San Francisco just became the first city in the country to mandate six weeks of fully paid family leave for all parents, a tremendously large win for fatherhood rights activists.

According to USA Today, the San Francisco Board of Supervisors passed the measure unanimously, extending California's already generous family leave law, which gave employees 55 percent of their salary for six weeks.

The San Francisco law will require businesses with more than 20 employees to pay both male and female workers their full salaries for the month and a half leave. It affects both part-time and full-time employees who work in the city and will take effect in January 2017.

“Our country’s parental leave policies are woefully behind the rest of the world, and today San Francisco has taken the lead in pushing for better family leave policies for our workers,” the bill's author, Supervisor Scott Wiener, said in a statement. “We shouldn’t be forcing new mothers and fathers to choose between spending precious bonding time with their children and putting food on the table."

Five Thirty Eight reported that "dads are big winners" with this law, because paternity leave is not included in most private businesses' benefits and is generally undervalued in U.S. society, despite the fact that early bonding has been proven to play an essential role in children's lives.

“Early bonding between father and child is strongly associated with a father’s later desire to want to maintain contact with that child,” David Popenoe wrote in his book Families Without Fathers.

Social conservatives who have lost with millennials on many issues over the last decade may be able to champion paid paternity leave as essential for creating deeper family bonds.