Since the work of Franz Boas and Bronisław Malinowski in the late 19th and early 20th centuries, social anthropology in Great Britain and cultural anthropology in the US have been distinguished from other social sciences by its emphasis on cross-cultural comparisons, long-term in-depth examination of context, and the importance it places on participant-observation or experiential immersion in the area of research. What did Margaret Mead advocate for?