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 middleborn children were underrepresented. The data suggest that this birth-order effect is weaker among younger generations and is more pronounced among women.KEY WORDS: birth order, political leadership, parent-child interactions, sibling interactions Ever since Adler (1928), scholars have put forward hypotheses linking an individual's birth-order position to a range of personal characteristics, including politically relevant traits such as intellectual development, a need for power, liberalism, and rebelliousness. There is now substantial and growing support for one such proposition-that birth order, and in particular being a first-born child, affects the likelihood of attaining public office. First-born children-those with one or more younger siblings-have been found to be overrepresented among U.S. presidents, British and Australian prime ministers, leaders of countries in all parts of the world, members of the U.