The term "Twinkie Defense" was coined by the San Francisco press during the 1979 trial of Dan White for murders committed by him in November of 1978.
In 1979 in San Francisco, a former police officer, Dan White, was tried for the murders of Mayor George Moscone and an administrative aide named Harvey Milk. That White committed the murders was never in doubt inasmuch as he shot both men at mid-day inside City Hall; however, the defense argued that White was not responsible for his acts because he was, at the time of the acts, temporarily insane and acting with "diminished capacity." White's attorney attempted to prove that severe depression had put White into an altered state that changed his behavior and rendered him incapable of being found culpable for his criminal acts. One of his arguments in support of his client's alleged "diminished capacity" was that the previously health conscious defendant began to eat Twinkies and other junk food as a result of his severe depression. He argued that this junk food had exacerbated the defendant's depression; and, thus, was a causal factor in the defendant's "diminished" mental state at the time of the murders.
One psychiatrist testified: "I think that on the day of the crimes he really had no meaningful, rational capacity to carefully weigh the considerations for and against and rationally decide on a course of action. He couldn’t think carefully about what he was going to do."
Although the defense was dubbed the "Twinkie Defense" by the local press, the correlation between junk food and White’s behavior was never made at the trial. This was a falsehood that has been repeated many times in hundreds of press accounts of the trial. But the characterization stuck and the Dan White case will probably always be remembered as the time junk food caused a man to go crazy and to commit murder.
White was convicted of manslaughter, and, as a result, he received a much lighter sentence as opposed to a 1st degree murder conviction. Thus, for Dan White, the "deminished capacity" defense actually succeeded in mitigating the verdict and sentence. He spent 5 years in prison for a double murder and was paroled in 1985. A few months later, Dan White committed suicide.
As to the legitimacy of the defense, Mark Gado has written extensively on the insanity defense and its variations. Please see:
http://www.crimelibrary.com/criminal_min… [and subsequent chapters, particularly:
http://www.crimelibrary.com/criminal_min…
In his examination of the "Twinkie Defense," he states: "Of course, this type of defense is not as common as the press believes, but it demonstrates what can be done under the elusive definitions of the Durham Rule and 'diminished capacity' defenses. Eventually, the Durham rule was abandoned [1982] in favor of the 'substantial capacity test.' This criteria was, in effect, a combination of the M’Naghten Rule and the Irresistible Impulse test. Under M’Naghten alone, a defendant was required to show a complete failure to distinguish between right and wrong. This compromise, at least to some, seemed acceptable to the courts. For Dan White though, the 'diminished capacity' defense worked."