Learning from Quasi-Experiments In their early work, Money and colleagues treated gender identity as if it formed without biological infl uence. By 1972, in their classic book Man and Woman, Boy and Girl , Money and his colleague Anke Ehrhardt were less certain of the idea of total plasticity, but still emphasized the high degree of malleability in gender identity formation in the fi rst two years of life (Money & Ehrhardt, 1972). Regardless of what combination of biological and social forces contributed to gender identity formation, Money and Ehrhardt believed that gender identity became fi xed at some point around 2 years of age. Th eir concept of early identity fi xation became the underpinning for the view that “corrective” surgery for children born with ambiguous genitalia needed to be done swiftly. In more recent years, many have criticized the practice of early surgery (Dreger, 1998a, 1998b; Fausto-Sterling, 2000; Kessler, 1998).