Walter Mischel, a student of Julian Rotter and colleague of Albert Bandura, was a prominent personality psychologist known for two major contributions. First, his extensive review of empirical literature led him to conclude that personality traits are not as consistent across situations as previously believed, sparking the 'person-situation debate'. Second, he made significant contributions to the understanding of self-regulation, or willpower, particularly through his research on delayed gratification, exemplified by his famous marshmallow test.