Once marketed "for milady" and available only for passengers, vanity mirrors now come on both the left and right sunvisors of most vehicles sold in the United States. Among the 2,192 trim levels offered on 2013-model cars and trucks, dual visor mirrors are standard on 81 percent, according to Chrome Data Solutions, a Portland, Ore., company that provides vehicle descriptions for online shopping tools.
But while many well-appointed Rams, Silverados and Jeeps provide bilateral vanity mirrors, 262 versions of new vehicles do not. They have one visor mirror, for the front-seat passenger.
"That just seems crazy," said David Gartman, a sociology professor at the University of South Alabama and author of "Auto Opium: A Social History of American Automobile Design" (Routledge, 1994). "It could just be dumb tradition. That's where the vanity mirror has always been, because when couples ride together, the man has traditionally done the driving."
Automakers may assume that "since women stereotypically are the vain sex, they need the mirror where they sit, the passenger side," he added.
Even if women clamor louder than men for vanity mirrors -- as one big automaker says they do -- placing just one on the passenger's side makes little sense, Dr. Gartman said, "since families now have a car for each adult in the household, which means women are probably driving themselves and their children."
For the first time, more women than men are in the driver's seat, according to a study released last year by the University of Michigan's Transportation Research Institute. In 2010, the report said, 105.7 million women were licensed to drive in the nation, compared with 104.3 million men, reversing a longtime gender gap.
Yet some automakers aren't catering to those women at the wheel. On the Nissan Versa sedan, a passenger-side vanity is optional on the SV version and standard on the SL, with no driver-side mirror available for either model. And for all Smart cars, a vanity is standard for the passenger but not even an option for the driver.
"It certainly has nothing to do with gender roles," John Curl, a regional product manager for Nissan, said of the Versa, about half of which are bought by women. "We had only enough money for one mirror, and we decided it should go on the passenger's side," he said, adding that "regardless of gender, the driver can use the rear-view mirror," while a vanity mirror could be a "driver distraction."
"It would be exceedingly odd if you only had one to put it on the driver's side," he added. "I think you would get customer complaints." But according to Chrome Data, versions of three 2013 vehicles -- the Chevrolet Spark, Dodge Journey and Honda CR-Z -- did just that.
Dr. Gartner, the sociologist, said the boom in men's cosmetics and plastic surgery suggested that "men are just as vain about their appearance as women." But if vanity is gender-neutral, requests for vanity mirrors are not, said Justin Healy, the global overhead systems supervisor for engineering at Ford. He said that when Ford surveyed S.U.V. drivers in 2009 about their favorite visor feature, 54 percent of all respondents asked for a mirror. Among women, the figure was 70 percent.
He said dual vanity mirrors were standard in 90 percent of Ford vehicles, the exceptions being commercial vehicles and entry-level cars. But in those with a solo mirror, why give it to the passenger?
"We're hedging our bets that the driver will use the rear-view mirror for a quick check," Mr. Healy said. Or maybe not: Mary Sipes, vice president for global planning and program management at General Motors, looked into that in 2005, when she was a vehicle line director. She asked engineers to climb into an S.U.V. and check their teeth after eating broccoli. Pamela Flores, a G.M. spokeswoman, recalled that the men used the rear-view mirror; the women went for the vanity on the visor.
This article originally appeared in The New York Times.