In several cities, nanny agencies decline to serve certain geographic areas — not because of redlining, these agencies say, but because the nannies, who decide which jobs to take, do not want to work there. “I can’t service everyone,” said Maria Christopoulos-Katris, owner of Nanny Boutique, an agency that turned down Ms. Freeman’s request, even though it claims to cater to the city of Chicago. “I don’t discriminate.”

Image Kimberly McClain DaCosta, a Harvard sociologist, left, preparing dinner for her three children with her new Dutch au pair, Sharon Ceder. Credit... Jodi Hilton for The New York Times

Ms. Freeman finally found a friend, another black mother, to watch her children.

Similarly, Ms. Jackson was told by some of the best-known nanny agencies in Washington that they did not serve Prince George’s County, Md., a largely black area bordering the District of Columbia.

“We have problems getting people to certain areas because of logistics,” said Barbara Kline, the owner of White House Nannies, which Ms. Jackson contacted. “I’m always worried people will interpret it the wrong way.” She added, “Nannies like to go where other nannies go or where their previous jobs were.” Ms. Jackson noted that White House Nannies served other suburbs, and that a bus stopped just minutes from her house.

Agencies represent only a small slice of nannies; most work through informal arrangements, further out of reach of civil rights and labor laws. (Because so many nannies are illegal, no one can say with certainty how many work in this country, let alone work for black families.)

In visits, telephone calls and e-mail exchanges across the country, nannies of all colors spoke of parents in sweeping ethnic generalizations: the Jews this, the Indians that. Viola Waszkiewicz, a white sitter in Chicago, has cared for black children, but explained that many fellow Eastern European nannies would not.

“We come here, and we watch TV and the news, and all we see is black people who got hurt, got murdered,” she said. Most of the nannies she knows “think all black people are bad,” she said. “They’re afraid to go to black neighborhoods.”

Pamela Potischman, a social worker in Brooklyn who specializes in parent-nanny relationships, said, “You rely on what’s familiar, so you’re going to rely on these vast generalizations to be self-protective.” She added, “The nannies talk, and they say, ‘This is what’s O.K. and what to watch out for.’ ”