Developmentally disabled people, despite their quirks and oddities, are not so different from the rest of us, in fact, in many ways, they are exactly the same as us because their behaviors stem from very universal feelings, identical emotions that we all possess.
In the film, The Punishing Business, all of Addie's actions, from the tiniest to the largest, reflect this. Some people chew their fingernails, or smoke cigarettes, we all have our own weird little nervous habits -- Addie holds out a fist and says 'wanna knuckle sandwich.' Some people have low self worth and hate their body image and go to great lengths because of these negative thoughts -- Addie slaps a pretty person after looking at herself in the mirror, a literal manifestation of an emotion everyone has felt at one time or another. We all want to be liked and do things we think will make people like us -- Addie smilingly waves a gun around because she saw it on TV and thinks they will like her for it. Everybody in the world understands loneliness and fear of abandonment, and craves companionship -- Addie simply expresses it literally by physically trying to prevent Ellen from leaving. The same with grief, a universal emotion-- Addie enacts it literally by bashing her head because the physical pain is more bearable to the pain she feels inside. Similarly, grief is a concept/emotion that often defies language and words -- Addie reflects this by putting her 'mom' in her purse.
Pain doesn't ever really go away, but being alone and torturing one's self makes it even more unbearable; companionship, especially based on shared similarities, can help alleviate suffering.
Ellen and Addie share many similarities, they are two outsiders, both looked down on by society, both with hardened exteriors, both know what it's like to do harmful things without having intended harm, who are both struggling with their internal demons and consequently come to realize that through each other, their demons can perhaps hurt a little less.