The best theory I've heard is that, among your ancestors, the women who were okay with being "war brides" or who desired the most violent men in the tribe left more and healthier offspring.
You have to compare these two groups of women:
- when the conquering tribe rolls in and slaughters your men, you lay down and let them rape you - if they offer to take you with them, you accept. If there's no conquering tribe, but there are especially violent men in your tribe, you find those men especially attractive
compared to:
- when the conquering tribe rolls in and slaughters your men, you fight back ...and are killed. If there's no conquering tribe, you are attracted to a meek, quiet male who will treat you well
Between those two groups, which women leave more offspring? These women's offspring inherit their mother's instincts.