Francesco Moro cannot say with certainty how his 65-year-old father contracted the coronavirus, but he figures it has something to do with the fact that he and his older brother both fell ill a week earlier. Though the two sons are grown, they live with their parents in a modest home outside Bergamo, the epicenter of the Italian outbreak.

“The kitchen and the living room, the rooms that we share the most, are quite small.” said Mr. Moro, 28. “The probability of transmitting it is much higher.”

That logic has captured the attention of social scientists who are exploring a theory that may partly explain why the pandemic has proved especially deadly in Italy and Spain. In those countries, large numbers of working-age people live with their parents, and the younger people may be bringing the virus home and spreading it to their far more vulnerable, elderly parents.

Far from universally accepted, this hypothesis is the subject of fierce debate, after the publication of a research paper purporting to find a link between incidence of adults living with their parents and deaths from the virus. Some economists have challenged the rigor and validity of that analysis. Italians and Spaniards are pushing back on the notion that an element of cultural pride — multiple generations of families living under the same roof instead of offloading older people to senior homes — is now being construed as a deadly vulnerability.