession compared to other European (or global) monarchs.

Think about figures like the French or British monarchs which sport very long lasting patrilineal direct lines of descent. Of course, the Windors, Habsburgs, and Romanovs of the modern era technically came from female lines, but their crowns mostly descended from father-son or mother-son, even during pre-industrial times. One extreme of this is the emperor of Japan which claims to have descended from a single male line since at least the 500's CE.

In comparison, biological parent-child succession in the Roman Empire was noticeably uncommon. Sure, there were cases like Vespasian-Titus-Domitian or Marcus Aurelius-Commodus, but when you actually crunch the numbers, father-son cases of succession more often did not occur than they did. A really unique fact is that until the Constantinians of the 4th century CE (Constantius I-Constantine I-Constantine II/Constantius II/Constans I), no imperial dynasty actually survived kept the throne into the third generation (grandfather-father-son). For the relevant dynasties before the Constantinians, Domitian, Commodus, and Caracalla were all deposed at fairly young ages before they could advance their families further. Meanwhile, other emperors simply did not have biological sons for whatever reason

And even for Roman history, the uniqueness of this situation is made even clearer the further you go down the timeline into the middle-late Byzantine period. The Herakleian dynasty of the 7th-8th centuries had a father-son chain (with some others ruling as senior emperor between this line) of Herakleios I-Constantine III-Constans II-Constantine IV-Justinian II-Tiberius (a co-emperor, so not numbered). The Macedonian dynasty of the 9th-11th centuries had Basil I-Leo VI (allegedly)-Alexander I-Constantine VII-Romanos II-Basil II/Constantine VIII. There's also the Komnenoi dynasty of the 11th-12th centuries and the really long line of the Palaiologoi dynasty in the last centuries of the empire that I won't write down. So the way later Romans were more than capable of having sons to a greater degree. Something else had to have prevented father-son succession in the earlier centuries other than simply infant mortality in my opinion.

2. Emperors were always (aside from one exceptional case) succeeded by designated as heirs their biological sons if they existed

This phenomenon of emperors not having sons gets really weird when you consider that if an emperor had a biological son who survived childhood, they almost always chose to make them their heir. The only instance in which an emperor explicitly passed over their biological son for succession was during the early tetrarchy in which the Caesars Constantius I and Maximian passed over their sons Constantine I and Maxentius for non-dynastic appointments. But even this comes with caveats, the big ones being that this was a conscious choice by Diocletian in how he wanted the tetrarchy to operate; it was not the traditional norm. And in the end, Constantine and Maxentius both asserted their dynastic claims to power, and Constantine would later overthrow the tetrarchy and establish dynastic control over the empire.

The two factors above are why I think this is such a odd issue. There is a very clear awareness that emperors wanted members of their family, and especially their sons, to succeed them, but for some reason, they're more often than not failing to produce male children.

There are some explanations that I can think of, but I don't think they fully explain the entire situation.

• ⁠Adoption: This was one method emperors used to keep their diadems within the family. Adopt, for example, a nephew or some other distant relative and secure the succession that way (ex. Julio-Claudians and Nerva-Antonines). It allowed a ruling emperor to carefully pick out who they wanted to succeed them and do it in a relatively prepared manner.

• ⁠The influential powers of the empire (Army, important courtiers, the Senate especially) often chose old, perceivably weak, childless men to rule: In cases of interregnum, the power blocs that be elevated such men to avoid a strong, centralized emperor. See cases like Nerva or Anastasius I, among others. (I'll admit, this one's a bit more speculative)

Both these explanations run into difficulties when you get past the tetrarchy though, which I'll highlight to finish this comment. What I want to get at here is the idea that dynastic succession was simply not as important for the emperors as we would expect from most European monarchs. Let's look at the case of Constantius II and Julian I in the mid 300's. Both were in power with a strong precedent of dynasticism having been established by Constantine I (Constantius' dad and Julian's uncle) and affirmed by the succession of his sons in an imperial college (Constantine II, Constantius II, and Constans I, Constantine II and Constans were both killed in their twenties). In fact, that dynastic sentiment was so strong that the three sons of Constantine began their joint rules in 337 by massacring most of their paternal relatives (with the exception of the child brothers, Gallus and Julian, their cousins) as potential threats. At this point, out of three brothers in their twenties, that there would be biological sons to succeed them would have been a foregone conclusion, so they were free to kill off any surplus family members. Fast forward about two decades, and all that's left is Constantius II and Julian as co-emperors (Gallus, the older brother of Julian, was earlier appointed co-emperor but executed by Constantius for an unknown crime), 2 Constantinians. 5 years after his elevation, Julian would usurp Constantius, with Constantius dying of an unrelated illness during the civil war. Julian would also die about two years later during a campaign against Persia. Constantius died at 44 and Julian at 31/32. Constantius had one daughter and Julian had no children. After all this time you would think that one of these two would go to some effort to ensure a dynastic succession past their generation, but that simply wasn't the case. It's even weirder with Constantius given that his last brother died in 350 CE, meaning that he went 11 years until his death in 361 CE as the last child of Constantine I without having a son. Of course, we could perhaps assume that Constantius expected the dynasty to live on with Julian, who died unexpectedly.

