It also turns out that our periods have a huge say in who or what we want. "When we are ovulating we value promiscuity, aggression, dominance, willingness to engage in battle… When we aren't ovulating, we value loyalty, stability, and intelligence."
"Younger women like badasses for procreation, but they like kindness for monogamy," adds Tom Murray, PhD and director of counseling services at UNC School of the Arts.
To dumb it down, in our 20s we want the bad boys inside of us -- but a nice guy to hold our hands on Valentine's Day. So what do men with sisters have over the rest? A few key things that are integral to foundations of relationships.
They know how to listen
"Men with sisters tend to be better listeners," says Murray. "By and large, women use more words than men do. If you're around people who talk more, then you're exposed to a lot more words. But boys, as a consequence, may not express themselves so much through words, but through behavior and their physicality.