Eugenics aimed to improve hereditary traits through selective breeding. It encompassed positive eugenics, encouraging reproduction among the genetically fit, and negative eugenics, which sought to restrict reproduction among those deemed unfit. The eugenics movement gained acceptance in the early 20th century in both the US and UK due to fears over poverty, immigration, and the lower classes outbreeding the elite. By the 1920s, over 30 US states had passed involuntary sterilization laws targeting those with disabilities or low IQs. However, the eugenics movement declined after World War 2 due to its association with the Nazi regime and discrimination.