What: Title XV of the Education Amendments of 1972, which modified Title IX of the Civil Rights Act of 1964. The law begins, “No person in the United States shall, on the basis of sex, be excluded from participation in, be denied the benefits of, or be subjected to discrimination under any education program or activity receiving Federal financial assistance.”

Why: Not surprisingly for an act that can’t even get its own number right, Title IX doesn’t even say what it came to be most connected with. Its intent was to eliminate discrimination against women in education—not sports in education, but education itself. In the 1960s, it was not even universally agreed upon that women should have equal opportunity to go to school. But by the 1970s, in the wake of the bra-burning revolution, the country was ready to codify the simple concept that women should go to school. The law had a subtle consequence, though. “Any education program or activity” meant that the traditional rampart of male exclusivity—athletics—was about to be turned on its head. Sports, especially team sports, was not then viewed as ladylike. But with Title IX, allowing play was viewed as mandatory. If women wanted to play soccer, play basketball, play volleyball, play softball—now they could. Now they would.

Impact: Greater than any federal administrative law since the Civil Rights Act itself. The collective psyche of more than fifty percent of the U.S. population changed irrevocably, allowing women to feel comfortable in competitive arenas like law and medicine and the military. As for sports, it’s gotten to the point today where much of America is gathered around their TV sets to watch a women’s soccer team go for a World Cup championship. Nobody’s burning any bras now; the ones that matter are those under the jerseys of the new Brandi Chastains. With women’s sports expanding ninefold in the past four decades, the presumption that women could be competitive, could sweat, could swear, could leave it all on the pitch, could break you in half if they wanted—all of that is okay now. All of that, for want of a better term, is ladylike.

Personal Connection: I am of the right age to have seen American girls change because of Title IX. In the late 1970s, girls in my school became among the first to expect the opportunity to play. In high school, the girls I hung out with played basketball because they could. In college, the women I dated built a national championship program in lacrosse because they could. And now that those women have daughters, the games that I make are expected to be played by everyone. A game designer can write a game for boys, but it will be played by girls. And boys, I hate like the dickens to break it to you, but they will beat you. Because they can.

Other Contenders: the Endangered Species Act of 1973, which among other victories, gave us our bald eagles back; the Do-Not-Call Implementation Act of 2003, which criminalizes telephonic harassment; the ever-expanding list of laws, currently in Argentina, Belgium, Canada, Iceland, the Netherlands, Norway, Portugal, South Africa, Spain, Sweden, Mexico City, and parts of the USA, that allow everyone to marry; the laws which provide universal health care in countries such as Iraq and Afghanistan—which in those cases are provided by the United States.