At the time, she was completing her master's degree in psychology with research based on her mother's studies on Italian immigrants and working as an assistant to William Fielding Ogburn, helping him to edit the Journal of the American Statistical Association. She had decided that her doctoral studies would be anthropology. Anthropologists were concerned that opportunities to study cultures in secluded parts of the world would soon vanish. She decided to do field work in Polynesia and Boas. Her focus was on adolescents, rather than on the ethnography, or description, of an entire culture. Her father, who had previously offered her a trip around the world if she didn't marry Cressman, now agreed to pay her cost to Samoa. .
She was a very small and lively young woman, who was never very skillful at learning new languages. She set off alone for Samoa in 1925 to carry out the first study ever made of adolescent girls in an native society. Her documentation of that trip, Coming of Age in Samoa, became a best-seller and made her reputation, both as an anthropologist and as a writer. Furthermore, she thought that she had proved her thesis: the intensity and stress that westerners associate with adolescence were the products of western society, not inherent in human societies. .
Leaving Samoa, she sailed for Europe and her reunion with Cressman. But on the voyage she met Reo Franklin Fortune, a New Zealander on his way to Cambridge to study psychology. Cressman and her returned to New York and she then became a keeper of ethnology at the American Museum of Natural History, a position she would hold for the rest of her life. She divorced Cressman in 1928. Next she headed for New Zealand to marry Fortune. .
Her and Fortune left to do field work together in Melanesia. This time she studied infants and children and together she and Fortune developed new methodologies for field studies in anthropology.