In a popular film, an actor plays a success-driven sports agent whose marriage begins to crumble. Attempting to win his wife, Dorothy, back, he looks into her eyes and says, “You complete me.” It’s a heart-warming message that echoes a tale in Greek philosophy. According to that myth, each of us is a “half” that must find our “other half” to become whole.

The belief that a romantic partner “completes” us is now part of popular culture. But is it true? I talk to many married couples who still feel incomplete because they haven’t been able to have children, and others…