More infected men than women seem to be dying from the new coronavirus, according to data from countries hit by the pandemic, but an incomplete data set is clouding scientists’ ability to understand why.

The pattern underscores the role that sex—and the associated social norms and behaviors—plays as an indicator of risk and response to infection and disease.

“There are profound sex differences in immune systems, and this pandemic is revealing them once again,” said Marcia Stefanick, a professor of medicine at the Stanford Prevention Research Center, part of Stanford University School of Medicine. “What is biology versus what are our social norms and gender behaviors confounds our ability to understand what’s going on.”