The ability to understand others' mental states carries profound consequences for mental and physical health, making efforts at validly and reliably assessing mental state understanding (MSU) of utmost importance. However, the most widely used and current NIMH-recommended task for assessing MSU - the Reading the Mind in the Eyes Task (RMET) - suffers from potential assessment issues, including reliance on a participant's vocabulary/intelligence and the use of culturally biased stimuli. Here, we evaluate the impact of demographic and sociocultural factors (age, gender, education, ethnicity, race) on the RMET and other social and non-social cognitive tasks in an effort to determine the extent to which the RMET may be unduly influenced by participant characteristics. In total, 40 248 international, native-/primarily English-speaking participants between the ages of 10 and 70 completed one of five measures on TestMyBrain.org: RMET, a shortened version of RMET, a multiracial emotion identification task, an emotion discrimination task, and a non-social/non-verbal processing speed task (digit symbol matching). Contrary to other tasks, performance on the RMET increased across the lifespan. Education, race, and ethnicity explained more variance in RMET performance than the other tasks, and differences between levels of education, race, and ethnicity were more pronounced for the RMET than the other tasks such that more highly educated, non-Hispanic, and White/Caucasian individuals performed best. These data suggest that the RMET may be unduly influenced by social class and culture, posing a serious challenge to assessing MSU in clinical populations given shared variance between social status and psychiatric illness.