Simplify the following text for a Grade 10 native English-speaking person.
Original: Laboratory rats have also proved valuable in psychological studies of learning and other mental processes ( Barnett , 2002 ) .
Simplified: Laboratory rats have also proved valuable in psychological studies of learning and other mental processes ( Barnett , 2002 ) , as well as to understand group behavior and overcrowding ( with the work of John B. Calhoun on behavioral sink ) .