Does the relationship between the victim and person committing the offesne remain an important predictor of the likelihood of police notification in rape cases? An NIJ-funded study  examined this question and found that police notification rates by third parties and by victims who had been raped by an acquaintance or intimate partner increased significantly between 1973 and 2000. Using data from the National Crime Survey (NCS) from 1973–1991 and the National Crime Victimization Survey (NCVS) from 1992–2000, Baumer found that overall reporting rates continued to increase during the 1990s, and that differences in rates of reporting between stranger and non-stranger rapes diminished.
These changes coincided with large-scale media and social campaigns that focused attention on "hidden" rapes. Legal reforms and the growth in services available to rape victims have been influential in increasing the likelihood that women will report a rape to police. 
The most recent research, however, indicates that a majority of rape victims still do not report their attacks to police. Further study is needed to understand what impact various policies and practices have on reporting behavior and system response and to precisely identify the practices that would facilitate higher rates of notification.
For more about rape reporting, see "Has Rape Reporting Increased Over Time?" from the NIJ Journal.
The majority of sexual assaults are not reported to the authorities.
The Bureau of Justice Statistics (BJS) reports that the majority of rapes and sexual assaults perpetrated against women and girls in the United States between 1992 and 2000 were not reported to the police. Only 36 percent of rapes, 34 percent of attempted rapes, and 26 percent of sexual assaults were reported.  Reasons for not reporting assault vary among individuals, but one study identified the following as common: 
- Self-blame or guilt.
- Shame, embarrassment, or desire to keep the assault a private matter.
- Humiliation or fear of the person who committed the crime or other individual's perceptions.
- Fear of not being believed or of being accused of playing a role in the crime.
- Lack of trust in the criminal justice system.
In the NIJ funded Sexual Assault Among Latinas Study (SALAS), it was found that victims did not commonly seek help from the criminal justice system, but did seek informal sources of help such as family and friends. However, one third of the women included in the study did not report their victimization to anyone.
[note 1], [note 2] Baumer, E.P. Temporal Variation in the Likelihood of Police Notification by Victims of Rapes, 1973–2000. Washington, DC: U.S. Department of Justice, National Institute of Justice, November 2004, NCJ 207497.
[note 3] Rennison, C.M. Rape and Sexual Assault: Reporting to Police and Medical Attention, 1992–2000. Washington, DC: U.S. Department of Justice, Bureau of Justice Statistics, August 2002, NCJ 194530.
[note 4] Du Mont, J., K.L. Miller, and T.L. Myhr. "The Role of 'Real Rape' and 'Real Victim' Stereotypes in the Police Reporting Practices of Sexually Assaulted Women." Violence Against Women 9(4)(April 2003): 466–486.