From a reputable source:

The U.S. Air Force investigated 556 allegations of rape. 27% of the women admitted they had lied, either just before or just after taking the lie detector test. As the panel continued to investigate the evidence they came up with some very disturbing results. Dr Charles P. McDowell, Supervisory Special Agent of the Office of Special Investigations stated, "To my considerable chagrin, we found that at least 60 percent of all the rape allegations were false." (Quoted in W. Farrell. The Myth of Male Power. Random House. p. 322). The study found the following motivations for false complaints: 20% spite or revenge; 20% to compensate for feelings of guilt or shame; 13% thought she might be pregnant; 12% to conceal an affair; 9% to test a husband's love; 9% mental/emotional disorder; 4% to avoid personal responsibility; 4% failure to pay or extortion; 3% thought she might have caught VD; 6% other.