Every dozen or so seconds in the United States, a woman is beaten, assaulted, or strangled. Domestic violence is the top cause of injury for American women between the ages of 15 and 44. Many of these victims know their assaulter: Nearly 30 percent of all murdered women are killed by husbands, exes, or boyfriends. (Less than five percent of males are slain by wives, exes, or girlfriends.)

You also know these women. One in four females will be the victim of domestic violence. She could be your mother, sister, friend, or co-worker, stuck in a controlling relationship in which her partner uses manipulation, humiliation, violence, and other means to maintain control over her. (Ninety percent of all victims are women and most of the perpetrators are male.) Most shockingly, domestic violence is so vastly underreported, you may never actually know.

It wasn’t until the 1970s that many states finally agreed a wife could be raped by her husband. Until then, the law and conventional wisdom said a wife could not refuse her husband sex, or that once she’d given consent after the wedding day it couldn’t be retracted. Thanks in part to the women’s liberation movement, the law caught up with the reality of wives who were being assaulted by the very men they vowed to stay with through sickness and in health. What seems intuitive now—that a wife and husband each have equal legal power over their bodies—was an ideological nuclear bomb in the ’70s.

In 1986, domestic violence was finally identified as a “public health issue”—one costing $5.8 billion a year in the United States. Four years later, then-Senator Joseph Biden first proposed the Violence Against Women Act. It took four years to pass. (In 2012, conservatives stalled VAWA’s renewal over ideological differences about extending the law to include same-sex couples and some provisions to aid illegal immigrants. After months of squabbling, VAWA was renewed in February 2013.)

The laws remain difficult to enforce, partly because the term “domestic violence” is a misnomer. Abusive relationships often are less about actual violence than about control and power, which makes abuse even harder to define, enforce, and convict. Domestic violence involves a warped dynamic that—whether or not a criminal act has been committed—is often misunderstood by people outside the relationship. Still, as U.S. Supreme Court Justice Potter Stewart once quipped about porn: “I know it when I see it.”