To study beliefs about ability and STEM major choice, I conduct a field experiment where I provide students with information that they are above average in their top fields of study. I find that STEM students are more likely to switch out of their major and that non-STEM students fail to switch into STEM at the same rates as other fields. I also find that learning you are above average in your top field of study increases STEM major choice by almost a third, as STEM students appear more like to persist and non-STEM students increase their switching into STEM fields.