Up-to-date comments are critical for the successful evolution of a software application. When modifying a function, developers may update the comment associated with the function or may not update it. For example, comments associated with a complex function are likely to be updated more often when the function is modified to prevent the code and the comments from drifting apart. Nevertheless, the rationale behind updating a comment has never been studied. In this paper, we present a large empirical study to better understand the rationale for updating comments. We recover the code change history for four large open source projects (GCC: a compiler, FreeBSD: an operation system, PostgreSQL: a database management system, and GCluster: a clustering framework) with an average code history of 10 years. Using the Random Forests algorithm, we investigate the rationale for updating comments along three dimensions: characteristics of the changed function, characteristics of the change itself a...