"I think men actually do want to do this"

At Harvard, Dr. David Clapham, a neurobiologist, discovered that sperm tails contain calcium ion channels, with electrically charged atoms “turbo-charging the sperm” to reach eggs, he said. He is developing a drug to disable the channel.

“You just turn off the motor, rather than alter the people in a car,” he explained.

Elaine Lissner, director of the Male Contraception Information Project, formed a foundation to develop other approaches. One, “reversible inhibition of sperm under guidance,” or Risug, involves injecting gel into the scrotum to inactivate sperm. Another involves briefly heating the testes with ultrasound, which can halt sperm production for months, she said.

“I can imagine a world where you take your car in every six months to get your oil changed and go next door and get your ultrasound for 50 bucks,” she said.