Which early-20th-century anthropologist studied samoan adolescents, calling into question the assumption that adolescent psychological development was similar everywhere?

The answer is  Margaret Mead
Margared Mead choose the samoan civilization because they're secluded for the rest of the culture in that time (1925)
Her study showed that compared to western adolescents, Samoan Adolescents are far less stressful because they felt no restrictional standards that found in western society.

Margaret Mead was the anthropologist who studied samoan adolescents. She was a mass speaker in the 1960's and in the 1970's. When she died she was the most famous anthropologist in the world.

Whatever you believe