There is significant variation in lay people's beliefs about the nature of intelligence: Some believe that intelligence is relatively fixed and innate, whereas others view intelligence as more malleable and affected by experience. However, most studies in this domain do not explicitly define intelligence when probing about beliefs about intelligence and aptitude. Thus, variation in beliefs may reflect variation in how intelligence is defined. To address this issue, we conducted 3 studies examining individuals' beliefs about fluid versus crystallized intelligence. Study 1 used a modified version of Dweck's (1999) mindset questionnaire and found that people have more fixed views about fluid intelligence than either crystallized intelligence or intelligence in general. Study 2 used a switched-at-birth paradigm and found that individuals hold more essentialist beliefs about fluid intelligence than crystallized intelligence. Study 3 added a survey that probed participants' beliefs about mathematics achievement. It found that when reasoning about mathematics achievements, participants' attributions of ability and effort were differentially associated with their crystallized and fluid mindset beliefs. Specifically, mindsets of fluid intelligence were more associated with effort for professional-level mathematics achievements, whereas mindsets of crystallized intelligence were more associated with elementary-level mathematics achievements. Together, the present studies highlight the importance of considering the definition of intelligence when assessing related beliefs about malleability, inheritance, and achievement. (PsycInfo Database Record (c) 2021 APA, all rights reserved).