But by 2012, the ratio had tumbled to one in 12, according to the study conducted by the Criminological Research Institute of Lower Saxony (KFN).
"These findings are problematic for a state built upon the rule of law," the authors of the study said in a statement.
Case overloads in the criminal justice system, a rise in reported rapes within families and a federal court ruling setting a much higher standard of proof have made it harder for victims to put their attackers behind bars.
The institute found that the authorities were often overwhelmed with accusations of sex crimes, with strong variation from region to region.
"The greater the workload for police officers, prosecutors and courts, the less frequently the trial ends in a conviction of the perpetrator," they said.
The authors found that jail time was a deterrent, noting that the higher the conviction rate for rape in a region, the lower the number of reported rapes.
The number of sex crimes allegedly committed by relatives, partners and acquaintances soared to nearly 28 percent of all cases in 2012 from 7.4 percent in 1994.
The authors attributed this rise in part to a legal change in 1998 making rape within a marriage a crime.
But they said such cases were harder to prove in court, despite major advances in DNA profiling.
"The accused men usually admit to having had sexual intercourse but say that it was consensual," they said, leading to "he said, she said" testimony.
The authors also highlighted a federal court ruling from 2006 where the judges overturned a rape conviction in which the defendant ripped the woman's clothes off and had sex with her against her "explicitly expressed will".
"Since this ruling, there have been several cases in which prosecutors and courts have interpreted rape laws in a similarly rigid way," they said, noting that women's rights groups had launched a petition calling for legislative reform.