1. A method for measuring engagement comprising: presenting a first audiovisual stimulus to a first subject, a second subject, and a third subject; wherein presenting the first audiovisual stimulus comprises presenting the first audiovisual stimulus to the first subject, to the second subject, and to the third subject at substantially similar rates; capturing fMRI data from the first subject, the second subject, and the third subject, during presentation of the first audiovisual stimulus; calculating a first cross-brain correlation for the first subject and the second subject using the fMRI data; calculating a second cross-brain correlation for the first subject and the third subject using the fMRI data; calculating a third cross-brain correlation for the second subject and the third subject using the fMRI data; and generating an averaged measure of engagement from the first cross-brain correlation, the second cross-brain correlation, and the third cross-brain correlation.