Final answer:
Trait theory suggests that Joe's niceness can be attributed to his high agreeableness trait.
Step-by-step explanation:
Trait theory focuses on identifying stable characteristics and ways of behaving in individuals. According to trait theory, Joe's niceness can be associated with the trait of agreeableness. Agreeableness is characterized by being pleasant, cooperative, trustworthy, and good-natured. People who score high on agreeableness tend to exhibit positive interpersonal relationships and may have higher incomes as well (Judge, Livingston, & Hurst, 2012).