Yesterday Argentina's Supreme Court unanimously ruled that punishing adults for private marijuana use that does not harm others is unconstitutional. It therefore rejected the arrests of five men caught with a few joints in their pockets. The Cato Institute's Juan Carlos Hidalgo says the ruling hinged on the Argentine constitution's privacy clause (PDF):
The private actions of men which in no way offend public order or morality, nor injure a third party, are only reserved to God and are exempted from the authority of judges. No inhabitant of the Nation shall be obliged to perform what the law does not demand nor deprived of what it does not prohibit.
It's a matter of opinion, of course, which actions "offend public order or morality," but it's implausible to argue that private (and therefore unobserved), consensual activity of any sort does so. Likewise, one would be hard pressed to show how an adult smoking pot in the privacy of his home thereby harms third parties. According to CNN, "Supreme Court Justice Carlos Fayt, who at one time supported laws that make personal use of marijuana illegal, told the state-run Telam news agency that 'reality' changed his mind." Similar reasoning underlies the 1975 Alaska Supreme Court decision that legalized private possession of marijuana in small amounts, based on the state constitution's privacy clause (although that ruling left the door open to new evidence concerning the harm that pot smokers cause to themselves).
The Argentine Supreme Court's decision came less than a week after Mexico eliminated criminal penalties for possessing small quantities of marijuana and various other drugs. Hidalgo says Brazil and Ecuador may take similar steps.