Here’s an article about a couple arrested for domestic violence because the police couldn’t determine who was the primary aggressor. I’m including this article as a chance to mention “primary aggressor.” Some of the men’s rights activists are ticked off that police determine and arrest the primary aggressor. They’d much rather think the violence was mutual because, according to their viewpoint, all domestic violence is mutual.
While this particular article indicates the woman committed violence, the problem with dual arrests is that victims can be arrested too. So perhaps this is not the best example to use – but I want to take this opportunity to include some citations to research studies here about primary aggressors and dual arrests.
Section 12 — Who is the primary/predominant aggressor?
A substantial percentage of victims of domestic violence hit their perpetrators back.  In Massachusetts, 37.3 percent of the female victims fought back in the incident in which their male abuser was arrested. However, most (59.1 percent) of those females who fought back found that this made their abuser more violent.  A substantial number of victims will not self-disclose their victimization.  Consequently, determination of primary or predominant aggressor may not be self-evident. Nonetheless, data on police action in 2,819 jurisdictions in 19 states reveal that only 1.9 percent of incidents resulted in dual arrests for intimate partner violence and intimidation. In other words, less than 4 percent of all intimate partner arrests were dual arrests in which law enforcement could not determine a primary or predominant aggressor. 
Studies suggest that officers’ determination of primary or predominant aggressor is particularly problematic when the intimate partner violence occurs between same-sex couples. Although police are equally likely to make arrests in same-sex as in heterosexual partner abuse cases, a study of more than 1,000 same-sex intimate partner violence reports from departments across the country found that officers were substantially more likely to arrest both parties in same-sex cases. Specifically, 26.1 percent of female same-sex cases and 27.3 percent of male same-sex cases resulted in dual arrests, compared to only 0.8 percent with male offenders and female victims, and 3 percent with female offenders and male victims. 
Research on the impact of primary aggressor policies, either mandated by state statute or by individual law enforcement agencies, reveals that such policies significantly reduce the percentage of dual arrests from an average of 9 percent to 2 percent of domestic violence arrests. 
Implications for Judges
In dual-arrest cases, judges should insist that prosecutors provide evidence that one of the parties was the primary or predominant aggressor and the other the victim. This may be particularly important, as advocates caution that female victims who are arrested along with their abusers may nonetheless plead guilty in order to be able to return home to care for minor children. Furthermore, it appears that law enforcement finds it particularly challenging to determine the primary/predominant aggressor with same-sex couples. (Research basis: The most significant dual-arrest study was based on examination of all assault and intimidation cases in the 2000 NIBRS database as well as more detailed examination of these data from 25 diverse police departments across the country.)