After reading Freud thinking, I totally agree with his thinking. For those kids, who have mental problems, were usually have family conflict, or I shall say it starts with a family problem. For kids, as the only child, they received all love and attention from parents, once the second child developed, that kid will get received parent’s partial love, and attention. The kid will feel lonely, their parents doesn’t love them anymore. (I feel lonely too, after my sister’s birth, feel like my parents don’t like me anymore, all they do is complaining about me.) Those kids will do things to get back those loves, or else personality reform will be shaped. They feel like since their parents don’t like them anymore, they can do whatever they want no matter what the consequence is as long as they were satisfied. The personality of jealous was formed.