Little Caius grew up in the army camps of his father Germanicus Caesar, who was the lionized nephew and adoptive son of Tiberius. His mother was Agrippina the Elder, a granddaughter of Augustus. When he was still a toddler, Agrippina dressed him in a Centurion's uniform, and the child strutted around giving orders, which, of course, must be obeyed. The official line was that the troops affectionately called him Caligula (meaning "little boot"), but it's easy to imagine other less flattering scenarios.
Some analysts say it was simply a case of giving too much power to an immature mind -- he was only about 25 when Tiberius died and he came to power with the support of the Praetorian Guard. The ancient historians Suetonius and Tacitus say that Sutorius Macro, Tiberius's own Praetorian Prefect, acting on behalf of Caligula, smothered Tiberius while he was reviving from an unxplained coma, but they produce no evidence other than the fact that Macro immediately became a strong supporter of Caligula. By that time, Germanicus, Agrippina, and his two elder brothers, Julius Caesar Nero and Drusus Caesar were dead -- all rumored to have been eliminated by the increasingly paranoid Tiberius. Caligula was living with Tiberius (perhaps "cohabiting" or perhaps a prisoner -- take your pick) in his Capri Island retreat. No restraints had ever been imposed on the behavior young Caius, either in the army camps or in the court of Tiberius, and during the years on Capri he had watched Tiberius indulge his wildest sexual and sadistic fantasies. Caligula had learned to take what he wanted with impunity and to demand satisfaction of every desire, no matter how perverse or violent. It's easy to understand why he was the ultimate unrestrained punk.
Or he might have had a medical problem. Caligula actually made a good start as emperor when he took over in march of 37 AD -- so good, in fact, that there was general mourning and dismay in Rome when he fell ill with what was described as "brain fever" in October of that year and general rejoicing when he finally recovered a few months later. But after the illness he quickly went on a rampage, exhausting the treasury that Tiberius had laboriously filled, ordering executions (long and brutal and personally witnessed by Caligula), and debauching women regardless of rank or age. The descriptions of Caligula's behavior changes parallel some post-epileptic syndromes, and it is generally accepted that the Julio-Claudian dynasty had inherited epilepsy. Or the "brain fever" might have been encephalitis. Behavior modification after brain damage caused by encephalitis is well documented. The medical jury is still debating whether encephalitis or epilepsy may have contributed to Caligula's "madness."
And, of course, there are historians who simply follow the ancient source and say that it was simply "madness". The madness school of thought can easily draw parallels between symptoms of mental diseases (usually schizophrenia), and Caligula's behavior as emperor. As mentioned above, in some diagnoses, the "medical" and "mental" aspects merge: he could have been suffering, for example, from post-encephalitic schizophrenia.
For the people of Rome, it didn't matter what caused the problem. The results were obvious, and all Romans did everything they could to avoid the emperor's notice. It was rumored that people were executed for looking down at his sparse hair from rooftops or mentioning that anything was "hairy" -- he was said to be sensitive about his baldness and his more-than-adequate body hair. If he noticed your wealth, especially after he had emptied the state treasury, you were in grave danger of post-mortem confiscation. Wives, sisters, and daughters -- even grandmothers -- were kept out of sight as much as possible. No one, of course, made any "political" comments or did anything political that might have attracted Caligula's attention. He was mercurial and might turn up anywhere.
Conspirators in several assassination plots discovered this, to their fatal disadvantage. But just two months short of four years into his reign one of the plots succeeded. Caligula, his wife and young daughter were all killed by members of the Praetorian Guard on orders from high ranking Guard officers. Several prominent Senators were also involved in the plot, including Marcus Vicinus, the husband of Caligula's sister, Julia Livilla. The Guard then put Claudius on the throne. except for the "I Claudius" novels by Robert Graves and the immensely popular television series made from them, Claudius, of course, would have remained unknown, an often drunk cipher, sane only in comparison to Caligula and Nero, who followed him.
A man as mad/evil/sick as Caligula would never slip past show biz. Several renditions of his reign have been produced. The most palatable and, until recently, most available production is the 54-minute video biography Caligula: Reign of Madness by the American "Arts and Entertainment" TV network. It is suitable for general audiences, only as deep as the 54 minute TV format allows, and perpetuates the totally false story that he appointed his favorite horse, Incitatus, as Consul. (The fable started with Suetonius who reported that "It was said that he intended to make him Consul". Dio Cassius, a later historian interpreted that as "He promised to appoint him Consul, etc." That ultimately became "He appointed his horse Consul.")
The two most notable stage renditions were those by Alexander Dumas, père (1831) and by Albert Camus (several editions in the 1940's.) Neither is performed often, but the full texts of both are on the Internet (in English). Both of these versions are based on Suetonius, but they also include cultural and idiosyncratic biases of their own times.
The most notorious portrayal of Caligula is in the 1980 film co-produced by Franco Rossellini and Bob Guccione (of Penthouse Magazine, which provided the money) and directed by Tinto Brass. It has been described as the first and only 15-million-dollar porn film, and it's definitely among the most perverted, nasty, violent, and disturbing movies ever made. It had a short run in the movie "art houses" when it was first issued, and then, to general sighs of relief, it disappeared. Gore Vidal provided a carefully researched script but took his name off the film when Guccione filmed additional scenes. Rossellini, who is not particularly squeamish about sex and violence, also took his name off the credits. It had top name and distinguished stars: Malcolm McDowell, John Gielgud, Peter O'toole, and Helen Mirren. Two versions were released, a 156 minute version for the European market and an American version about an hour shorter. Perverted, crude, violent? Yes. Worth seeing? Only if you have a extremely strong stomach. Critics almost universally despised it, but it is surprisingly accurate both in portraying background information and the perversion, crudity, and violence of Caligula -- and it is now again available as a digitally re-mastered 20th Anniversary DVD issued last September.
Caligula Internet links:
Suetonius Bio: http://www.fordham.edu/halsall/ancient/suetonius-caligula.html. The Tacitus bio of Suetonius is completely lost and known only through references. Only short exerpts from Dio Cassius, book 59, are on the net.
Caligula from "madrome": http://www.xs4all.nl/~kvenjb/madrome.htm#caligula
Britannica on Caligula: http://www.britannica.com/bcom/eb/article/0/0,5716,18971+1+18678,00.html
Vroma.org (McManus) -- "Caligula: Historical background" (with pictures): http://www.vroma.org/~bmcmanus/caligula.html
De Imperitoribus Romanis: http://www.roman-emperors.org/gaius.htm
Caligula from Arts and Entertainment TV. You can buy the tape at: http://www.amazon.com/exec/obidos/ISBN%3DB000006QKF/thesilentsmajoriA/105-0987170-9318331
The Dumas play, full text in English: http://www.cadytech.com/dumas/stories/caligula.asp
The Camus play, final edition, full text in English: http://faculty.uccb.ns.ca/philosophy/caligula/frontpage.htm
The 1980 Caligula movie. You can buy the DVD at: http://www.amazon.com/exec/obidos/tg/stores/detail/-/dvd/B0000214F0/reviews/105-0987170-9318331