As a medical anthropologist, Koss works at the interface between anthropology, psychiatry and psychology. Her primary interest is the treatment of illness and emotional problems, whether traditional, alternative or psychotherapeutic. She had conducted fieldwork in Latino cultures in the U.S., Latin America, Puerto Rico, Spain and Thailand. She also analyzed data from a study of mood change and depression among older women in Andalucia, Spain, and researched spirituality as a coping mechanism among Latinas in the U.S. living with HIV. She continues to be interested in spiritism in Puerto Rico.
She served as a member of two boards of the Metanexus Institute for Religion and Science and has been an advisor to their Spiritual Transformation Scientific Research Program. She also served on the board of the Society for the Study of Psychiatry and Culture and on the Scientific Committee of the World Association for Cultural Psychiatry.