Jeremy Mayer, political science professor at George Mason University, says generational attitudes also play a role.
"The most opposed to legalization are the oldest, the most in favor the youngest," he says. "I expect this issue will follow the same pattern as racial integration and gay marriage. As the elderly die off, and are replaced, marijuana will become more legal."
Mayer also points out that at least the last three presidents have all admitted using marijuana.
"Is there anyone claiming that imprisonment would have helped any of these young men? Do we see evidence of damage to their mental abilities thanks to marijuana?" he asks. "Yet all three presided over criminal justice systems that sent citizens to jail for precisely the act that all three committed."
Rebekah Metzler is a political writer for U.S. News & World Report. You can contact her at firstname.lastname@example.org or follow her on Twitter.