Writing is an important skill for communicating knowledge in science, technology, engineering, and mathematics (STEM) and an aid to developing students' communication skills, content knowledge, and disciplinary thinking. Despite the importance of writing, its incorporation into the undergraduate STEM curriculum is uneven. Research indicates that understanding faculty beliefs is important when trying to propagate evidence-based instructional practices, yet faculty beliefs about writing pedagogies are not yet broadly characterized for STEM teaching at the undergraduate level. Based on a nationwide cross-disciplinary survey at research-intensive institutions, this work aims to understand the extent to which writing is assigned in undergraduate STEM courses and the factors that influence faculty members' beliefs about, and reported use of, writing-based pedagogies. Faculty attitudes about the effectiveness of writing practices did not differ between faculty who assign and do not assign writing; rather, beliefs about the influence of social factors and contextually imposed instructional constraints informed their decisions to use or not use writing. Our findings indicate that strategies to increase the use of writing need to specifically target the factors that influence faculty decisions to assign or not assign writing. It is not faculty beliefs about effectiveness, but rather faculty beliefs about behavioral control and constraints at the departmental level that need to be targeted.