Well, It is not my intention to defend or refute the data. Posting was merely for information purpose only. I agree that many such incident remains unreported in developing countries. But the assumption that every such incident is reported in US of A is is controversial and may be not true. Following data about rape in US is shocking, Please read  following. Some posters seems confused with sexual assult and rape. They are two different things

Reporting Rates
Sexual assault is one of the most under reported crimes, with 54% still being left unreported.¹

What happens to Rapists When They are Caught and Prosecuted?

54% of rapes/sexual assaults are not reported to the police, according to a statistical average of the past 5 years. Those rapists, of course, never spend a day in prison. Factoring in unreported rapes, only about 3% of rapists ever serve a day in jail.
 

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References

1. Justice Department, National Crime Victimization Survey: 2006-2010
2. FBI, Uniform Crime Reports: 2006-2010
3. National Center for Policy Analysis, Crime and Punishment in America, 1999
4. Department of Justice, Felony Defendents in Large Urban Counties: average of 2002-2006

http://www.rainn.org/get-information/statistics/reporting-rates