"What do Angela Merkel, Hillary Rodham Clinton, Christine Lagarde, Oprah Winfrey, Sheryl Sandberg, JK Rowling and Beyonce have in common?" was the headline in the English newspaper The Observer in 2014. "Other than riding high in Forbes list of the world's most powerful women," journalist Tracy McVeigh wrote in answer to her own question, "they are also all firstborn children in their families. Firstborn children really do excel." So what does it mean to be an eldest daughter? Firstborns Lisette Schuitemaker and Wies Enthoven set out to discover the big five qualities that characterize all...