I wouldn't say there is a "psychological theme" to Adam Bede so much as there is a psychological mindset that infuses the themes of work ethic, personal integrity, and the power of love.
As for work ethic, Eliot rewards characters like the peasants who are not only hard-working, but the results of their efforts produce tangible results (milling, farming, etc.) Conversely, "bad" characters are slothful and unproductive. Thus, psychological commitment of characters to be useful or wasteful is a part of what makes them who they are.
A character's ability to either give or receive love also is a component of his psychology. Here, Eliot's characters are allowed (or given the opportunity) to grow. The character Hetty is an example of emotional and psychological growth, as she realizes the errors of her ways, opens her heart, and transforms her thinking. Thus, Hetty is able to grow.
The psychology of beauty is a prominent theme in the novel. Here, characters who are outwardly beautiful, like Hetty are contrasted with characters like Dinah, who is not attractive. Others tend to judge on outward, rather than inward beauty, often to their detriment, for it is Dinah who is truly beautiful. Dinah creates beauty through her love and kindness towards others.