Surveys by the National Assn. of Home Builders show that less than 20% of new construction in recent years has been for entry-level properties. Before the recession, that share typically hovered around 30%. More than half of single-family houses sold in recent years have been 2,400 square feet or larger, compared with about 40% a decade ago.

“This is the first time in the supply history of housing where, for whatever reason, a giant new generation is not being served,” said G.U. Krueger, a Los Angeles housing economist who does research for CalPERS’ advisors and other investors. “To me, it’s incomprehensible.”

Whether the lack of supply is hampering demand, or the other way around, isn’t clear. But millennial-generation home buyers have not emerged in expected numbers, in part because they are staying single or getting married and having children later in life.