Ann Tucker is pushing a shopping cart through the produce section of a supermarket in Plainview, N.Y., when she turns to kiss her husband. The supermarket kiss is a regular ritual for the Tuckers. So are the restaurant kiss and the traffic-light kiss. "I guess we do kiss a lot," says Mrs. Tucker, a 39-year-old mathematician at a money-management firm.

Mrs. Tucker is living happily ever after, and scientists are curious why. She belongs to a small class of men and women who say they live in the thrall of early love despite...