Comedian Bill Cosby has been charged with sexual assault. The Post's Manuel Roig-Franzia explains how we got here and what this means for Cosby's already damaged legacy. (Erin Patrick O'Connor/The Washington Post)

Bill Cosby was charged Wednesday with aggravated sexual assault, with prosecutors accusing him of drugging and assaulting a former Temple University employee Andrea Constand in 2004. At least 50 other women have made similar accusations against Cosby, but most will never see their cases go to court. The statutes of limitations on rape and sexual assault legally nullify their decades-old accusations.

Authorities in Pennsylvania, where Constand's encounter with Cosby is said to have unfolded, had 12 years after an incident to pursue criminal charges. The expiration date on her case: January 2016.

Statutes of limitations, which vary widely by state, date back to Roman law. Legal scholars say they persist to protect the legal process from deteriorated evidence and witnesses' faded memories.

Several states — including Virginia, Maryland and North Carolina — impose no limit on when a rape or sexual assault can be prosecuted. The majority, however, set some kind of restriction.

As of 2014, 34 states and the District of Columbia had statutes of limitations on filing sexual assault charges, ranging from three years to three decades, according to the National Center for Victims of Crime. But since then, there has been a growing effort to end -- or at least lengthen -- the limits on prosecuting sex crimes across the country.

In the wake of the Catholic church child abuse cases, and now the accusations against Cosby, advocates have argued that these laws put an undue burden on victims and let rapists go unpunished too often.

This year, at least two states passed policies that make it easier to bring rape and sexual assault charges years after the alleged incidents occurred. 

Indiana enacted a policy in April that allows prosecutors to file rape charges after the state’s five-year time limit has expired if new evidence is discovered. "Jenny's Law"was named after Jenny Wendt Ewing, whose rapist turned himself in to police nine years after the attack. Because too much time had passed, authorities could not arrest him.

Florida’s “43 Days Initiative Act,” which became law in July, was inspired by Danielle Sullivan, who reported her rape 43 days too late and later spoke out against that state's deadline. The state’s prosecution window widened from four years to eight years.

[Gender and racial stereotypes derail rape investigations, attorney general says]

And after a group of activists pushed for reform in Oregon, arguing serial rapists used legal deadlines to their advantage, state lawmakers doubled the statute of limitations from six to 12 years, starting in January.

Meanwhile, lawmakers in California are moving to eliminate the statute of limitations on sex crimes altogether. The state’s current law prohibits prosecuting rape cases after 10 years, unless evidence like DNA is introduced.

“Survivors of sexual offenses, including rape, deserve to know that California law stands on their side as they seek justice,” state Sen. Connie M. Leyva (D), who plans to propose the new law in January, said in a public statement in November. “A sexual predator should not be able to evade legal consequences in California for no other reason than that the time limits set in state law have expired.”

Such deadlines are often blown in sexual assault cases. Some victims put off going to police because they fear family, friends or law enforcement will blame or judge them for what happened, said Monika Johnson Hostler, president of the National Alliance to End Sexual Violence.

“It takes a lot to come forward,” Hostler said. “We see that with victims of all ages. They need time to think about it. They may feel ashamed.”

The country's backlog of rape kits — roughly 400,000, according to a 2014 estimate from the Justice Department — can also stall legal proceedings. Some untested kits date back to the 1980s, sitting on shelves long after the statute of limitations has expired.

The case of Constand and Cosby is unique, said James Shellenberger, a law professor at Temple University. Media attention likely raised it from the legal grave, he said. The combination of public outrage and lurid details from recently unsealed court documents likely prompted a renewed investigation.

The charges came after the documents from 2005, opened in September, revealed Cosby gave conscious-altering drugs called quaaludes to women he saw as prospective sex partners.

“The fact that more and more of these incidents were being reported certainly caused this case to be seen as a more serious matter,” Shellenberger said. “We’re not thinking of a person who may have committed one sexual assault but a person who may have committed 50.”

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