Asking people who answer the survey what they think of it would create what Dr. Groves calls "a big positivity bias, " likely overstating support for the survey.

WSJ: Census Gets Questions on Its Mandatory Queries

As early as age 3, children have what researchers call a "positivity bias" a tendency to see themselves as smart regardless of their abilities, and to exaggerate positive traits in others, says a 2010 study in the journal Child Development Perspectives.

WSJ: The Case for Lying to Yourself