They have seen us go through it all
"Depending on whether or not a boy was an older or younger brother to a sister will change his reaction to watching her go through a bad breakup," says Murray. "An older brother might think, 'I need to kick the shit out of that guy,’ in a hyper-masculine response. Whereas a younger brother might be more intuitive, more sensitive, more interested, and more engaged. He might be trying to understand what it is like for you, as a female in relationships, so that he's better able to find a mate."
Granted, this doesn't mean every good man grew up alongside a sister. Like any science, there are exceptions to the rule -- and it's unfair to assume all men raised without sisters make crappy partners. Parents, friends, and other non-sisterly influencers can do a lot to ready young boys to coexist with this new generation of smart, strong, independent women.