Methods, apparatus, systems and articles of manufacture to adjust content presented to an individual are disclosed. An example method includes measuring, via sensors, responses of an individual to content during a first time, determining response classifications based on a comparison of the responses and respective thresholds, determining a first mental classification of the individual based on combining the response classifications, determining a baseline during the first time, measuring additional responses to the content during a second time, determining additional response classifications based on a comparison of the additional responses to respective additional thresholds, adjusting the baseline based on the additional responses in the second time, determining a second mental classification of the individual based on combining the additional response classifications, determining a mental state of a user based on a similarity of the first mental classification and the second mental classification, and modifying the content based on the mental state.