On the one hand, San Francisco is doing as good a job of keeping families in the city now as it did 30 years ago. But on the other, we weren’t very good at it even then.

A report from the San Francisco Planning Department this week found that only 18 percent of San Francisco households have children. That’s down from 18.9 percent in the 1980s.

The majority of San Francisco children (over 9,500 of them) live in Bayview and nearby southeastern neighborhoods, while the Excelsior and Crocker-Amazon neighborhoods have some of the highest concentrations of families.

Interestingly, so does Treasure Island, albeit with only 524 neighborhood kids.

What’s the problem? Planning suggests a lot of potential causes, among them the fact that young millennials and aging baby boomers are now the majority of the city’s population—32 percent and 29 percent, respectively.

But one alarming takeaway from the data is the fact that almost all of the homes that could suit large families are occupied already. Turns out 70 percent of homes with three bedrooms or more in SF are occupied by households with no children.

Meanwhile, 25 percent of families with kids are living in single-bedroom homes, studios, or SROs. And since 50 percent of SF households dubbed overcrowded are families, a lot of the ones in larger homes are surely a bit squeezed as well.

(Note that the city has 669 families in SROs alone, about two-thirds of them in Chinatown.)

The report does not suggest a conclusive reason why so many singles and couples remain in larger homes, even if the kids have already moved out or were never there at all.

Further, developers haven’t been building the kind of housing that’s good for families. A clear majority of studio homes and one-bedroom apartments have been built just in the last 11 years or so, but the construction of larger homes has been a dribble.

So with little supply of significant size coming in and most of what’s out there already taken, it’s hardly surprising that families don’t stick around. Where would they even stay if they wanted to?