Making a formal, legal, complaint is not a cost-free process. There are countervailing pressures that push against formal protests against discriminatory conduct. Martha Minow, Professor of Law at Harvard University, explained some of these dynamics in her fantastic book, Making All the Difference: Inclusion, Exclusion, and American Law:
“[A]ccording to one study, two-thirds of white women and members of minorities who report they have experienced discrimination on the job refrain from complaining to any third party, including legal officials, despite their rights under the laws against discrimination. Kristin Bumiller’s study of discrimination victims who do not sue concludes that they perceive the high costs of complaining and the real benefits [of] ‘lumping it,’ or absorbing the injury without complaint. Her interviews show that complaining through the civil rights laws means accepting the role of victim, which is itself demeaning and also ‘transforms the conflict into an internal contest to reconcile a positive self-image with the image of oneself as a powerless and defeated victim.’ In addition, complaining forces the individual into a visible role and, paradoxically, demands the differential treatment of public attention and dispute because of allegations of differential treatment. A new label, ‘troublemaker,’ also carries negative consequences for the individual. And besides risking a painful reconstruction of the discrimination event before an agency or court, the potential complainant may fear that the process will be unavailing. Other people may fail to confirm the story, or the legal system will prove unresponsive; meanwhile, the individual loses control over the incident and the process. There are special costs involved in hoping and then losing, costs that may even be more painful than never hoping at all.
Besides avoiding the negative consequences of complaining, people may discover direct benefits from enduring discrimination without complaint. Members of minority groups, especially minority women, come to expect discrimination as inevitable and may find an opportunity to exercise strength and pride in surviving without confrontation. The very act of submission may be an expression of autonomy and dignity precisely because it is a chosen response. Similarly, in her study of a religious Baptist town in Georgia, anthropologist Carol Greenhouse found women who tended to internalize conflicts within their families, coming to terms with such conflicts by refining their own roles and by focusing on their spiritual identities. Although this solution may work for some, it suggests complex reasons why people refrain from using the avenues of relief that law makes available. Most important, individual decisions to swallow injury fail to alter sources of hurt or discrimination, leaving those who cause harm undisturbed." (92-93)
We assume that if nobody is complaining (or if nobody is winning their complaints), then nothing is going wrong. This is a severely misguided perception. I believe there are four factors that might influence whether victims of illegal treatment file a formal complaint.
1) Resources: It requires resources (monetary, and otherwise) to file these sorts of complaints. Some people can afford it. Others can't. Insofar as certain types of mistreatment disproportionately effect people with fewer resources, we'd expect to see fewer complaints.
2) Likelihood of success: It takes significant investments of time, resources, and energy to pursue certain claims. Most people only will do so if they feel there is a real chance that they will be vindicated at the end. Obviously, part of this factor is simply whether or not the victim feels like he or she can garner enough evidence to make the case. But also playing in to this factor is the perception the victim has of the adjudicating body. If she does not have faith in the integrity of the judicial process, or does not believe that her complaint will be adjudicated fairly, then she might not want to put herself through the ordeal knowing that she'll get nothing out of it. Police harassment is often he said/she said, and the "he said" has the authority of a badge behind it (and the "she said" a low-income youth who may or may not have a criminal record). It is entirely understandable that many victims of real wrongs are skeptical of their ability to be vindicated in court, regardless of what actually happened.
3) Personal costs during the proceedings: Certain types of complaints are more likely to result in attacks on the victim during the proceedings than others. Rape cases would be the paradigmatic example here: rape survivors are put on the stand and berated about their sexual history, implied to be sluts, and are forced to re-enact an extraordinarily traumatizing event in exquisite (some might say pornographic) detail. There really isn't any parallel to this in, say, a robbery case. There is heavy stigma attached to making certain claims, and not everyone wants to cast themselves in that role.
4) Costs of losing: Sometimes, losing a case just means you lost a case. Other times, it means much more than that. A rape trial that doesn't lead to a conviction often brands the survivor a liar. Losing a racial discrimination lawsuit makes one a "whiner" who tried to "play the race card" to hide his own incompetence and/or criminality. If you sue your employer for racial discrimination and lose, you've pretty much just wrote a ticket out of your job. A friend of mine complained about sexual harassment in her school, and was retaliated against severely by both the student body and the administration, escalating both the harassment and branching into physical violence. And ultimately, she got no remedy at all (I forget whether the administration white-washed her complaint, or she "voluntarily" dropped it). I found this out when she relayed this information to a co-worker who wanted to complain about sexual harassment they were both facing at their job, four states away from her old school (I worked there too, but in a different division). She wanted no part of it, she didn't want to risk going through it again. In this way, the cost of losing significantly increases the risk in filing complaints, deters legitimate victims, and thus makes it less likely that real cases will see daylight.
Certain types of wrongdoing are more likely to implicate these factors to a greater degree than others. As a result, these crimes are less likely to be reported, and the public perception as to their prevalence is likely to be low compared to their relative frequency. Crimes that have a sexual or racial dimension are disproportionately likely to touch on these factors, and that is something that needs to be kept in mind when discussing and addressing them.