While birth weight may seem like just a number, it can affect the path of someone’s life. Children with a low birth weight have been found to have lower test scores, lower lifetime earnings, and higher rates of reliance on welfare programs throughout their lives. In a previous study, a different team of researchers examined twins in Norway whose birth weight diverged by 10 percent or more. The lighter twin was 1 percent less likely to graduate from high school and earned 1 percent less than their sibling through their life.

“Hydraulic fracturing has widely dispersed benefits—we are all paying lower natural-gas bills for heating, we’re all paying lower electricity prices, we’re all paying less for cheaper gasoline at the pump. And even if health was all that you care about, we’re all benefitting from decreased air pollution thats widely dispersed, because coal plants are closing,” said Michael Greenstone, a professor of economics at the University of Chicago and another authors of the paper.

But all those benefits, he said, were borne by the local communities that lived extremely close to hydraulic fracturing wells. “There’s this interesting trade off between the greater good and what are the costs and benefits for local communities,” he told me.

Oil and gas lobbying groups rushed to criticize the study. “This report highlights a legitimate health issue across America that has nothing to do with natural gas and oil operations. It fails to consider important factors like family history, parental health, lifestyle habits, and other environmental factors and ignores the body of scientific research that has gone into child mortality and birthweight,” said Reid Porter, a spokesman for the American Petroleum Institute, a trade organization that represents the oil and gas industry.

In the fracking study, researchers tried to separate the costs of fracking and socioeconomic status and parental health in several ways. First, they compared baby birthweight near fracking wells to those babies immediately around them, which they believe accounts for the wealth of various communities.

Second, they found that the connection held for siblings who were or were not exposed to a fracking well. “We follow the same mother over time and ask whether on average, children born after fracking starts have worse outcomes than their siblings born before fracking starts,” Currie told me. “In this case, since we follow the same woman over time, we are controlling for her underlying characteristics.”

Babies who gestated near a well had a reliably lower birth weight than their siblings who were not exposed to the well.

The researchers don’t yet know why this link between fracking and low birth weight exists, though they suggest that air pollution could be a possible contributor. The process of fracking may release chemicals into the air, for one, but many wells also run multiple diesel engines at once, and they can be a hub of local activity, with trucks regularly commuting to the sites.