Eighteen percent of the women in Sweden have at one time been threatened by a man.
Forty six percent of the women in Sweden have been subjected to violence by a man.
Thirteen percent of the women in Sweden have been subjected to aggravated sexual assault outside a sexual relationship.
Ninety two percent of women in Sweden who have been subjected to sexual assault have not reported the incident to the police.
These epigraphs are found at the beginning of each of the novel's four sections. They don't quite cut mustard as statistics, because they don't come with any citations to speak of. But they do comment on the novel's general vision that one big problem in Sweden is violence towards women. This is not to say that violence against women is more or less of a problem in Sweden than it is in other places throughout the world. As we know from Stieg Larsson's work advocating against violence towards women over the years, he viewed the problem as global (source).
Eva Gabrielsson, Larsson's partner of 32 years, says an event from his past fueled his interest:
"When he was 14 or 15, he was witness to a gang-rape committed by his so-called friends. This is the key thing. For him, the fight against violence against women was personal." (source)
The first two epigraphs claim that over half the women in Sweden have been either threatened with, or actually subjected to, violence by a man. The third and fourth, concerning sexual assault, are a bit trickier. According to number three, sexual assault is much rarer than non-sexual assault. But, according to number four, almost all of the women who are sexually assaulted don't report it. Hm.
We might think of the number three as a type of "official" statistic, and number four as an "underground" statistic that basically says: don't believe the official statistic. We can trace this issue of non-reporting back to Larsson's own experience – Gabrielsson says that Larsson didn't report the gang-rape he witnessed (source).
Similarly, neither Harriet nor Salander consider reporting their rapes to the police. Not only that, but Henrik and Salander basically force Blomkvist to keep silent about Martin and Gottfried's crimes, and about his own assault by Martin's hand. The social stigma and the shame of being a victim of sexual assault seem to drive the cover-up. Whether or not to report sexual assault to authorities, reasons people don't, and questions of vigilante justice, frame many of the novel's ethical dilemmas.