WASHINGTON — The Census Bureau released a surprising finding this week: The number of non-Hispanic Americans who identify as multiracial had jumped by 127 percent over the decade. For people who identified as Hispanic, the increase was even higher.

The spike sent demographers scrambling. Was the reason simply that more multiracial babies were being born? Or that Americans were rethinking their identities? Or had a design change in this year’s census form caused the sudden, unexpected shift?

The answer, it seems, is all of the above.

Multiracial Americans are still a relatively small part of the population — just 4 percent — but the increase over the decade was substantial and, the data shows, often surprising in its geography. The number of Americans who identified as non-Hispanic and more than one race jumped to 13.5 million from 6 million. The largest increase in non-Hispanic Americans of two or more races was in Oklahoma, followed by Alaska and Arkansas.

Americans who were mixed race recorded a wide range of identities. People who identified themselves as both white and Asian made up about 18 percent of the total number of non-Hispanic multiracial Americans in 2020. Those who reported their race as both white and Black accounted […]