Oldest children tend to be the most maladjusted & have the most psychological issues of all birth orders. Here's why:
(1) Oldest children were once only children at one time. When a succedent sibling or siblings are born, they are dethroned. Of course, this dethronement has an affect on them. They feel that they aren't important or loved anymore. Oftentimes, they regress to earlier behavior to gain parental attention. When they express their angst, they are dismissed or told to man up, not to be so "selfish" so they adopt passive-aggressive behaviors not only to their parents but covertly to their younger sibling or siblings.
(2) Oldest children are expected to be perfect & adult-like even when they are children themselves. Oldest children are oftentimes not viewed as children by their parents. They are held to more harsher, exacting standards than their younger sibling or siblings will ever be. They are the birth order treated the most disparately. They are considered the example setter, the trailblazer- so they had better not fall short-EVER! This inordinate pressure on the oldest child does something to the psyche but not in a positive way. They have to be THE ONE. They can never be relaxed nor be themselves it seems.
(3) Always putting EVERYONE first & themselves last. Oldest children must be there for their families. It is always THEM. They are the ones who carry the family. It isn't usual for oldest children to assume the emotional, mental, psychological, & even socioeconomic burdens of their families. Family is a tough town for oldest children. They MUST be there for everyone but SELDOM or NEVER, is anyone there for oldest children. In familial relationships, it is oftentimes a ONE-WAY street for oldest children. Overburdened is one apt word to describe the situation of oldest children in the family. It is also hard to never acknowledge one's own needs. Many oldest children are doormats as far as their families are concerned. They are afraid to so no or otherwise be assertive lest it is considered to be "selfish".