The share of those who choose to work part-time because of family and health constraints, lifestyle preferences, or other reasons, is around 13 percent, a level that has been relatively stable since at least the 1970s. In recent years that group has changed little in its demographics – about 60 percent female and skewed towards younger and older workers – and in the numbers who cite child care, medical or other issues as the reason for working part-time.

“It is striking how stable the relationships have been,” said Ariane Hegewisch, a research director at the Institute for Women’s Policy Research in Washington.

As it analyzes the steady fall in those forced into part-time work by the weak economy, the Federal Reserve now must judge whether the tight labor markets of the 1990s and early 2000s remain a good benchmark.