Despite mounting evidence that first-born children are overrepresented among incumbents in political office, there is no consensus about the cause of this overrepresentation. Some stress the impact of differential parenting, arguing that the first-born receive a larger share of parental resources and have a greater need to live up to parental expectations. Others emphasize the interaction among siblings, arguing that first-born children are better prepared for power struggles, having experience both as followers and as leaders within the family. This study, using birth-order data for nearly 1,200 incumbents in various offices in local and national politics in the Netherlands, found more support for the parental impact perspective. Singletons were also overrepresented among incumbents, whereas middle-born children were underrepresented. The data suggest that this birth-order effect is weaker among younger generations and is more pronounced among women.