Duress and necessity are affirmative defenses. With them, a defense attorney can—if the evidence agrees—argue that the defendant did something that's typically illegal, but that doesn't constitute a crime because of extraordinary circumstances. Some courts look at the policies behind the duress and necessity defenses and blend them together into one defense. These defenses, however, are historically different and many courts treat them as such. (U.S. v. Bailey, supra; State v. Cole, 403 S.E.2d 117 (S.C. 1991).)
Necessity, like duress, involves the defendant acting unlawfully in order to avoid a threat of immediate harm. Both defenses fail if the defendant had a reasonable alternative to violating the law. So, how are they different?
Duress, on the one hand, arises from the actions of other people—the classic example is one person forcing another to commit a crime at gunpoint. Courts typically explain necessity, on the other hand, as a choice between two evils. If circumstances give rise to a situation where the defendant must break the law in order to prevent harm, the necessity defense may be in play. For the defense to succeed, the harm the defendant causes can't be greater than the harm she prevents.
Example: Alice holds a gun up to Joan's head and tells her, "Drive me to the hospital as fast as you can, or I'll kill you." They get in the car, and Joan drives with the gun pointed at her head. If Alice is stopped for and subsequently charged with careless driving, she may have a viable duress claim.
Example: Alice's child is having an allergic reaction and has stopped breathing. Alice is in a rural area without a working cellphone or any other means of contacting someone. So, she drives her child to the hospital as fast as she can and doesn't hurt anyone in the process. If she is stopped and later charged with careless driving, she may be able to invoke the necessity defense. She can argue that any harm caused by her driving was less than the harm she may have avoided: her child's death.