However, things get even stranger when you look at the last patrilineal ruler of the Theodosian dynasty, Theodosius II (living from 401-450 CE and ruling from 402-450 CE; yes, he became emperor at the age of 1/2), of Theodosian Wall fame (with the line of descent being Theodosius I-Arcadius-Theodosius II). Just like with the Constantinians, Theodosius II ruled the east as a member of an established dynasty, of which he represented the 3rd generation. He is generally regarded as a fairly weak, and therefore bad, emperor (though this has been disputed in some recent scholarship), but he ruled quite peacefully in Constantinople from 408 for 42 years, and lived to 49. He had two or three children, two confirmed daughters and a possible son named Arcadius who died as an infant if he existed in the first place. Theodosius II would die in a horse riding accident having done nothing to designate any sort of heir at all. He didn't adopt a relative. He didn't try to have more kids. He in fact absolutely hated his wife, Aelia Eudokia (who mutually despised her husband), but made no attempt to divorce her even when she went off to live in Jerusalem in 443 for reasons of religious devotion. The Theodosian male line died here.

These two cases in particular make me think "what the fuck" when trying to wrap my head around how dynasticism functioned in the pre-medieval Roman Empire. I can explain on further points if asked. That said, I would just like to conclude by saying that the essential importance of there being a direct male line for monarchical legitimacy just didn't seem to exist for these emperors, at least as we know it for typical European monarchs (think Henry VIII, as an extreme example).

Pasting some relevant studies here due to char limit:

• ⁠Borm's "Born to be Emperor: The Principle of succession and the Roman monarchy" in Wienand's Contested Monarchy (a very useful volume overall for understanding the emperor in late antiquity) https://www.academia.edu/9101127/Born_to_be_Emperor_The_Principle_of_succession_and_the_Roman_monarchy_in_J_Wienand_ed_Contested_Monarchy_Oxford_Oxford_University_Press_2015_pp_239ff

• ⁠Icks' "The Inadequate Heirs of Theodosius: Ancestry, Merit and Divine Blessing in the Representation of Arcadius and Honorius" https://www.academia.edu/6903816/The_Inadequate_Heirs_of_Theodosius_Ancestry_Merit_and_Divine_Blessing_in_the_Representation_of_Arcadius_and_Honorius

• ⁠Omissi's Emperors and Usurpers in the Later Roman Empire, especially the earlier chapters about Constantine and how he used dynastic ideology against the other tetrarchs

• ⁠Burgess' "The Summer of Blood: The "Great Massacre of 337" and the Promotion of the Sons of Constantine" looks at who was most responsible for the slaughter of the Constantinian family (spoiler: Constantine I's sons and especially Constantius II) and how Constantine I's decision to split the empire between his sons influenced such an outcome

https://www.academia.edu/3432092/_The_Summer_of_Blood_The_Great_Massacre_of_337_and_the_Promotion_of_the_Sons_of_Constantine_Dumbarton_Oaks_Papers_62_2008_5_51_appeared_in_2010_

Edit: I want to add one extra part here. Having seen some of the deleted comments on this thread, there seems to be a belief that biological sons were often avoided because they could turn on their fathers later in life. I'll admit, I've never really considered this theory too much and I don't think any scholar has argued this in the case of the Romans. That said, a look at all the instances where an imperial son existed, I'd have to say that this concept is completely wrong. True, having too many sons could complicate the succession (Constantine's sons or the tricky succession of Manuel I over his older brother in the 12th century). But these messes occurred after their fathers died or while he was dying. Of all the emperors that had sons, they all took great lengths to ensure that their children would rule. Emperors after those of the Valentinians even started appointing their sons to consulships and co-emperorships at infancy or early childhood. Imperial sons were almost always loyal to their ruling fathers considering that the son's political legitimacy derived primarily from his father. Most of the political messiness happened when they succeeded their fathers. The only cases I can recall involving anything like this would be the father-son rivalry of Basil I and Leo VI in the 9th century and the mother-son conflict between Irene of Athens and Constantine VI in the 8th century, far less common than the opposite.