“Helen Fisher, a women’s cult figure and an anthropologist, has long argued that falling in love—and falling out of love—is part of our evolutionary biology and that humans are programmed not for lifelong monogamy, but for serial monogamy. (In stretches of four years, to be exact, approximately the time it takes to get one kid safely through infancy.)

Why Him? Why Her? explains the hormonal forces that trigger humans to be romantically attracted to some people and not to others (a phenomenon also documented in the animal world). Fisher posits that each of us gets dosed in the womb with different levels of hormones that impel us toward one of four basic personality types:

The Explorer—the libidinous, creative adventurer who acts “on the spur of the moment.” Operative neurochemical: dopamine.

The Builder—the much calmer person who has “traditional values.” The Builder also “would rather have loyal friends than interesting friends,” enjoys routines, and places a high priority on taking care of his or her possessions. Operative neurotransmitter: serotonin.

The Director—the “analytical and logical” thinker who enjoys a good argument. The Director wants to discover all the features of his or her new camera or computer. Operative hormone: testosterone.

The Negotiator—the touchy-feely communicator who imagines “both wonderful and horrible things happening” to him- or herself. Operative hormone: estrogen, then oxytocin.”