But after a while, they said, family members shrugged off the cameras and relaxed.

The same cannot be said of the fieldworkers, most of them childless graduate students seeing combat for the first time. “The very purest form of birth control ever devised. Ever,” said one, Anthony P. Graesch, a postdoctoral fellow, about the experience. (Dr. Graesch and his wife have just had their second child.)

In one house, Dr. Graesch was recording locations when an escalating argument threatened to get ugly. He bailed out for air and continued to track people inside by peeking through the windows. “Luckily it was a one-story bungalow,” he said.

In weekly meetings, the researchers discussed what they were witnessing.

“Every time we met, I felt like I was on the defensive,” said Tamar Kremer-Sadlik, the research director, who herself has two children and a working husband. “I mean, it’s not like I approved of everything these parents were doing. But I could relate to them. I knew exactly what they were going through.”

Continual negotiations, for one. Parents generally were so flexible in dividing up chores and child-care responsibilities — “catch as catch can,” one dad described it — that many boundaries were left unclear, adding to the stress.

The couples who reported the least stress tended to have rigid divisions of labor, whether equal or not. “She does the inside work, and I do all the outside, and we don’t interfere” with each other, said one husband.

The videotapes reveal parents as at-home teachers, enforcing homework deadlines. As coaches and personal trainers, sorting through piles of equipment. As camp directors, planning play dates and weekend “family time.”

“The coordination it takes, it’s more complicated than a theater production,” said Elinor Ochs, the U.C.L.A. linguistic anthropologist who led the study. “And there are no rehearsals.”