HATTIESBURG, Miss. — For generations here in the deepest South, there had been a great taboo: publicly crossing the color line for love. Less than 45 years ago, marriage between blacks and whites was illegal, and it has been frowned upon for much of the time since.

So when a great job beckoned about an hour’s drive north of the Gulf Coast, Jeffrey Norwood, a black college basketball coach, had reservations. He was in a serious relationship with a woman who was white and Asian.

“You’re thinking about a life in South Mississippi?” his father said in a skeptical voice, recalling days when a black man could face mortal danger just being seen with a woman of another race, regardless of intentions. “Are you sure?”

But on visits to Hattiesburg, the younger Mr. Norwood said he liked what he saw: growing diversity. So he moved, married, and, with his wife, had a baby girl who was counted on the last census as black, white and Asian. Taylor Rae Norwood, 3, is one of thousands of mixed-race children who have made this state home to one of the country’s most rapidly expanding multiracial populations, up 70 percent between 2000 and 2010, according to new data from the Census Bureau.