I’ve just read a striking article recounting cases of violence associated with delusions about household pets. Although the academic paper is locked, a copy is available online as a pdf.
The curious study was published in a 1987 edition of Behavioral Sciences and the Law and includes three extended case studies of defendants charged with aggressive crimes who had “psychotic perceptions and delusions involving their pet animals”.
Several weeks before the alleged killing, Mr. A’s cat appeared to be pregnant, so he made a nesting box for it. He said his wife disliked his cat because of its grey color. One day Mr. A dropped a jar of molasses, thereby spilling the contents. He lamented to himself that he no longer had control over picking up and holding objects. Upon seeing the cat’s strange green eyes, he concluded that voodoo was being perpetrated against him through the cat as a medium, and the cat was therefore responsible for his loss of control. It occurred to him that Cleopatra and the ancient Egyptians were surrounded by cats. He associated cats with the ancient past and evil spirits. He decided to shoot the cat in order to “break the spell” against him. However, killing his cat failed to dispel his sense of being fragmented and persecuted.
From the time he shot his cat until his wife was mortally shot several days later, Mr. A’s psychosis worsened; his thinking became more disorganized and lacking in reality adherence. Time seemed to have stopped. He perceived a striking deterioration in his wife’s appearance. “She looked so grey (her criticism of the cat), like a craven image… she looked sick.”
The author notes that the first description of this phenomenon was actually in the Edgar Allen Poe short story The Black Cat where the narrator develops delusions about his cat and eventually kills his wife.
Many thanks to Keith Laws for finding this unusual footnote in the forensic psychiatry literature.
pdf of article on violence and pet delusions (via @Keith_Laws)
Link to locked academic paper in journal.
Mind Hacks 
Interesting! See also: How Your Cat Is Making You Crazy – http://goo.gl/b86E2 – Any links there?
Pets make pretty easy recipients for any sort of narcissistic “projection”, and arguably that’s probably why some folks become so, um “obsessed”, referring to them as their “children”, “best friend”, etc.. Which kinda makes you wonder what some of those critters might really have to say if they could talk!
“Oh man, you should see what that crazy broad does to me when no one’s around…!” ;-p
I doubt it’s coincidence that a lot of this stuff revolves around cats, historically suspected of being in league with the devil. I heard an interesting Radio Lab podcast on the ability of cats to survive falls from high buildings, and how this fed into the superstition that they were supernaturally endowed.
Strange coincidence: After posting the above, I stumbled across a report in the Atlantic indicating that maybe your cat really IS making you crazy! Check it out: http://bit.ly/x8Tm4k
Looking for cause and effect is inside all humans.
“Magical thinking”is causal reasoning that looks for correlation between acts or utterances and certain events.
“Street light interference phenomenon” :people believe street lights go off/on when THEY are nearby. When the lights are randomly turning off or on.
In the “Mr. A” story, dropped a jar of molasses he needed to blame it on something- find a reason. He decided to blame it on the cat. Listened to his imaginations idea (cause and effect) as credible. Shows his reasoning skills were going bad.
1) Make mistake
2) Don’t detect the mistake
3) Don’t correct the mistake