A century ago, this image of men left alone with children was horrifying enough to spur an anti-suffrage movement. So what happened? How did single fatherhood go from terrifying to increasingly normal?

According to the Pew study, we can attribute a large part of this to the U.S. judicial system, and its shifting standards for child custody cases. Until recently, U.S. courts would almost always rule “in the best interest of the child” (slang for “in favor of the mother”). But since the early 2000’s, many states have been adopting legislation that moves away from the “best interest” policy. Most new legislation provides for “joint parenting” or joint physical custody, policies that encourage both parents to spend equal time with the child (Oregon, Minnesota, Arizona, Iowa, and Maine have particularly strong joint parenting laws). But rather than prompting divorced parents to split their child’s time 50-50, these policies seem to have prompted a dramatic increase in the number of single fathers.

A 2011 study published in the Journal of Empirical Legal Studies explains why this is happening. The study analyzes the case of Oregon, which in 1997 became one of the first states to formally enact joint parenting legislation. Before this change, Oregon’s custody law was pretty standard. It favored joint legal custody (the parents both have a say in decisions made about the child), but not joint physical custody. The decision about physical custody was made “in the best interest of the child,” which generally meant that the child lived with one parent or the other (and most often with the mother). After the new legislation, Oregon courts defaulted to joint parenting, encouraging the child to spend half her time with dad, and half her time with mom.

But this new law didn’t turn out the way legislators expected. Instead of increasing the number of families that split parenting time equally, it increased the numbers of fathers with sole custody.

There are a few reasons why the state push for joint parenting is resulting in more single dads.

It empowers fathers to ask for more, and believe they deserve it. William Fabricius, Professor of Psychology at Arizona State University and chair of Arizona’s committee on child custody statutes, says that most men want to share parenting time equally, but assume courts have a strong maternal bias.

“Dads think that the courts will favor mom, and so they will settle for less parenting time. They see other dads in the neighborhood who spend time with their kids every other weekend, and assume that’s the way it is. They don’t ask for more parenting time because they don’t think it’s the norm,” Professor Fabricius said.

Since only five percent of child custody cases ever make it to trial, these perceptions of the norm are important. If men realize that courts are granting more parenting time to fathers than they have in the past, they’ll be more likely to ask for more time with their child, and fight to get it. Men used to assume that there was no way they’d get custody of their kids—and that maybe it was for the best.