Instant messages are ubiquitous and convenient, but something primal may be lost in translation.

When girls stressed by a test talked with their moms, stress hormones dropped and comfort hormones rose. When they used IM, nothing happened. By the study’s neurophysiological measures, IM was barely different than not communicating at all.

“IM isn’t really a substitute for in-person or over-the-phone interaction in terms of the hormones released,” said psychologist Leslie Seltzer of the University of Wisconsin, a co-author of the new study. “People still need to interact the way we evolved to intera...