Dr. Garfinkel said the results of the Fragile Families study “provide no support for the culture of poverty thesis,” which posits that different values and behaviors among the poor trap them in self-perpetuating cycles of privation. Rather, he said, “circumstances and opportunities,” not value differences, have the greatest impact.

Though her findings rankled some advocates for single mothers, Dr. McLanahan continued to publish books and papers on the topic.

“We reject the argument that people should not talk about the negative consequences of single motherhood for fear of stigmatizing single mothers and their children,” Dr. McLanahan and the researcher Gary Sandefur wrote in their 1994 book, “Growing Up With a Single Parent: What Hurts, What Helps.” “While we appreciate the compassion that lies behind this position, we disagree with the bottom line. Indeed, we believe that not talking about these problems does more harm than good.”

Sara Frances Smith was born on Dec. 27, 1940, in Tyler, Texas. Her father, Norman Smith, was a general manager for a local oil company. Her mother, Iredell (Brown) Smith, was a homemaker.

She attended Robert E. Lee High School in Tyler, now known as Tyler Legacy High School. A gifted pianist, she studied at the Aspen Music Festival in Colorado for a summer. She attended Bennett Junior College in Irvington, N.Y., and went on to Smith College. After a year at Smith, she dropped out in 1962 and married Ellery McLanahan. They had three children, Sara, Ellery and Anna Bell, all of whom survive her. The family moved to Houston, and the couple divorced in 1972.

Dr. McLanahan returned to school and graduated from the University of Houston with a bachelor’s degree in sociology in 1974. She received her masters and doctoral degrees in sociology from the University of Texas at Austin.

She began her postdoctoral fellowship at the University of Wisconsin in 1979. It was there that she met Dr. Garfinkel and focused her research on single motherhood. (She had avoided the topic in her doctoral thesis at the University of Texas because, she said, it felt too close to home.) She and Dr. Garfinkel married in 1982.