Definition of jury nullification: A sanctioned doctrine of trial proceedings wherein members of a jury disregard either the evidence presented or the instructions of the judge in order to reach a verdict based upon their own consciences. It espouses the concept that jurors should be the judges of both law and fact.
When the law that is responsible for the indictment of a defendant is found to be unfair, unjust, an overstep of legislative action, unsound, unreasonable, archaic, or for any other reason goes against a person's decent conscience...The jury can ignore the evidence (even if guilt is proven beyond a reasonable doubt) and refuse a conviction. Jury duty gives everyday citizens the right to participate in indirect legislative action as well as judicial action.
There are many laws our elected public officials have signed into law that American citizens in large disagree with. Whether the law is excessive, unfair, petty, violates moral sensibility or otherwise is unpopular among American citizens, jury duty is the last chance for the citizen to exercise their sovereign democratic power to refuse to convict a defendant accused of violating it, in spite of overwhelming evidence from the state. What good is a law that most juries won't render a conviction of an accused violator of said law? We all know of laws that fall into this category. Whether it is non-violent marijuana users, or having a magazine for your firearm that has a capacity that is 3 rounds over the legally authorized amount.
The courts, the lawmakers, and criminal system retain their power from the people in the USA. When oppressive or unreasonable laws stand to destroy a defendant's life, 12 private citizens can nullify that law. If enough of the population disagrees with this law, imagine how many jury pools that will effectively render this law unenforceable. No prosecutor is going to waste the time and effort bringing a case that they know won't earn a conviction.
The judge will always in struct the jury that they are only to evaluate the evidence and not judge the merits of the actual law. That the jury is merely supposed to decide if the law was broken and render their verdict accordingly. But here is what judges are NOT permitted to do if a verdict they disagree with is rendered.
1) Can't force jurors to change verdict or reconsider changing verdict
2) Can't force jurors to give an account as to how they arrived at that verdict
3) Can't punish jurors for a verdict rendered including holding them in contempt of court
There is no negative consequences for juries to exercise this type of power.