Psychologists claimed that notions about human behavior and mental processes should be supported by proof. In the late 1800s psychological laboratories were established in Europe and the United States. In these laboratories, psychologists studied behavior and mental processes using a sequence of experiments to test a single theory—methods similar to those Lavoisier had used to study chemistry. In 1879, most of the historians of psychology marked it as the beginning of psychology as a modern laboratory science. In that year, German psychologist Wilhelm Wundt recognized his laboratory in Leipzig.