Walker also took an interest in demographics later in his career, particularly towards the issues of immigration and birth rates. He published "The Growth of the United States" in 1882 and "Restriction on Immigration" in 1896 arguing for increasing restrictions out of concern about the diminished industrial and intellectual capacity of the most recent wave of immigrants. Walker also argued that unrestricted immigration was the major reason behind nineteenth-century Native American fertility decline, but while the argument was politically popular and became widely accepted in mobilizing restrictions on immigration, it rested upon a surprisingly facile statistical analysis that was later refuted. Writing on immigrants from southern Italy, Hungary, Austria, and Russia in "The Atlantic", Walker claimed,