On Thursday, FiveThirtyEight published a story I wrote about the challenges of estimating abortion rates. As I discovered while reporting the piece, it’s a data problem that will only get more difficult as states like Texas pass legislation that effectively forces abortion clinics to close. As the issue becomes more politicized, states are less willing to hand over abortion data, making it hard for us to know whether policy changes are resulting in fewer abortions.

History does tell us one thing, though: Women have gone to great lengths to get around past attempts to make abortions more expensive or difficult to obtain. Ted Joyce, a professor of economics at the City University of New York, analyzed abortion data in the years before and after the Supreme Court legalized abortion in 1973. He found that before Roe v. Wade, when abortion was only permitted in a handful of states, women traveled hundreds of miles to get the procedure.

After New York legalized abortion in 1970, the state became a destination for pregnant women looking to terminate. In 1971, when abortion was legally available in six states and Washington, D.C., 84 percent of the known abortions performed outside of a woman’s state of residence took place in New York. Getting to New York was easy — if you could afford it — even from Midwestern states like Illinois and Michigan. “There was a plane for women who wanted abortions that went from Detroit to Buffalo,” Joyce explained when we spoke recently. “The doctors bundled it all together, so the flight was included in the price of the abortion.”

Distance, according to Joyce’s analysis, deterred some women from coming to New York. But for most, it wasn’t a deciding factor. To test this, Joyce looked at the average distance women traveled to New York in the study’s 13-state sample — 233 miles (excluding those traveling within New York) — and did a regression analysis that calculated the difference in abortion rates for women traveling 50 miles on either side. An extra hundred miles of travel — 283 compared to 183 — resulted in only a 12.2 percent drop in the average abortion rate for the 12 states for which New York was the likeliest site for abortion in 1971 (New Jersey, Connecticut, Rhode Island, Massachusetts, New Hampshire, Vermont, Oregon, Michigan, Maine, Indiana and Illinois). In other words, the rate fell by about 1 abortion per 1,000 women when the distance increased.

Non-white women, however, were more sensitive to distance. Joyce hypothesized that this was largely due to travel costs. Many white women, who tended to be wealthier, could afford to fly from Detroit to Buffalo. Women of color, who tended to have lower incomes, often couldn’t.

Forty years later, as the number of abortion clinics shrinks in states like Texas and Mississippi, Joyce predicts that there will be an uptick in women traveling to obtain abortions outside the states where they live. But, he adds, the same socioeconomic divide seems likely to emerge. Abortion is expensive — on average, women pay about $500 for a first-trimester procedure — even without the added cost of a plane ticket or a long drive. “Most people will find the money, because they want to avoid taking on that lifetime commitment,” he said. “Where you’re going to see an impact is with women who are economically vulnerable.”