Modern gender norms and biases in Europe have deep historical roots dating back to the Middle Ages and beyond, research finds.

The findings—published in the Proceedings of the National Academy of Sciences—highlight why gender norms have remained stubbornly persistent in many parts of the world despite significant strides made by the international women’s rights movement over the last 100-150 years.

Using dental records of more than 10,000 people from 139 archaeological sites throughout Europe, researchers found that individuals who live in areas that historically favored men over women display more pro-male bias today than those who live in places where gender relations were more egalitarian centuries ago—evidence that gender attitudes are “transmitted” or handed down from generation to generation.

These biases outlasted monumental socioeconomic and political changes such as industrialization and world wars. Researchers found one exception to the rule, however: In regions that experienced abrupt, large-scale population replacement—such as a pandemic or natural disaster—transmission of these values was interrupted.

“The median age of the skeletons in this study is about 1,000 years dating back to the medieval era. It is therefore remarkable that the patterns of gender bias that existed during those times and earlier are still replicated in contemporary attitudes,” says Margit Tavits, professor at Washington University in St. Louis.

“Given the enormous social, economic, and political changes that have taken place in Europe during this time, our findings speak to the power of cultural transmission of gender norms.”

The incredible stability of these norms over hundreds, if not thousands, of years also explains why it has been difficult in some regions to move the needle toward gender equality.

“There has been a widespread belief that gender norms are a byproduct of structural and institutional factors like religion and agricultural practices. Our findings draw attention to