<p> This study examined if diagnostic overshadowing occurred with lay people in regard to individuals with an intellectual disability, as well as investigating how lay people essentialize different categories. It was hypothesized that essentialistic thinking could be offered as a partial explanation for diagnostic overshadowing because certain mental health disorders would be categorized as having a strong, unchangeable biological component to them. Three hundred and thirty undergraduate general psychology students from the University of Central Arkansas completed the Essentialism Belief Scale on nine different concepts, read different case descriptions of an individual with or without an intellectual disorder, and gave their impressions of the individual as experiencing anxiety, depression, and if the person had an intellectual disability. Contrary to expectation, lay people did not demonstrate diagnostic overshadowing. Therefore, the relationship between essentialistic thinking and diagnostic overshadowing could not be confirmed and suggests that diagnostic overshadowing may occur for reasons other than essentialistic thinking. Interestingly, when essentialistic thinking was analyzed using a principal components analysis, a three-factor solution for essentialistic thinking was found, accounting for 72.22% of the variance, with the three factors appearing to demonstrate a biological, non-biological, and mental health grouping.</p><p>