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There are many different defenses used by a criminal attorney at criminal trial. Each trial is different and each trial is based upon a different set of circumstances. Thus, it is best to be relatively broad in my discussion of the most common/popular defenses used at trial. I will follow each example with a case cite.
The common law rule of duress, provides that the defense of duress is available to a defendant coerced to commit a criminal act through the use of, or the threat of force against his person or the person of another. Commonwealth v. Yoder, 3 Pa. D. & C.4th 60. A claim of duress must involve the threat of imminent peril of death or serious injury, operating on the defendant's mind and overbearing his will. Threats to third persons may qualify as well.
According to the rule of self defense, a defender may use deadly force only if he reasonably believes that such force is necessary to avoid death or serious bodily injury; he did not provoke the use of force against himself in the same incident; and finally, he could not retreat with complete safety. If any of these factors is negated; i.e., the defender did not reasonably believe deadly force was necessary; he provoked the incident, or he could retreat with safety, then his use of deadly force in self-defense was not justifiable and he may be prosecuted for injuries or death he inflicts on the assailants or on bystanders. If his use of deadly force was justifiable, he may not be prosecuted for either. Commonwealth v. Fowlin, 551 Pa. 414.
In many, jurisdictions, intoxication may negate specific intent, a particular kind of mens rea applicable only to some crimes. For example, lack of specific intent might reduce murder to manslaughter. Voluntary intoxication nevertheless often will provide basic intent, e.g., the intent required for manslaughter.On the other hand, involuntarily intoxication, for example by punch spiked unforeseeably with alcohol, may give rise to no inference of basic intent and would be considered an affirmative defense.
Lastly, I wanted to cover insanity as it is quite popular in our pop culture, but is actually rarely used in the real world. The insanity defense can be broken down into two different sub-categories. (1) "Mentally ill." One who as a result of mental disease or defect, lacks substantial capacity either to appreciate the wrongfulness of his conduct or to conform his conduct to the requirements of the law. (2) "Legal insanity." At the time of the commission of the act, the defendant was laboring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing or, if he did know it, that he did not know he was doing what was wrong. Addington v. Texas, 441 U.S. 418, 431-34,XXXXX 1804, 1812-13, 60 L. Ed. 2d 323 (1979). The insanity defense is rarely utilized because it is so difficult, and expensive to prove.
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Well, legal insanity is a much more stringent concept. It is really difficult to prove someone is legally insane. For example, an individual may have a medically verifiable mental illness (bi-polar, schizophrenia, etc.), but the fact an individual has a medically verifiable mental illness does not necessarily mean the individual is legally insane for criminal matters.
A person is generally considered legally insane for criminal purposes if the defense can show the defendant should not be held responsible for his actions only if, due to his mental disease or defect, he (i) did not know that his act would be wrong; or (ii) did not understand the nature and quality of his actions. One can see that, using the legal definition of insanity, an individual could be suffering from a mental health condition, but still realize an act is wrong, or be capable of understanding the nature and quality of his/her actions. This is why the insanity defense is not used very often in criminal cases; it is just too hard to show the individuals mental health issues bring him/her up to the level required by law.
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