Sméagol (Gollum) is a single, 587 year old, hobbit-like male of no fixed abode. He has presented with antisocial behaviour, increasing aggression, and preoccupation with the “one ring.”… …His forensic history consists of Deagol’s murder and the attempted murder of Samwise Gamgee. He has no history of substance misuse, although like many young hobbits he smoked “pipe weed” in adolescence. Sméagol has forgotten many memories of his childhood, and we have limited collateral history on his premorbid personality. Before obtaining the ring he was an inquisitive child with odd interests, who enjoyed causing mischief and solitary activities such as burrowing under trees to look at roots. He dislikes himself, stale raw fish, and “hobbitses.”
Several differential diagnoses need to be considered, and we should exclude organic causes for his symptoms. A space occupying lesion such as a brain tumour is unlikely as his symptoms are long standing. Gollum’s diet is extremely limited, consisting only of raw fish. Vitamin B-12 deficiency may cause irritability, delusions, and paranoia. His reduced appetite and loss of hair and weight may be associated with iron deficiency anaemia. He is hypervigilant and does not seem to need much sleep. This, accompanied by his bulging eyes and weight loss, suggests hyperthyroidism. Gollum’s dislike of sunlight may be due to the photosensitivity of porphyria. Attacks may be induced by starvation and accompanied by paranoid psychosis.
Via Ilya Somin at the Volokh Conspiracy, who argues that Gollum was simply engaging in rational (in the economic sense), if unusual choices:
If we assume that Gollum valued long life, power, and wealth above companionship, socializing, and conventional morality, his actions seem perfectly rational. True, the Ring didn’t ultimately make him wealthy. But it was reasonable to assume that it might when he first stole it. And it did give him a much longer life and greater power than he would have had otherwise. As for his supposed multiple personality disorder, perhaps inventing a second personality was a good way to pass the time during his long years of living alone. When he met Sam and Frodo, the supposed second personality was a good excuse for evading responsibility for his deceptions and efforts to steal back the Ring. If not for the alternate personality, Frodo might have let Sam kill Gollum or drive him away. Finally, Gollum’s theft of the ring and his obsessive guarding of it afterwards was arguably a rational response to the extremely poor enforcement of property rights in Middle Earth.