My own best guess about the asymmetry of parental love lies in a metaphor borrowed from the sciences.

BBC: A Point of View: The pain when children fly the nest

"Romantic love and parental love are overlapping systems, " says Parrott, who has studied jealousy and envy for more than 25 years.

CNN: I'm jealous of my baby -- a Mother's Day confession

Parental love, I think, is infinite.

BBC: A Point of View: The pain when children fly the nest