A new study suggests the person who left your message unanswered tends to feel worse about it than you imagine, and worse still when the silence was accidental — in a scenario version of the situation, a one-line "Sorry, but I can't talk right now" narrowed the gap without closing it

A person sits alone at a window at dusk, seen from behind, holding a lit phone screen in one hand while city lights blur outside.

"What do you think of this?" That is the last message in one version of an experiment funded by the University of California, Berkeley. It goes to a friend you don't know all that well, a...