According to Edward Gibbon, 180 AD is where he draws the end of the so-called Pax Romana when the last of the Five Good Emperors, Marcus Aurelius, dies and is succeeded by his incompetent son Commodus. How would the social structure of the Romans look like, did their economy thrive, or was it stagnant/in decline? Did the succession by Commodus directly affect those affairs, or will the consequences of Commodus' bad rule be felt later on? If you're able to, please provide some good sources as well.
Thanks in advance!
To start off, I'm going to list a structure of the civil commands in the later Roman empire to give you an idea of the structure behind the Emperor:
Now by the time of Marcus Aurelius, things were already starting to turn downhill. Rome's external enemies were growing in strength and number, and there was a plague in 169, and even though his predecessor, Antoninus Pius (138-161) was aiming for fiscal conservatism and better relations with the senate, the senate at this point was still pretty marginalised and had been since the Flavians. On the one hand the Antonines secured the peace, dealt with foreign invasions effectively and increased social mobility. But Marcus Aurelius broke down that merit-based succession which had worked so well for the Antonines in the past, when Commodus became emperor. After Commodus' assassination in 192 there followed a brief struggle and Septimius Severus rose to power, setting up a dynasty that would last a good 30-40 years. It was a pretty stable dynasty, especially considering the rest of the 3rd century crisis before Diocletian comes along. Still this whole time period of the 3rd century would be wracked with financial issues: inflation being the big one.
There would have been a number of ways that the Romans would have battled inflation. Their thoughts around inflation primarily revolved around the value of precious metals in coins. By the 3rd century most of the population didn't trust the currency, and would hoarde those older coins that had considerable more "worth" (for an example of the extreme measure of coin hoarding, go look at the Hoxne Hoard). But the very act of hoarding the coins took that precious metal out of circulation, which created this viscious cycle of depreciating coin value and corresponding distrust in currency. There were a number of currency reforms that tried to rectify this, such as the creation of the antoninianus by Caracalla, which was supposed to be worth twice as much as the common denarii (think of the denarii as being the rough equivalent of the $20 bill), but which slowly degenerated in value as time went on.
By far one of the best examples of a great effort being put into the Roman economy during the end of the 3rd century, however, is Diocletian's Price Edict. It attempted what other emperors had done before with currency decrees, but it also put price caps on hundreds of products and wages and the like in an attempt to control spending and earning.
What most people didn't really understand about inflation, though, is that the value of money does not rely upon the material of the money itself, but upon the productivity of the person or nation or organisation it represents. Coins made of gold or modern paper money made of plastic both have limited intrinsic value (if you want a good modern example, look at the Doge Coin). When Diocletian put a cap on wages, workers found that the money they were earning was essentially worthless, which resulted in more basic forms of trade, such as barter or taxes being paid in kind -- since the value of an actual physical product would have been far greater than any currency at the time. (We can speculate here and say that perhaps rather than institute a price cap, they should have simply increased supply). The Romans understood that there was inflation; they saw the effects of it critically in the 3rd century and beyond. But for the most part their attempts to rectify the situation were futile.
In times of great inflation, people aren't after money, they're after goods. Because goods and land and the like hold value far better than money does. Keith Hopkins in his article Taxes and Trade in the Roman Empire makes the rather astute claim that, "...the Romans' imposition of taxes paid in money greatly increased the volume of trade in the Roman empire...[and] Secondly, in so far as money taxes were levied on conquered provinces and then spent in other provinces or in Italy, then the tax-exporting provinces had to earn money with which to pay their taxes by exporting goods of an equal value." (Hopkins, 101). Things like taxes in kind, which was a rampant practice in the 3rd century, but which was largely discontinued by Constantine, would have had more of an effect on the Roman economy than what percentage of coins was comprised of gold or silver, since taxes in kind severely limit trade and undermine not the intrinsic value but the representative value of money.
And there would have been other things that stimulated the economy again in the 4th century. Constantine for instance discontinued the long-standing practice of clearing everyone's debt to the state upon the succession of a new emperor. Instead he called all those debts in and had them dragged, kicking and screaming, to pay their enormous debts. There was also changes to the tax system; by 350 AD Peter Brown notes that, "[Taxation] reached more than one-third of a farmer's gross income" (Brown, The World of Late Antiquity, 36).
But all of this can't be attributed to Commodus. Poor Commodus gets a bad rep. He ruled for almost 13 years, yet most people seem to think he didn't last very long. Moreover his reign was actually pretty peaceful. We get a bad image of Commodus because of the rhetoric of ancient authors. Here a few good examples: Epit. Caes. 17, Eutr. Brev. 8.15, Zos. NH 1.7, Orosius, Hist. 7.16 (you also might want to compare Eutropius' accounts on Nero (Brev. 7.14) and Domitian (Brev. 7.23) just to see how similar the rhetoric is). My last point is that it's not all Commodus' fault. The 3rd century crisis was due to systemic issues.