In the very first jury trial before the U.S. Supreme Court, Chief
Justice John Jay instructed the jury in this way: "It is presumed, that
juries are the best judges of facts; it is on the other hand, presumed
that courts are the best judges of law. But still both objects are
within your power of decision... you have a right to take it upon
yourselves to judge of both, and to determine the law as well as the
fact in controversy". State of Georgia vs. Brailsford (3 Dall 1)
Jurors not only have the duty to decide guilt or innocence, but also the
"equity", or "inequity", of the law for which the defendant is accused.
This means that juries have the ability to acquit a defendant based upon
their own ideas of right and wrong, fairness, and their own conscience.
If they feel that the laws against marijuana possession are unjust and
unfair, then jury nullification suggests that they vote for acquittal,
regardless of the judge's instructions.
Because a jury's guilty verdict must be unanimous, it takes only one
vote to effectively NULLIFY an unjust law. Regardless of legislative
intent, or a judge's instructions to the jury, the jury, not the court,
will make the decision to acquit. Though this may only cause a "hung"
jury, and it won't be a true acquittal, at least the defendant will not
be convicted of violating an unjust or unconstitutional law. Very often,
defendants are not retried by the government, due to the high cost
involved, and in this way juries send a very clear and powerful message.