It's this thing based on neurotransmitter levels and how they have an effect on temperament, personality, and mate selection.
"People who express dopamine —I call them Explorers —tend to be risk-taking, curious, creative, impulsive, optimistic and energetic. The traits associated with the serotonin system express themselves in what I call Builders. They're cautious but not fearful, calm, traditional, community-oriented, persistent and loyal. Directors have traits associated with activity in the testosterone system. These people tend to be very analytical, decisive, tough-minded; they like to debate and can be aggressive. The fourth type is the Negotiator. Men or women who express activity in the estrogen system tend to be broadminded imaginative, compassionate, intuitive, verbal, nurturing, altruistic and idealistic."
I believe explorers tend to be attracted to one amother. Builders to one another and directors to negotiators and vice versa. What type do you think you are and have you seen this play out in the relationships you've seen develop in your own life?