This study investigated, by means of an ethnographic approach, what exactly occurs when Japanese novice writers are socialized into the undergraduate thesis genre and how the processes differ among disciplines. Data were collected from 10 undergraduate students enrolled in various humanities disciplines at two Japanese universities. The study identified the genre chains in psychology and non-psychology consisting of the thesis genre and the related genres to capture the whole picture of their socialization processes, and then examined cognitive and sociocultural aspects of their learning. All of the four psychology students were systematically inducted into research and experimental reports, a genre similar to undergraduate theses, in their first 2 years with emphasis on acquisition of knowledge and skills of the research process and report writing, which appear to follow the tradition of the natural sciences. In the last 2 years, the students were then weekly guided by their supervisors specifically in writing their undergraduate thesis in groups. The non-psychology students were socialized into the undergraduate thesis only in the last 2 years, with a particular focus on subject-matter knowledge, which was solely dependent on their supervisors, in the group and/or individual sessions. On the other hand, the psychology students experienced less difficulty in writing their theses than the non-psychology students. The study concludes that the highly prescriptive and regulated training for writing in the disciplinary context at the early stage via the written genres similar to the thesis were likely to account for the psychology students' success.