Paul Bowman Popenoe was a researcher, writer, and social advocate who studied breeding in the United States during the twentieth century, first in plants and then in humans. Popenoe advocated for eugenic policies in California and beyond in the early twentieth century, and he introduced and promoted marriage counseling as a professional service in the US during the mid-twentieth century. The US eugenics movement, in which Popenoe participated, was a scientific and political project in which eugenicists attempted to improve human populations through selective breeding. Participants of the movement often advocated for the implementation of compulsory sterilization laws for certain “undesirable” populations. Though largely disavowed as of 2026, eugenics persisted well into the twentieth century. Through dozens of publications and a lifetime of public advocacy, Popenoe supported policies that led to the forced sterilization of thousands of people in California and popularized counseling as an intervention into people’s marriages and reproductive choices.

Theodosius Dobzhansky studied genetics and evolutionary biology, primarily using fruit flies, in the Soviet Union and the United States during the twentieth century. Scholars credit his widely read 1937 book Genetics and the Origin of Species with providing one of the earliest and most impactful integrations of Charles Darwin’s theory of evolution by natural selection with Gregor Mendel’s theory of genetics and therefore contributing significantly to the Modern Synthesis. In his research, Dobzhansky studied wild populations of organisms such as fruit flies and found that populations of the same species can differ in the frequency of one or more genetic traits. Understanding the role of such genetic diversity in evolution became a central theme of his research. Later in his career, he wrote about the social implications of his research, including the relationship between biological and cultural notions of race. Through both his scientific papers and popular writings, Dobzhansky shaped the course of evolutionary biology in the twentieth century and influenced public discussions of race in the years following World War II.