It goes like this: You strike up a conversation with someone you've never met before, and whether you admit it to yourself or not, after two minutes or so, you know: You're attracted to him or you're not. If you're single, you're intrigued; if you're happily involved with someone, you're... careful.
While long-term compatibility depends a lot on factors like status and life history, what causes the sparks to fly, or not, during that first conversation is how well your personality types match up.
Some might say four chemicals play leading roles in determining who we are and who we're drawn to: two sex hormones, testosterone and estrogen; and two neurotransmitters, dopamine and serotonin. The influence of these molecules on our romantic lives are the building blocks to construct four distinct personality types. The Explorer, defined by high dopamine activity, is adventurous, novelty-seeking, creative. The Builder, with high serotonin activity, is cautious, conventional, managerial. The Director, pumped up with testosterone, is aggressive, single-minded, analytical. The Negotiator, more estrogen-influenced, is empathetic, idealistic, a big-picture thinker.