SAN FRANCISCO — Dr. Paul Turek was on his way to speak to employees at a cryptocurrency investment firm one recent afternoon about a growing anxiety for the men in the office: what’s going on with their sperm?

Is there enough? Is the existing supply satisfactory? Are we men enough?

“They’re worried, right?” Dr. Turek said. “And we’re O.K. with the worry.”

Last summer, a meta-analysis of 185 studies in which semen was collected over the past 40 years indicated that sperm concentration seemed to have consistently and remarkably declined in the course of a generation.

Different clusters of people — urologists, anthropologists, men’s rights activists and start-up founders — became quietly concerned about the state of sperm. Quiet, probably, because Americans are more used to talking about women and fecundity. And also quiet because there has not been much research aimed at discovering if anything is actually happening.