The Romans had an obsession with displaying virtue; in the later Republic, young successful men would order busts that made them seem older, more wrinkled, as age was a sign of wisdom. Augustus may have changed the trend of worshiping the past by introducing youthful hopeful traits into his busts and statues (most notably Augustus of Prima Porta), yet he kept the typical Roman appearance - a bland short cut on his head and a clean shave.
From what I've learned in my Arts history and Roman history classes, lavish hairstyling, or any sign of obsession about outward appearance was seen as overly effeminate, and Roman men sought the appearance of a soldierly scholar, embellishing Roman virtues. Beards were looked down on as being barbaric, and a Gallic feature to be avoided (like the Roman fear of wearing trousers).
The busts and statues of Marcus Aurelius and his son Commodus strike me as being completely detached from canon of Roman sculpting. They have lushious curls on their hair, magnificent beards, and their likeness is captured in very relaxed, emotional poses. Marcus Aurelius was a great emperor, yet his appearance in these sculptures is relaxed, indifferent, a departure from the stoic, thinking faces we see on late Republic busts like that of Cicero. So clearly, the style was not meant as a reflection of the character of Marcus Aurelius.
So I wonder, where did this change in Roman sculpting canon begin? Why did it become popular? Was it a symptom of the decadence of the middle and late Empire that Gibbon would have us believe, or was it inspired by the Greeks, like many other Roman canons?
It became much more common around the time of the Punic Wars and thereafter (as Rome was expanding, integrating and becoming more and more of a regional hegemon) to have Greek tutors teach upper-class Romans. This gave them a more refined and Hellenistic appreciation for the arts. Note that this didn't mean they didn't have art and refinement in Rome before, but the culture was extremely militaristic and not as mature/worldly as the Greek world.
Greek styles came with that package. And it was quite common for Greeks to wear there hair long and have beards. This didn't become mainstream style until closer to Marcus Aurelius' time. At that point Rome had become quite wealthy and was clearly the center of the Mediterranean world. As such an increase in wealth and a change in how to obtain status (being close to the emperor as opposed a system based more on honoring and exaggerating your noble ancestry and winning acclaim in military combat-- though this last route certainly disappear by Aurelius' time).
Much of this came from Carthage Must Be Destroyed by Richard Miles as well as a few of Adrian Goldsworthy's books, such as "Augustus" and "Caesar". If you want a really comprehensive understanding of the Late Republic and Early Roman Empire, you can't get better than Goldsworthy. If you'd preference the drastic and society-shattering effects the Punic Wars had on Rome and the Mediterranean world (and a solid treatment of Carthage that has few solid primary sources to it's name to work with unfortunately), I can recommend Carthage Must be Destroyed as an enjoyable read.
The Punic Wars and the Civil Wars of the Late Republic altered the makeup of Roman society and especially the ruling class in almost unimaginable ways. Antonine Hair styles among them.
I'm also interested in literature on the evolution of Roman art and architecture from the end of the Republic to the end of the Western Empire, if anyone is able to share.