Surely you have heard the phrase. Is it merely a statement of the obvious? The phrase subliminally declares that boys are expected to behave differently. The details of exactly how differently boys were, and continue, to be expected to behave has been a socio-cultural conversation that has evolved enormously in recent history.
Looking specifically for modern western origins of masculine and juvenile behavior, one cannot overlook the historical context of rapid social change, mass-migrations, newly reformed institutions of education and governance, and an increasingly urban, industrialized economy. Historian Steven Mintz points out that "wrenching social and economic changes… the explosive growth of cities and industry, the rapid movement into the trans-Mississippi West, the sharp increase in foreign immigration… produced patterns of schooling, play and work that differed dramatically by class, ethnicity, gender, race and region."
Lest we forget: millions of lives lost in multiple world wars, the end of the frontier, and the expansion of civil rights, all of which had great impact on the life of young men. Throughout the 20th century, in the midst of so much change, young men faced ever-changing notions of what it meant to be an upright, contributing member of society.