Results from several decades of long-term follow-up of scores of unselected XYY males identified in eight international newborn chromosome screening studies in the 1960s and 1970s have replaced pioneering but biased studies from the 1960s (that used only institutionalized XYY men), as the basis for current understanding of the XYY genotype and established that XYY males are characterized by increased height but are not characterized by aggressive behavior. Though the link currently between genetics and aggression has turned to an aspect of genetics different from chromosomal abnormalities, it is important to understand where the research started and the direction it is moving towards today.