Methods, apparatus, systems and articles of manufacture to adjust content presented to an individual are disclosed. An example system includes a first modality sensor to measure a first response of an individual to first content during a first time frame and a second modality sensor to measure a second response of the individual to the first content during the first time frame. The first modality sensor is to measure a third response of the individual to first content during a second time frame, and the second modality sensor is to measure a fourth response of the individual to the first content during the second time frame. The example system also includes a mental classifier executing instructions to determine a first mental classification of the individual based on a first comparison of the first response to a first threshold and a second comparison of the second response to a second threshold. The mental classifier also is to determine a second mental classification of the individual based on a third comparison of the third response to a third threshold and a fourth comparison of the fourth response to a fourth threshold. In addition, the mental classified is to determine a mental state of the individual based on a degree of similarity between the first mental classification and the second mental classification. The example system also includes a content modifier to at least one of modify the first content to include second content or replace the first content with second content based on the mental state.