Unlike some other virtual twins, Sara and Julie were not dressed alike or treated as twins in school. “They are totally different,” Mr. Curry said. “They offset each other.” As the family sat around their dining room table on a recent afternoon, Mr. Curry turned to his daughters and said: “It has to be more nature. You grew up so close, you were together 24-7.” He turned to Sara and said, “She’s a stabilizer to your drama.” And to Julie, “She brings excitement to you.”

Image MOTHER NATURE Dr. Nancy L. Segal studies virtual twins for the material they contribute to the nature-versus-nurture question. She says that evidence suggests nature is winning. Credit... Ann Johansson for The New York Times

“Yeah,” Julie said to her sister, “you bring me out of my shell.”

Some virtual twins in the study, however, have been deliberately raised as twins from birth. Julie Dykstra, a former nurse who lives in Belmont, Mich., had two adoptions come through within the same week. Her sons, now 6, were due on May 1 and May 3. Each was born 8 pounds 2 ounces and 21 inches long, she said, 10 days apart. “I never felt like I didn’t have twins,” she said. “I felt like I was immersed in it.”

They have grown up feeling like twins, and the family has told their school that they are twins.

“They have twin language,” Mrs. Dykstra said. “They know what the other is thinking and going to say before he says it. They met at 13 days and 3 days old.”

Dr. Segal said that while “genetics do not tell the whole story,” even if parents treat their virtual twins as biological twins or if the children show similarities early in life, her research has found that environment still has “minimal or no effect” on them in terms of behavior and intelligence. Virtual twins can seem very similar in their early years, she said, but in the long term a shared environment is not “going to have a lasting impact.”

Mrs. Dykstra and her husband, Todd, a pastor at Maranatha Bible Church, later adopted another set of virtual twins, two girls who are 20 days apart and are now 2. One was adopted from China and did not join the family until she was 10 months old. Mrs. Dykstra said she does not feel the girls are twins because one had been in an orphanage in China and came into the family so much later than the other. She added that the girls are still young. Six months ago the couple adopted another girl at birth, the biological sibling of one of their adopted daughters, and Mrs. Dykstra said she was excited to find out how the three sisters will be different. “We are like a little petri dish,” she said.

Some critics and adoption agencies say that having virtual twins (sometimes called pseudo twins or artificial twins) should be avoided by parents when possible, so that each child receives adequate parental attention. Some say that parents are not being truthful or fair when they attempt to arrange more than one adoption from birth mothers who might not choose them if they knew another infant would be brought into the home within the first year.

“People need to be very well counseled,” said Joyce Maguire Pavao, the chief executive and founder of the Center for Family Connections in Cambridge, Mass., which provides counseling and other services to what the center calls blended families.