One of the main arguments against marijuana legalization is that it will inevitably increase underage people’s access to marijuana, therefore increasing their use of the drug. But two studies released in the last month — one about Colorado and one about Washington, both of which legalized in 2012 — refute that claim.

Colorado:

Anonymous surveys given to about 40,000 Colorado students before and after legalization showed “no significant change” in marijuana use by children under 18 in the preceding 30 days. Among high school students, use went from about 23 percent in 2005 to about 20 percent in 2014. Similarly, there was no significant change in use by kids younger than 13 in recent years.

Washington:

Researchers compared 2010 and 2014 data from the Washington State Healthy Youth Survey. Each year’s survey included questions about ease of access to marijuana, alcohol, cigarettes and other illicit drugs. There was virtually no change in the proportion of teens who reported it was “easy” to access marijuana in 2010 (55 percent), compared to 2014 (54 percent) after the new law was enacted, according to the study.

We’re learning a lot from the experiences in Colorado, Washington, Oregon, DC, and Alaska. Luckily, one of the things we’ve learned is that marijuana legalization can be done responsibly, in a way that does not increase the amount of marijuana consumed by young people whose brains are still developing. The initiative in Massachusetts will regulate marijuana very tightly, and there is no reason to expect that our results will be any different.