Until quite recently, drinking in America has been strongly associated with maleness. According to one study, as late as 1990 only 80 percent of Americans thought it was acceptable for a woman to be drinking at a bar with friends, compared with 85 percent who thought it was fine for a man to do so.

We can thank many things for the change in social norms: The increase in college attendance among women, feminism, and of course, Sex and the City and its river of cosmopolitans.

But some places were ahead of their time. I recently came across this map, which shows that until 1961 in Illinois, the drinking age was lower for women than it was for men. It was the only state where women could knock one back at 18, but men had to wait until 21.

To find out why, I reached out to Joy Getnick, a State University of New York at Geneseo who has studied drinking ages. It turns out the Illinois provision was tied to the age of majority—when someone is considered an adult—which was lower for women.

“The premise behind the old laws had been that women matured faster than men, and perhaps married younger than men,” Getnick said in an email. “By 1961 those views had changed. Women matured (or didn't) similarly to men, and there were growing concerns about younger women buying older (but still not yet legal) men drinks, and all of the social (and legal) problems that went with that.