For every three men hopping on a bike, just one woman does the same, according to an analysis by BuzzFeed’s Jeremy Singer-Vine, who collected data from the three largest bike-share programs in the United States — New York, Chicago and Boston — and mapped the gender balance in each. Although slightly more women check out a bike on weekends, overall, they still make up only 24.7 percent of riders.

This gender gap isn’t unique to bike-share programs or these three cities. The most recent National Household Travel Survey shows that 24 percent of bike trips in 2009 were made by women. The national data also shows that women are slightly less likely to cycle now than they were in 2001; researchers found “the prevalence of any cycling declined signiﬁcantly for children (by 1.5 percent) and women (by 0.3 percent).” That needn’t be the case: In Germany, 49 percent of cyclists are women. In the Netherlands, that number is 55 percent.

Readers were quick to offer explanations for the lack of female cyclers:

@anya1anya @BuzzFeedBen @jsvine well, and the obvious oversight: street harassment. Impacts women's use of all transit, especially at night. — Anil Dash (@anildash) June 16, 2014

In fact, most theories fell into one of two categories: fashion or fear.

But what’s the evidence for each?

For her master’s thesis in urban planning at the University of Washington, Anne Broache examined which influenced women’s decisions about cycling. In 2012, she surveyed 365 women in Seattle — where 28 percent of bike commuters are female, according to the city’s Department of Transportation — about fashion and road safety. One-third of the nondaily riders and one-fifth of daily riders reported general concerns about “grooming issues, bringing spare clothes, helmet hair, and arriving at destinations red-faced and sweaty.” But safety was “by far the leading concern for all riders” — 79 percent of the women cited “distracted driving” as the biggest barrier to them cycling.

Although it didn’t ask about fashion, the 2010 Women’s Cycling Survey asked a lot more women about their cycling choices; it, too, found the No. 1 concern was “distracted driving,” which was cited by 73 percent of the 11,453 women questioned.

But there’s more to personal safety than the risk of traffic accidents. As @anildash suggests (and Helen Pidd’s personal account on The Guardian’s Bike Blog illustrates), female cyclists might fear sexual assault and harassment. The Women’s Cycling Survey found that 13 percent of women said “stranger attacks” were a concern.

And just as fear is a complex issue, so, too, is fashion. Lifestyle barriers affect women’s decision-making around cycling in ways more challenging than footwear or hem length. When the Bikes Belong Coalition surveyed almost 2,000 U.S. adults, they found that women were twice as likely as men to report an “inability to carry children or other passengers” as a factor that discouraged them from cycling. Convenient transport is important for moms, because they spend 3.7 minutes more per day than dads ferrying kids around.

Finally, one more reason women aren’t getting on bikes can’t be captured by the fear or fashion explanations; like most lifestyle choices, cycling decisions are affected by wealth. And women earn less than men. As Clarissa Ersoz at the Bicycle Paper explains, “Even a reasonably priced bike is a significant up-front expense for disadvantaged households.” A 2001 report, “The Socioeconomics of Urban Travel,” found that households earning less than $20,000 were no more likely to use bicycles as a mode of transport than those earning $75,000 – $99,999.

When it comes to the why of America’s cycling gender gap, the data suggests that fashion really isn’t front of mind for most women. Instead, road safety and practical lifestyle issues are the biggest obstacles to female riders. That makes it all the more disappointing that the recent bike-share data shows that rental systems have been unable to address those concerns.

CORRECTION (June 18, 3:59 p.m.): An earlier version of this post incorrectly stated that, according to “The Socioeconomics of Urban Travel,” 13.5 percent of households earning less than $20,000 used a bicycle as a mode of transport. The table in the report actually shows that 13.5 percent of all people who use a bicycle as a mode of transport live in households earning less than $20,000.