Girls with fraternal male twins may earn less and be less likely to thrive in society, new research suggests. Researchers in Norway and at Northwestern University posit that exposure to their brother’s testosterone in the womb may damage the female twin, resulting in poorer cognitive functioning and fertility.

Though studies on humans vary, it may be that this testosterone exposure alters the way females develop, leading to more masculine traits that could explain the lower probability that these girls find partners and marry later in life.

Understanding these outcomes is more important now than ever, the researchers say, as IVF has let to a near two-fold increase in the number of twins born annually.