This study is the first to provide evidence of birthday manipulation. In South Korea,approximately 13,000 children born in the month of December between 2006 and 2015 were falsely registered as having been home-born in January of the subsequent year. These children differed significantly in terms of certain characteristics from those home-born in other months. Some parents likely manipulated their child’s birthday to leverage the benefits enjoyed by the oldest children in a class cohort.