To illustrate, Fisher cited her own relationship

Even sense of humor can be broken down by type, with questions like “Do you sometimes make faces at yourself in the mirror?” (people with a sense of humor do) and “At the zoo, which do you generally prefer to watch?” (the reply “monkeys and apes” indicates more of a funny bone than “lions and tigers”). According to Fisher, a Director likes people to laugh at his or her jokes; a Negotiator likes to be around someone funny so he or she can laugh at that person’s jokes; an Explorer is spontaneous and laughs at just about anything; and a Builder, she suspects, generally isn’t as funny as the others.

This is why she decided to include an item on the Chemistry questionnaire that asks about the traits of a person’s partner in his or her most successful former relationship: Was that person an Explorer, a Builder, a Director, a Negotiator?

But how to match people up according to Fisher’s four personality types, and under what circumstances, isn’t so straightforward. Another question, for instance, presents four smiling faces and asks:

Fisher says that people with high levels of estrogen-usually women-have better social skills, and are better at reading other people. So users who choose the correct “real” smiles (pictures two and three) will be the Negotiators. This, Fisher says, is an area where “complementarity” might be important. “We also want someone who masks our flaws,” she explained. “For example, people with poor social skills sometimes gravitate toward people with good social skills. I’m an Explorer, so I don’t really need a partner who is socially skilled. That’s not essential to me. But it may be essential to a Director, who’s generally less socially skilled.”

Chemistry’s compatibility questionnaire also examines secondary personality traits. “I’m currently going out with a man,” she said, “and of course I made him take the test instantly. We’re both Explorers and older. I’m not sure two Explorers want to raise a baby together, because nobody will be home. But in addition, I’m a Negotiator and he’s a Director type. Our dominant personality is similar, but underneath, we’re complementary.”

The problem with sites like eHarmony, she believes, is that they place too much emphasis on similarity, whereas, in her view, falling in love depends on two elements: similarity and complementarity

Determining which works best-similarity or complementarity-may change with the circumstances. A young woman who’s an Explorer, Fisher said, might be attracted to a Builder, someone who’s more of a homebody, loyal, dependable, and protective. But the pair will be more compatible if their secondary personalities match-maybe they’re both Negotiators underneath.

“Nobody is directly locked into any one of these temperament types,” Fisher said. “That’s why we provide each person with both a major and a minor personality profile. Do Explorers go well together? Do likes attract likes? Sometimes they do and sometimes they don’t.”

If this sounds a bit, well, unscientific, Fisher is the first to admit it. “I have theories about what personality type a person would be most ideally suited with,” she told me, “but I also trust people to tell me what they are looking for. All throughout the questionnaire are checks and balances to what are just Helen Fisher’s theories.”

“Anybody can match somebody for values. But I’m hoping to create a system so that five years later they still fascinate each other.”

At the same time, Fisher wants couples to be fascinated by each other early on. In other words, why waste time e?mailing back and forth to get to know a potential match over the course of several weeks, as eHarmony encourages its users to do, if there won’t be any chemistry when they finally meet? Chemistry’s guided 1-2-3-Meet system provides a step-by-step structure to get couples face to face as soon as possible for that all-important “vibe check.” Then there’s a post-meeting “chemistry check ,” where each person offers feedback about the date.