Why do we fall in love with one person and not another?

This question has vexed philosophers, psychologists and poets for generations. The theories—proximity, pheromones, timing—don’t fully explain the mystery. We can be in a room full of attractive, available strangers—and be open to love—and still choose one person over all others.

A decade ago, the biological anthropologist Helen Fisher set out to answer this question of how we choose whom we love. Dr. Fisher is known for her research scanning the brains of people in various stages of love, and she went looking for neurological clues.

She found them—and, in the process, she developed a broad personality test that, unlike many others, is based on brain science rather than psychology. The Fisher Temperament Inventory measures temperament, which comes from our genes, hormones and neurotransmitters. It can help people understand themselves and why they are attracted to certain people, both romantically and as friends or colleagues. (It turns out some personality types are attracted to people who are the opposite of them, while other types are attracted to people who are similar.) And when you know the personality type of loved ones or co-workers, you will understand how they prefer to communicate and will be able to tailor your approach to what works best. The inventory is so comprehensive that now some companies are using a version of it to help understand and motivate their employees.

To develop her test, Dr. Fisher spent two years reviewing medical and academic literature, searching for the personality traits linked to a biological system. She identified four systems, each with its own host of traits: the dopamine, serotonin, testosterone and estrogen systems. Dopamine and serotonin, which are neurotransmitters, govern our “stay or go” scale, which decides how comfortable we are exploring unknown risks or whether we prefer the familiar. Testosterone and estrogen are hormones and determine the extent to which our brains express male or female traits.