In this world, the nuclear family took a different route. The wife of a household would marry multiple husbands. This would usually be two, although wealthier wives might have more if they can support them. These husbands would usually be related (brothers, cousins, etc), but having no blood ties with the woman. 

In our world, this is unlikely to happen. Males are unlikely to tolerate such a situation, and fight each other for dominance. In this case, I want the multiple-husband to one wife setup to actually favor cooperation over competition when it comes to who gets to reproduce. After thousands of years natural selection would favor males who can share a mate instead of fighting over one. This would eventually lead to a much less confrontational species.

I want this to be the common situation across the human species. What set of factors (biological, environmental) would allow me to make this possible in the human race?